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Side-effects of electrotrawling: Exploring the safe operating space for Dover sole (Solea solea L.) and Atlantic cod (Gadus morhua L.)
... Low frequency pulses (5 Hz) induced a muscle contraction at every pulse and elicit a flight response Soetaert et al. 2016b). Increasing the pulse frequency to 20 Hz or more elicited a cramp response that immobilized the fish as long as the duration of the pulse (Soetaert et al. 2016b. ...
... Low frequency pulses (5 Hz) induced a muscle contraction at every pulse and elicit a flight response Soetaert et al. 2016b). Increasing the pulse frequency to 20 Hz or more elicited a cramp response that immobilized the fish as long as the duration of the pulse (Soetaert et al. 2016b. After cessation of the pulse stimulus, fish often showed a strong swimming response (de Haan et al. 2015;Soetaert et al. 2016b). ...
... Increasing the pulse frequency to 20 Hz or more elicited a cramp response that immobilized the fish as long as the duration of the pulse (Soetaert et al. 2016b. After cessation of the pulse stimulus, fish often showed a strong swimming response (de Haan et al. 2015;Soetaert et al. 2016b). When the field strength or pulse duration was raised, a stronger response could be elicited, such as an epileptic seizure or sedation. ...
Between 2009 and 2021 almost the complete beam trawl fleet of the Netherlands switched from conventional beam trawls (BT) to pulse trawls (PT) using electrical stimulation to catch sole, Solea solea. Electric fishing, being banned in the EU in 1988, was made possible in 2006 under a derogation. Over the years stakeholders expressed concern about ecosystem effects. Here we review the research conducted. PT improved the selectivity of the fishery and reduced the ecological side effects. PT caught more sole per hour fishing but less discards and benthos than BT. The transition to PT reduced the surface area swept (lower towing speed), sediment depth of disturbance and associated benthic impacts, as well as fuel consumption. Laboratory experiments with 9 fish and 17 benthic invertebrate species showed that exposure to a commercial bipolar pulse stimulus did not result in harmful effects except in cod. Autopsy of cod sampled from PT revealed that 40% had an internal injury. Injury rates in other roundfish species was low (< 2%) and absent in flatfish. Electrical-induced impacts on biogeochemistry were not observed. The transition increased competition with other fishers which fed the resentment against PT. Governance arrangements under which the number of temporary licenses expanded, undermined legitimacy of the gear, resulting in a ban in 2021. Although questions about the ecological impact of electrical stimulation remain, adverse effects are considered negligible in comparison with the benefits.
... It is unknown if fish can respond to low electric field strengths generated by pulse trawlers. Common solethe target species for pulse trawlersresponds to high field strengths by curling their body upwards in a U-shape during which they are immobilised, without obvious damage (Soetaert et al., 2015b;Soetaert et al., 2016b). Whole-body muscle contractions in non-target species, especially bilaterally-symmetrical round fish, may however lead to spinal injuries and internal haemorrhages, as studied in the laboratory (de Haan et al., 2016;Soetaert et al., 2016a;Soetaert et al., 2016b;Soetaert et al., 2018;Schram et al., 2022) and under field conditions (van Marlen et al., 2014;Soetaert et al., 2016c;Boute, 2022;Boute et al., 2022;Boute et al., 2023). ...
... Common solethe target species for pulse trawlersresponds to high field strengths by curling their body upwards in a U-shape during which they are immobilised, without obvious damage (Soetaert et al., 2015b;Soetaert et al., 2016b). Whole-body muscle contractions in non-target species, especially bilaterally-symmetrical round fish, may however lead to spinal injuries and internal haemorrhages, as studied in the laboratory (de Haan et al., 2016;Soetaert et al., 2016a;Soetaert et al., 2016b;Soetaert et al., 2018;Schram et al., 2022) and under field conditions (van Marlen et al., 2014;Soetaert et al., 2016c;Boute, 2022;Boute et al., 2022;Boute et al., 2023). Whole-body muscle contractions occur at field strengths above 37 V m -1 in Atlantic cod (Gadus morhua) resulting in a 50% spinal injury probability in larger specimens at 80 V m -1 (95% CI: 60-110 V m -1 ) (de Haan et al., 2016). ...
... These species represent round fish and flatfish as well as bycatch and target species of the pulse-trawl fishery, and inhabit areas trawled by pulse vessels (Heessen et al., 2015). In addition, small-spotted catshark, European seabass, and common sole have previously been studied in electrotrawling research, albeit with a focus on injuries (Soetaert et al., 2016b;Desender et al., 2017;Soetaert et al., 2018). Although many fish species with different body shapes and sizes may encounter pulse trawls, our aim is to quantify individual and species-specific behavioural thresholds and compare electroreceptive with nonelectroreceptive species to make a first assessment of behavioural sensitivity differences. ...
Electrical pulse trawling is an alternative to conventional beam trawling for common sole (Solea solea), with substantially less discards, lower fuel consumption, and reduced impact on the benthic ecosystem. Pulsed electric fields are used to drive the fish from the seafloor and immobilise them in front of the nets. Concerns exist, however, that the electric fields may affect fishes outside the trawl track. Here, we address these concerns by measuring amplitude thresholds for behavioural responses and by comparing these response thresholds to simulated field strengths around fishing gear. Electroreceptive small-spotted catshark (Scyliorhinus canicula) and thornback ray (Raja clavata) as well as non-electroreceptive European seabass (Dicentrarchus labrax), turbot (Scophthalmus maximus), and common sole were, one at the time, placed in a ø2.5 m circular tank with seven, individually controlled, evenly spaced electrode pairs, spanning the diameter of the tank. Behavioural responses were assessed from camera recordings for different pulse amplitudes and for different fish positions relative to the stimulating electrodes. Electrical stimulation consisted of a Pulsed Bipolar Current at 45 Hz and 0.3 ms pulse width, similar to that used in commercial gears. Computer simulations of the electric field, verified with in situ measurements, were used to determine the field strength at the location of the animal. Thresholds for different species varied between 6.0 and 9.8 V m–1, with no significant difference between electroreceptive and non-electroreceptive species. The thresholds correspond to a distance of maximally 80 cm from the electrode arrays in simulated electric fields around commercial fishing gears. These findings suggest that electrical pulses as used in pulse trawling are unlikely to elicit a behavioural response outside the nets that surround the electrode arrays.
... Marine electrofishing is likewise known to inflict internal injuries (Soetaert et al ., 2015 ). Relatively high spinal injury occurrences (7-11%) have been reported in Atlantic cod ( Gadus morhua ) caught in PUL trawls or trawls with electrified benthos release panels (van Marlen et al ., 2014 ;Soetaert et al ., 2016c ) and between 0-37% in controlled laboratory conditions depending on fish length and electric field strength (de Haan et al ., 2016 ;Soetaert et al ., 2016a ;Soetaert et al ., 2016b ). By comparison, injury occurrence in whiting ( Merlangius merlangus ) was low. ...
... Van Marlen et al. (2014) reported a single injured whiting out of 57 filleted specimens sampled from pulse gear catches, whereas Boute et al. ( 2022 ), in a more extensive study with X-radiography, showed that < 2% of the whiting sampled from commercial PUL trawl catches showed a spinal injury. In experiments where fish were exposed to a commercial pulse stimulus in controlled laboratory conditions no spinal injuries were found in sole, dab ( Limanda limanda ), European seabass ( Dicentrarchus labrax ), lesser sandeel ( Ammodytes tobianus ), and greater sandeel ( Hyperoplus lanceolatus ) (de Haan et al ., 2015 ;Soetaert et al ., 2016b ;Soetaert et al ., 2018 ;Schram et al ., 2022b ). ...
... Major spinal injuries ranged between a subluxation or compression of several vertebrae (e.g. spinal misalignment) and fractured, or clearly dislocated vertebrae, as regularly observed in laboratory exposure experiments in the context of marine electrotrawling (de Haan et al., 2016 ;Soetaert et al., 2016a ;Soetaert et al., 2016b ) and in freshwater electrofishing studies (Fredenberg, 1992 ;Hollender and Carline, 1994 ;Snyder, 2003 ). Ma-jor haemorrhages (h2) covered a substantial area as typically found in pulse-exposed cod (de Haan et al., 2016 ;Soetaert et al., 2016a ;Soetaert et al., 2016b ). ...
To improve the ecological and economic sustainability in the Dutch beam trawl fishery, tickler chains were replaced by electrical pulse stimulation to drive common sole (Solea solea) out of the seabed. Because electrical stimulation may cause internal injuries, we quantified this risk by sampling fish species from commercial beam trawlers and recording spinal injuries and haemorrhages from X-radiographs and autopsy. To distinguish mechanically-induced and electrical-pulse-induced injuries, we compared injuries in ten species sampled from pulse (PUL) and tickler-chain (TCK) trawlers and four species sampled from PUL trawlers with the stimulus switched on or off. Co-occurrence of a major spinal injury and major haemorrhage at the same location was only observed in PUL samples, and were frequently (40%) observed in Atlantic cod (Gadus morhua) and in low numbers (0–2%) in whiting (Merlangius merlangus), grey gurnard (Eutrigla gurnardus), and greater sandeel (Hyperoplus lanceolatus), but not in flatfishes and other species. In cod, injury occurrence correlated with fish length, with lower probabilities for small fish. Major spinal injury or major haemorrhage occurrence in PUL (range: <1–16%) was lower than in TCK (range: <1–42%) in eight of the ten species studied. Population level consequences of pulse-induced injuries are considered negligible.
... The available scientific knowledge did not support these claims. In particular, laboratory experiments did not find an increased mortality rate among marine organisms exposed to the electric pulse used in the fishery for sole (Smaal and Brummelhuis, 2005;Soetaert et al., 2015b;Soetaert et al., 2016a;Soetaert et al., 2016b;Soetaert et al., 2018;de Haan et al., 2009;de Haan et al., 2015), whilst field studies showed that over 90% of undersized plaice, common sole, turbot, brill and thornback ray caught in a commercial pulse trawl is alive when landed on deck (Schram and Molenaar, 2018, submitted). The only adverse effect of electrical stimulation shown by laboratory and field studies is the occurrence of spinal-injuries in cod (Van Marlen et al., 2014;De Haan et al., 2016;Soetaert et al., 2016a;Soetaert et al., 2016b). ...
... In particular, laboratory experiments did not find an increased mortality rate among marine organisms exposed to the electric pulse used in the fishery for sole (Smaal and Brummelhuis, 2005;Soetaert et al., 2015b;Soetaert et al., 2016a;Soetaert et al., 2016b;Soetaert et al., 2018;de Haan et al., 2009;de Haan et al., 2015), whilst field studies showed that over 90% of undersized plaice, common sole, turbot, brill and thornback ray caught in a commercial pulse trawl is alive when landed on deck (Schram and Molenaar, 2018, submitted). The only adverse effect of electrical stimulation shown by laboratory and field studies is the occurrence of spinal-injuries in cod (Van Marlen et al., 2014;De Haan et al., 2016;Soetaert et al., 2016a;Soetaert et al., 2016b). However, direct mortality in the wake of pulse trawlers was never studied under realistic field conditions. ...
... Exposure to electrical pulses may inflict internal injuries, such as spinal injuries and hemorrhages, which could result in a delayed mortality (De Haan et al., 2016;Soetaert et al., 2016a). Because the recorded injury rate in pulse trawl catches was low (<2.5%) in 14 dominant North Sea fish species sampled (Boute, 2022;Boute et al., 2022) it is highly unlikely that pulse-induced internal injuries will result in mass mortality. ...
Between 2009 and 2020, beam trawlers in the North Sea switched to electrical stimulation to target sole (Solea solea). The transition to pulse trawling raised widespread concern about possible adverse effects of electrical stimulation on marine organisms. Environmental NGO’s and small scale fishers claimed that it would electrocute marine life and create a ‘graveyard’ in the wake of pulse trawlers. This paper uses realistic field experiments to investigate the ‘graveyard’ hypothesis. In cooperation with fishers, a field experiment was designed where we simultaneously sampled marine organisms in the wake of pulse trawlers and in untrawled control areas. The impact was quantified by estimating the direct mortality among three dominant fish species and four dominant invertebrate species. In total, nine experimental tows were conducted in two years. Direct mortality among fish and invertebrates was low (0-10%) and did not differ between the pulse trawl track and the untrawled controls. Equally, no impact of the pulse trawl was found on external damages and vitality scores. The limited effects observed are likely due to the mechanical impact of the pulse and the sampling gear. The results of experiment do not support the claim that pulse trawling results in mass mortality among marine organisms in the trawl track. Throughout the research period, the concerns of small-scale fishers on pulse fishing shifted from being focused on biological effects to political and managerial issues. This can partly be attributed to the engagement in and the results of our research and has increased its credibility and salience. By integrating fishers’ knowledge and examining their perceptions through transdisciplinary research, we were able to show the importance of untangling the intricate relation between perceived knowledge gaps and political or management related concerns.
... High spinal injury probabilities, however, were detected for Atlantic cod (Gadus morhua), lesser sandeel (Am-modytes tobianus), and greater sandeel (Hyperoplus lanceolatus). The spinal injuries observed in the sampled cod are caused by pulse-induced muscle contractions as shown in laboratory experiments (de Haan et al., 2016;Soetaert et al., 2016a, b). For sandeels, the cause of spinal injuries is less clear as injury probability was elevated in catches of both pulse and tickler-chain gears, with the highest injury probability in the tickler-chain catches. ...
... The two electrodes array were spaced 42.5 cm apart on top of a 0.3 m layer of coarse sand (1.0-1.6 mm) in a polyester tank (2.05 × 1.40 × 0.9 m; l × w × h) with a water level of 0.6 m above the sediment. A pulsed bipolar current (PBC) stimulus was generated by a LPG or Delmeco pulse generator as used in earlier experiments (de Haan et al., 2016;Soetaert et al., 2016a;de Haan and Burggraaf, 2018). Both pulse systems generated alternating positive and negative rectangular-shaped pulses that were evenly distributed in time (Soetaert et al., 2019). ...
... sediment, animals will experience lower internal FS, and are therefore presumably less affected than animals in the water column that are exposed to the same external FS (ICES, 2020;Boute, 2022). The electrical properties of the body depend on factors such as body shape, insulating characteristics of the skin, body fluids, muscle fibre type, and subcutaneous fat layers (Snyder, 2003;Polet, 2010;Soetaert et al., 2016a;Boute, 2022). These factors may set different thresholds for inducing double-sided muscle cramps. ...
Electric stimulation was used in the North Sea beam trawl fishery for common sole to reduce its environmental impact. Because electrical stimulation may cause internal injuries in fish, a laboratory experiment was conducted to study the effect of pulse exposure on lesser sandeel (Ammodytes tobianus) and greater sandeel (Hyperoplus lanceolatus), important mid-trophic species in the North Sea ecosystem. We exposed 244 sandeels between two electrodes to a pulsed bipolar current for 2 s in an experimental cage with 5 cm sediment; 221 control fish were
handled similarly but not exposed. The occurrence of spinal injuries and internal haemorrhages were scored using X-radiography and dissection. None of the sandeels exposed to a field strength of up to 600 V m–1 showed spinal injury or haemorrhage. Equal numbers of minor spinal abnormalities were found in exposed and control fish. In the absence of spinal injuries, we estimated by bootstrapping the field strength below which spinal injuries are unlikely to occur, i.e. the lower limit threshold, and the corresponding limit dose–response relationship between field strength and injury probability. We conclude that it is unlikely that pulse trawl fishery will have an ecologically significant adverse effect on the population abundance of sandeels, because of the low probabilities of exposure and injury.
... Other advantages include increased selectivity ; van Marlen et al., 2014), higher discard survival (van der Reijden et al., 2017), and higher net revenues (Batsleer et al., 2016). However, concerns have been expressed regarding the occurrence of internal injuries in non-target fish species exposed to the pulsed electric field (de Haan et al., 2016;Desender et al., 2016;ICES, 2018;Quirijns et al., 2018;Soetaert et al., 2016bSoetaert et al., , 2016a; van Marlen et al., 2014). ...
... Although pulse fishing primarily targets common sole, non-target fishes may also be exposed to high electric field strengths in close proximity to the electrodes (de Haan et al., 2016;de Haan and Burggraaf, 2018). Simultaneous muscle contractions on both sides may lead to spinal injuries and haemorrhages (de Haan et al., 2016;Soetaert et al., 2019Soetaert et al., , 2016aSoetaert et al., , 2016bvan Marlen et al., 2014). For large fish, this may be irrelevant as they are retained and processed on board, but for small specimens, internal injuries may compromise survival due to a reduced chance for escaping, or increased risk for predation (e.g. ...
... Few studies have investigated internal injuries due to electrical pulse exposure in sole-targeting marine electrotrawling. Common sole and European seabass (Dicentrarchus labrax) were found insensitive to electrical-pulse induced internal injuries (Soetaert et al., 2018(Soetaert et al., , 2016b. In Atlantic cod (Gadus morhua), high occurrences (7-11%) of spinal injuries and haemorrhages have been reported for fish caught with pulse gears or electrified benthos release panels (Soetaert et al., 2016c;van Marlen et al., 2014). ...
Electrical pulse fishing has been widely adopted by Dutch fishers as an economically viable alternative to tickler-chain trawling for common sole (Solea solea) in the North Sea. Concerns exist, however, that the use of electrical pulses may cause spinal injuries and haemorrhages, as previously shown for Atlantic cod (Gadus morhua). To find out whether other gadoids are similarly affected, we studied injury occurrences in whiting (Merlangius merlangus) catches on commercial vessels. To distinguish mechanically and electrically-induced injuries, we compared (1) injuries for pulse gears with electrical pulses either turned on or off and (2) injuries from pulse-trawl catches with those in tickler-chain trawling. Spinal injuries were visualised with X-radiography and internal haemorrhages with subsequent dissection. Injuries were categorised on a severity scale and their location was quantified along the anteroposterior fish axis. Major spinal injury occurrence in (1) pulses-on and pulses-off samples were lower than 1% and not significantly different between catch methods. Major spinal injury occurrence was slightly higher in (2) tickler-chain catches (2.4%) than in pulses-on samples (1.1%). Major haemorrhage occurrences were also low. The slightly higher occurrences of these haemorrhages in pulses-on samples (1.8%) compared to fish caught with tickler chains (0.3%) and their locations suggest that they may be partly related to electrical-pulse exposure. Overall, our results indicate that injuries in whiting are rare and primarily due to mechanical impact. These findings suggest that pulse trawling is unlikely to impose increased mortality on whiting populations compared to the tickler-chain technique.
... The electric field near to the electrode array, within the surrounding nets, induces muscle cramps in the fish . These muscle cramps may consequently lead to spinal injuries and haemorrhages (van Marlen et al., 2014;de Haan et al., 2016;Soetaert et al., 2016aSoetaert et al., , 2016bSoetaert et al., , 2016cSoetaert et al., , 2018. Internal injuries, however, may also be caused by an external mechanical load acting on the body of the fish. ...
... The percentage of fish with at least one spinal injury are provided in Table 7 Our results corroborate that Atlantic cod (Godus morhua) is sensitive to pulse-induced injuries, as has previously been found in laboratory studies (de Haan et al., 2016;Soetaert et al., 2016bSoetaert et al., , 2016a, and field studies (van Marlen et al., 2014;Soetaert et al., 2016c). Atlantic cod do not appear highly sensitive to mechanically-induced injuries. ...
... Exposure experiments with sole, plaice, sea bass, small-spotted cat shark did not find evidence for direct mortality of fish exposed to a commercial pulse stimulus (de Haan et al., 2015;Soetaert et al., 2016a;Desender et al., 2017b;Molenaar, pers comm), nor caused ulcers in dab (de Haan et al., 2015). Elevated injury probabilities were also recorded for lesser and greater sandeel in pulse trawl catches. ...
This report presents the results of a four year research project “Impact Assessment Pulse trawl Fishery (IAPF)“ on the biological and ecological effects of electric pulse trawls used in the fishery for North Sea common sole (Solea solea).
... The pulsed bipolar (PBC) and pulse alternating current (PAC) used in pulse trawls that target common sole can affect organisms variably (Soetaert et al. 2015a. Exposure experiments with fish did not reveal injuries or mortality in sole (Soetaert et al., 2016a), European sea bass (Soetaert et al., 2018), small-spotted catshark Desender et al., 2017b), while no effect on ulcers was detected in dab (de Haan et al., 2015). Spinal injuries and heamorrhages were observed in exposure experiments with cod, but the frequency of occurrence varied substantially (de Haan et al., 2016;Soetaert et al., 2016b). ...
... The electric field near to the electrode array, within the trawl track, induces muscle cramps in the fish . These muscle cramps may consequently lead to spinal injuries and haemorrhages (van Marlen et al., 2014;de Haan et al., 2016;Soetaert et al., 2016aSoetaert et al., , 2016bSoetaert et al., , 2016cSoetaert et al., , 2019. In addition to pulse-induced injuries, internal injuries may also be caused by an external mechanical load acting on the body of the fish. ...
... The percentage of fish with at least one spinal injury of categories 3 to 5 are provided in Table 8.3 per species and catch method (Boute et al. in prep). These spinal injuries correspond to those previously reported in experimental studies (van Marlen et al., 2014;de Haan et al., 2016;Soetaert et al., 2016bSoetaert et al., , 2016a. Our results corroborate that Atlantic cod (Godus morhua) is sensitive to pulseinduced injuries, as has previously been found in laboratory studies (de Haan et al., 2016;Soetaert et al., 2016bSoetaert et al., , 2016a, and field studies (van Marlen et al., 2014;Soetaert et al., 2016c). ...
The Working Group on Electrical Trawling (WGELECTRA) works on improving knowledge of the effects of electrical or pulse fishing on the marine environment. At the 2020 meeting, the working group considered the Scottish Ensis fishery, ongoing work on shrimp pulse fishery study and analysed the possible contribution of pulse trawling to reducing or increasing the ecosystem/environmental impacts of the North Sea sole fishery and its fuel consumption. Substantial efforts were invested during the last 10 years to examine the effect of pulsed currents at the individual level on a range of species, species groups and life stages. Exposure to the pulsed bipolar current (PBC), used in pulse trawling for sole, does not result in direct mortality in fish and invertebrates, but may cause spinal injuries in fish. Pulse induced injury rate is low (<=1%) in the twelve fish species studied and population level effect will be negligible. Injury probability in cod is 36% and seems to decrease in small cod. The population level consequences are considered negligible. Adverse effect on electroreceptive species is unlikely because they are sensitive for low frequency direct current and not to high frequency PBC. Non-lethal effects are considered unlikely due to low exposure. No adverse effects (mortality or lesions) were found for the benthic invertebrate species exposed to the sole pulse, and animals returned to normal behaviour less than one hour after exposure. This made any long-term ecological effect unlikely. The low exposure probability and short duration implies no chronic exposure to pulse stimuli. Pulse trawling has less mechanical impact on the benthic ecosystem than conventional beam trawling. The lower towing speed of pulse trawls led to reduced mobilization of sediments, and resulted in a smaller footprint and a reduced surface area swept when exploiting the sole quota. The replacement of tickler chains by electrodes reduced the depth of disturbance of the trawl and likely reduced the average mortality imposed on benthic invertebrates. Although no specific experiments have been carried out on Natura 2000 species, the available knowledge suggests that the probability of exposure is likely to be (very) low. Natura 2000 habitats will have been exposed less by pulse trawls compared to conventional beam trawls. CO2 emissions of pulse trawlers are lower than those of conventional beam trawlers due to an estimated reduction in fuel consumption by ~50% per unit of sole quota and ~20% per unit of total landings. Pulse trawls catch, per hour, more sole and less plaice and other species and can contribute to a reduction in the bycatch of undersized fish (discards) and benthic invertebrates. Pulse trawling does not impose a risk to the sustainable exploitation of sole if the stock is well managed, although an increase in local fishing pressure was observed in the southern North Sea following introduction of the pulse trawl.
... Similarly, ragworm (Alitta virens) and brown shrimp exposed up to 4 times to the shrimp pulse did not show a reduced 14 day survival (Soetaert et al. 2014). This result was confirmed in a follow up study in which brown shrimp was exposed 20 times to the shrimp pulse in a commercial setting without showing adverse effects on survival, moulting or the number of egg carrying females 14 days after the start of the experiment (Soetaert et al. 2016a). ...
... In addition, the number of melanomacrophage centres (MMCs) in the spleen of exposed cod was significantly higher than in the non-exposed animals. No haemorrhages, MMCs or other lesions were observed in sole and cod 14 days after a 2 s exposure to the crangon pulse as well as other much stronger pulse stimuli between plate electrodes (Soetaert et al., 2016a). This indicates these histological deviations were reversible and healed after 14 days. ...
... In addition, the same electrodes and configuration were used as those previously applied by IMARES. But this time, no injuries were observed (Soetaert et al., 2016a). There was also no mortality during the first two weeks following exposure, and no other injuries could be detected. ...
The Working Group on Electrical Trawling (WGELECTRA), works on improving knowledge of the effects of electrical or pulse fishing on the marine environment. In this report the group provide details of ongoing work including preliminary results, upcoming research projects, and possibilities for international collaboration and scientific publications. A living document, overviewing the current published scientific knowledge on pulse trawling, was updated and attached as annex.
Highlights of ongoing research included presentations about laboratory and field experiments on the effects of electrical stimulation on fish, benthic invertebrates and biogeochemical processes. Results were presented of a monitoring project in which pulse trawl skippers record the catch and effort by tow. This information provides insight in the dynamics of pulse trawlers exploiting local aggregations of sole and can shed light on the effects of pulse fishing on local aggregations of fish and on possible competitive interactions between vessels. Further, pulse stimulation can also be used to increase selectivity in the net instead of a capture technique. This was shown by the studies of the Instituut Voor Landbouw-, Visserij-, en Voedingsonderzoek (ILVO) showing that a 200 mm benthos release panel equipped with an electrical stimulus (eBRP) has the potential to release up to 75% of the benthos and debris immediately after capture without significant losses of marketable Dover sole and other species. Other work focused on pulse trawling targeting brown shrimp. The preliminary results of an elaborate Dutch study including active pulse trawlers targeting shrimp confirmed that a pulse trawl with a straight bobbin rope and 12 bobbins can obtain similar or slightly higher catches of marketable shrimp, while showing drastic reductions in the bycatch of other invertebrates and fish. Finally, the group discussed a number of upcoming research projects including the study of the changes in catch rate of various fisheries in relation to emergence of pulse fishing.
... Current European regulations ban the use of AC waveforms and advise on <60 Hz pulsed direct current (PDC) where used in freshwater electrofishing (Anon., 2003). However, these settings are used by at least one marine equipment manufacturer of pulse trawls targeting common sole, which may explain why injuries in by-catch of Atlantic cod are encountered in this fishery and not in pulse trawls targeting brown shrimp using a 5 Hz square-wave PDC startle pulse (Desender et al., 2016;Soetaert et al., 2016a). Hence marine electrotrawling targeting common sole may be optimized further by learning from electrofishing methods used to capture fish in freshwater environments. ...
... Moreover, the extremities of an animal placed in a homogeneous field will have a constant potential difference, regardless of their position, as long as their orientation remains unchanged. Hence, homogeneous electric fields are used in laboratory set-ups to study the effects of electrical stimulation on organisms, since this design enables standardization with minimum variability in field strengths (Soetaert et al., 2014(Soetaert et al., , 2016a. Note that in a natural environment many factors can distort the idealized model of the electric field propagated from electrodes e.g. by conductive objects being within the field. ...
... In previous pulse trawling research, PAC has been used to refer to the waveform type where the polarization reversal occurred (almost) immediately followed by a long (inter pulse) interval time, whereas PBC was used when the interval time between the polarization reversal was equal ( Figure 5) (Soetaert et al., 2016a, b). We propose to make the distinction threshold between PAC and PBC based on the length of the pulse width (PW ) and pulse break time (PB). ...
Electricity can be used to facilitate fish and invertebrate capture in both marine and freshwater environments. In freshwaters, electrofishing is largely used for research or management purposes. In marine environments electrofishing is principally used in the form of electrotrawling for the commercial capture of fishes and benthic invertebrates, in particular common sole (Solea solea L.), brown shrimp (Crangon crangon L.), and razor clams (Ensis spp.). The terminology and definitions used to describe the electrical stimulus characteristics and experimental setups have, so far, been diverse and incomplete, hampering constructive discussion and comparison of electrofishing studies. This paper aims to (i) harmonize existing terminology, abbreviations, and symbols, (ii) offer best practice recommendations for publishing results, and (iii) provide a concise and comprehensible reference work for people unfamiliar with this topic. By incorporating common practice in marine electric pulse trawling terminology and related freshwater electrofishing studies, based on existing terms where possible, we provide a framework for future studies. The suggested guideline is recommended by the ICES Working Group on Electrical Trawling as a constructive approach to improved communication standards in electrofishing and electrical pulse stimulation research and publications.
... This doubt was strengthened by the observation of spinal injuries in Atlantic cod (Gadus morhua L.) after exposure to the cramp pulse for sole (van Marlen et al., 2014;de Haan et al., 2016;Soetaert et al., 2016b, d). However, no negative side-effects on adult cod are reported after exposure to the startle pulse for brown shrimp (Desender et al., 2016;Soetaert et al., 2016a), nor in sole, plaice (Pleuronectes platessa L.), bull-rout (Myoxocephalus scorpius L.) and armed bullhead (Agonus cataphractus L.) (Desender et al., 2016;Soetaert et al., 2016a). Additionally, no sideeffects were demonstrated on the electro-sensitive organs of smallspotted catshark (Scyliorhinus canicula L.) which are used for prey detection, nor on other organs or its survival (Desender et al., 2017). ...
... This doubt was strengthened by the observation of spinal injuries in Atlantic cod (Gadus morhua L.) after exposure to the cramp pulse for sole (van Marlen et al., 2014;de Haan et al., 2016;Soetaert et al., 2016b, d). However, no negative side-effects on adult cod are reported after exposure to the startle pulse for brown shrimp (Desender et al., 2016;Soetaert et al., 2016a), nor in sole, plaice (Pleuronectes platessa L.), bull-rout (Myoxocephalus scorpius L.) and armed bullhead (Agonus cataphractus L.) (Desender et al., 2016;Soetaert et al., 2016a). Additionally, no sideeffects were demonstrated on the electro-sensitive organs of smallspotted catshark (Scyliorhinus canicula L.) which are used for prey detection, nor on other organs or its survival (Desender et al., 2017). ...
... Flatfish generally showed the opposite reaction. Indeed, adult plaice and the majority of sole (Solea solea L.) showed only minor reactions and remained close to the seafloor during laboratory experiments (Desender et al., 2016;Soetaert et al., 2016a). This lack of escape behaviour under the influence of the shrimp startle pulse, in combination with a reduced mechanical stimulation, explains their lower abundance in the pulse trawl's catches since they cannot end up in the net as long Fig. 5. Catch comparison between the experimental pulse trawl (grey) and the traditional shrimp trawl (white). ...
... However, concerns have been raised regarding possible negative side effects of these electric pulses on marine organisms, especially since spinal injuries have been observed in gadoids, particularly Atlantic Cod Gadus morhua (van Marlen et al. 2014;de Haan et al. 2016;Soetaert et al. 2016aSoetaert et al. , 2016bSoetaert et al. , 2016c. Nevertheless, tetany did not induce severe lesions in flatfish such as Sole (Soetaert et al. 2016b), Dab Limanda limanda (also known as Pleuronectes limanda; de Haan et al. 2015), or Small-Spotted Cat Sharks (also known as Spotted Dogfish) Scyliorhinus canicula (Desender et al. 2017). ...
... However, concerns have been raised regarding possible negative side effects of these electric pulses on marine organisms, especially since spinal injuries have been observed in gadoids, particularly Atlantic Cod Gadus morhua (van Marlen et al. 2014;de Haan et al. 2016;Soetaert et al. 2016aSoetaert et al. , 2016bSoetaert et al. , 2016c. Nevertheless, tetany did not induce severe lesions in flatfish such as Sole (Soetaert et al. 2016b), Dab Limanda limanda (also known as Pleuronectes limanda; de Haan et al. 2015), or Small-Spotted Cat Sharks (also known as Spotted Dogfish) Scyliorhinus canicula (Desender et al. 2017). Based on these results, it seems that osteichthyan fusiform fish are especially vulnerable to injury by electric pulses. ...
... The major aim of this study was to investigate the vulnerability of Sea Bass to electrically induced injuries and to reveal differences in susceptibility among fish species. Exposed fish exhibited tetany on being electrically stimulated, followed by an escape response similar to that reported for Atlantic Cod (de Haan et al. 2016;Soetaert et al. 2016aSoetaert et al. , 2016b; marine flatfish such as Sole, Dab, and Plaice Pleuronectes platessa (Stewart 1977;Soetaert et al. 2016b); and invertebrates such as brown shrimp C. crangon (Soetaert et al. 2014(Soetaert et al. , 2016d. ...
... Studies investigating the side effects of these electric pulses have revealed varying results for different marine species. Exposure of invertebrates (Soetaert et al. 2014), Sole (Soetaert et al. 2016), and European Seabass Dicentrarchus labrax (also known as European Bass Morone labrax ;Soetaert 2015) to the electric pulses used in the field did not elicit mortality or lesions. Gadoid roundfish such as Whiting (also known as European Whiting) Merlangius merlangus and Atlantic Cod Gadus morhua, on the other hand, displayed spinal injuries (De Haan et al. 2011;van Marlen et al. 2014;Soetaert et al. 2016), albeit with different levels of severity in terms of the number of animals affected. ...
... Exposure of invertebrates (Soetaert et al. 2014), Sole (Soetaert et al. 2016), and European Seabass Dicentrarchus labrax (also known as European Bass Morone labrax ;Soetaert 2015) to the electric pulses used in the field did not elicit mortality or lesions. Gadoid roundfish such as Whiting (also known as European Whiting) Merlangius merlangus and Atlantic Cod Gadus morhua, on the other hand, displayed spinal injuries (De Haan et al. 2011;van Marlen et al. 2014;Soetaert et al. 2016), albeit with different levels of severity in terms of the number of animals affected. These spinal injuries resulted from powerful convulsions of the body musculature when fish were exposed to electric pulses (Snyder 2003). ...
... In the same study, juvenile cod (12-16 cm) exposed to high field strengths of 250-300 V/m did not exhibit vertebral injuries (De Haan et al. 2011). In a recent laboratory study, no spinal injuries were observed when cultured and wild-caught cod were exposed to similar pulses in a homogenous electric field (Soetaert et al. 2016). These results suggest that the extent to which electric pulses exert a negative impact does not depend solely on the electric pulse parameters but may also be affected by fish-specific morphological differences or the experimental conditions (conductivity, electrode configuration, and so forth). ...
Pulse trawling is the most promising alternative to conventional beam trawls targeting Sole Solea solea (also known as Solea vulgaris), but due to the electric fields created by electrotrawls spinal injuries are reported in gadoid round fishes such as Atlantic Cod Gadus morhua. This study aimed to investigate the variability in the occurrence of electric-induced spinal injuries in cod. Four groups of cod, each originating from a different wild or farmed stock, were exposed to the pulses used by electrotrawls targeting Sole. Effects were analyzed based on behavior, mortality, and lesions up to 14 d after exposure, and morphological characteristics such as size, somatic weight, muscularity, the number of vertebral bodies, and the vertebral mineral contents of animals were compared among different cohorts. Second, the influence of factors such as water temperature, electrode diameter, and pulse type and amplitude were tested. Electrode diameter and pulse amplitude showed a positive correlation with the intensity of the fish’s reaction. However, the present experiments confirmed that cod also show variable vulnerability, with injury rates ranging from 0% to 70% after (almost) identical exposures near the electrode. This indicates that these injuries are not only determined by the pulse parameter settings but also by subtle, fish-specific factors. Although the absence of a sensitive group of cod did not enable the elucidation of the conclusive factor, the effect of physiological and morphological factors such as intervertebral ligaments and rearing conditions during early life merit further attention in future research.
Received September 3, 2015; accepted March 16, 2016
... Four out of 45 cod exposed caught by electrotrawls were reported to have paravertebral haemorrhages (van Marlen et al., 2014). This was confirmed in laboratory experiments showing 0-70% spinal injuries in cod exposed near the electrodes (Soetaert et al., 2016a;de Haan et al., 2016). However, no effects were demonstrated in an identical experiment with the non-gadoid roundfish European seabass (Dicentrarchus Labrax L.) (Soetaert, 2015b), dogfish (Scyliorhinus Canicula L.) and dab (Limanda limanda L.) (de Haan et al., 2015). ...
... However, no effects were demonstrated in an identical experiment with the non-gadoid roundfish European seabass (Dicentrarchus Labrax L.) (Soetaert, 2015b), dogfish (Scyliorhinus Canicula L.) and dab (Limanda limanda L.) (de Haan et al., 2015). Exposure of sole to a broad range of electric stimuli could also not provoque lesions or mortality (Soetaert et al., 2016a). Finally, no increased impact of electric cramp stimuli compared to conventional mechanical stimulation has been evidenced for invertebrate species either (Smaal and Brummelhuis, 2005;van Marlen et al., 2009;Soetaert et al., 2014;2016c). ...
... These conductors were attached transversally to the 240 mm BRP at a mutual distance of 0.48 m (four meshes in the BRP), starting from 0.6 m in front of the BRP to the very end of the panel ( Figure 2). Electric stimuli similar to those used in commercial electrotrawls targeting sole were applied (80 Hz bipolar pulsed current, 0.25 ms pulse duration) (Soetaert et al. 2016a), aiming for an 'immobilizing cramp reaction' in the sole's muscle to prevent it from escaping through the panel. The electric potential difference on the electrodes however, was limited to 40 V due to the capacity of the generator, whereas 655 V is used in the commercial trawls (Soetaert et al., 2015a). ...
Benthos release panels (BRPs) are known for their capacity to release large amounts of unwanted benthos and debris, which
can decrease mortality on these animals and eases the on board sorting process aboard demersal beam trawlers. They can reduce
the bycatch of undersized fish, which is desired once the European discard ban is implemented. However, unacceptable commercial
losses of sole (Solea solea L.) and damage to the BRP as a consequence of suboptimal and unsuitable rigging in the traditional beam trawl with chain
mat, is hampering a successful introduction in commercial beam-trawl fisheries. To eliminate these drawbacks, square-meshed
BRPs with different mesh sizes (150, 200, and 240 mm) were rigged in a trawl with square net design as used in electrotrawls
and tested for selectivity. In addition to this, the effect of electric stimulation at the height of the BRP to eliminate
the loss of commercial sole was examined. According to our observations, no abrasion of the net attributable to suboptimal
rigging occurred in any of the BRPs tested. The catch comparisons showed significant release of benthos and undersized fish
in all panel mesh sizes, but there was always a significant loss of marketable sole in the 150, 200, and 240 mm BRPs. Adding
a 80 Hz electric cramp stimulus to the BRP, resulted in equal catches of sole larger than 25 cm as the standard net, without
negatively affecting the release of benthos and most undersized commercial fish. This clearly demonstrates the promising potential
of electrified BRPs (eBRPs), but further optimization by using smaller BRP mesh sizes or optimized electric stimuli is warranted
to retain all marketable sole.
... Most beam trawls use mechanical stimulation to chase fish up from the seabed and concentrate them in the mouth of the net before catching them [3]. Alternatively, electrical stimulation, that either startle or cramp the fish, can be used to improve the selectivity and reduce the adverse effects from mechanical stimulation [38]. In this study, the application of electrical stimulation in the North Sea beam trawl fisheries for brown shrimps, Crangon crangon and common sole, Solea solea was investigated. ...
... Replacing the tickler chains with electrodes is expected to reduce these adverse effects. Electrical stimulation invokes a cramp response that causes sole to bend into a U-shape and prevents them from escaping from the approaching gear [38,45]. The pulse trawl is often used in combination with a wing-shaped foil (called SumWing) that allow to reduce drag of the gear. ...
... For each core exposed to electrical currents, two plate-shaped stainless steel electrodes were used to generate a homogenous electric field (Soetaert et al., 2016;Soetaert et al., 2019;Fig. 3). ...
... 1). For exposure experiments in the laboratory, a homogenous electric field using flat electrodes is preferred in order to expose the whole sample to the same electric field strength (Soetaert et al., 2016;Boute et al., 2020). To obtain a homogenous electric field inside an incubation core while exposing the full sediment column, we oriented the electrodes vertically, with one electrode attached to the bottom of the core and the other suspended above the sediment (Fig. 3), whereas flatfish pulse trawls and Ensis electrofishing gears have horizontal oriented electrodes. ...
Both physical and electrical impacts have been linked to North Sea fisheries activity. This study evaluates how these effects can influence marine ecological functioning by assessing their consequences on benthic pelagic coupling. Experiments were conducted on sediment microcosms taken from 9 North Sea and 2 Eastern Scheldt locations. Samples were subjected to physical disturbances by mechanically stirring the sediment surface or electrical stimulation with exposure to high frequency pulsed bipolar or direct currents. Electrical exposure times of 3 and 120-s were used to simulate in situ exposure times related to sole (Solea solea) and razor clam (Ensis spp.) electric fisheries respectively. Water column oxygen rapidly declined after sediment resuspension, inducing an immediate uptake ranging from 0.55 to 22 mmol oxygen per m2 of sediment disturbed. Mechanical disturbances released the equivalent of up to 94 and 101 h of natural ammonium and silicate effluxes respectively. Fresh organic material significantly predicted the magnitude of mechanical-induced oxygen, ammonium, phosphate and silicate changes. No biogeochemical effects from bipolar (3 s or 120 s) or 3-s direct current exposures were detected. However, significant changes were induced by 120-s exposures to direct currents due to electrolysis and ionic drift. This lowered the water column pH by 1-1.3 units and caused the appearance of iron oxides on the sediment surface, resulting in the equivalent of 25-28 h of sedimentary phosphate removal. Our findings demonstrate that prolonged (+1 min) exposure to high frequency pulsed direct currents can cause electrochemical effects in the marine environment, with implications for phosphorus cycling. Nevertheless, bi-directional pulsed currents used in flatfish pulse trawling and AC waveforms featured in Ensis electrofishing, seem to severely limit these effects. Mechanical disturbance, on the other hand, causes a much greater effect on benthic pelagic coupling, the extent of which depends on sediment grain size, organic matter content, and the time of the year when the impact occurs.
... Increasing voltage, frequency, or pulse duration could improve the usefulness of an electric field to repel sawfish. However, higher energy pulses (high voltage) would lead to very high field strengths around the electrodes, potentially leading to injury or death of animals (including sawfish) that get too close to the electrodes (Roth et al. 2003, Dolan & Miranda 2004, Soetaert et al. 2016a. Such high voltage would also require large amounts of electricity to be produced, making it expensive, potentially dangerous to humans, and unpractical to run. ...
... In teleosts, electric fields that elicit strong cramp reactions can lead to haemorrhages and spinal injuries (e.g. Snyder 2003, Soetaert et al. 2016a. Although the 2 studies on elasmobranchs available to date (to our knowledge) did not report negative effects (de Haan et al. 2009, Desender et al. 2017) on the small-spotted catshark Scyliorhinus canicula, we did not subject the sawfish to stronger electric fields due to ethical concerns. ...
Sawfishes are among the most threatened families of marine fishes and are susceptible to incidental capture in net fisheries. Since bycatch reduction devices currently used in trawl fisheries are not effective at reducing sawfish catches, new methods to minimise sawfish bycatch are needed. Ideally, these should affect sawfish behaviour and prevent contact with the fishing gear. We tested the effects of electric fields on sawfish behaviour to assess the potential of electric pulses in mitigating sawfish bycatch. Experiments were conducted in a tank where 2 electrodes were suspended in the water column, connected to a pulse generator, and placed across the swimming path of sawfish. Two largetooth sawfish Pristis pristis were tested in control conditions, in the presence of a baseline pulse, and of 5 variations of that pulse where 1 parameter (polarity, voltage, frequency, pulse shape, pulse duration) was altered at a time. Conditional inference trees were used to identify the effects of various parameters (e.g. treatment, individual) on reaction type, reaction distance, twitching presence and duration, and inter-approach times. Sawfish reacted to electric fields, but reaction distances were small (typically <1.2 m), and no field tested consistently led to reactions conducive to escaping from moving nets. The following parameters induced the most response in both individuals: bipolar current, rectangular shaped, 5-10 Hz, ~1500 µs duration, and 100 V. We recommend further research focussing on moving nets, testing a V-shaped electric array preceding the net mouth by at least 5 m, and testing a setup similar to electrotrawling.
... In pulse trawls, tickler chains were replaced by electrodes that fire pulses of alternating current at a frequency of 30-45 pulse cycles per second (de Haan et al., 2016;Soetaert et al., 2019). The pulses invoke a cramp response that causes sole to bend in a Ushape, facilitating their catch (van Stralen, 2005;Soetaert et al., 2016a). The use of pulse trawls reduced fuel costs and mitigated ecological impacts of the fishery (van Balsfoort et al., 2006;Soetaert et al., 2015b). ...
... Possible adverse effects of electrical stimulation on marine biota and the benthic ecosystem are currently being investigated. Except for the spinal injuries in cod and whiting (de Haan et al., 2016;Soetaert et al., 2016b), the available studies have not shown adverse effects on other fish species and benthic invertebrates (Soetaert et al., 2015a;Soetaert et al., 2016a;Desender et al., 2017a, b). ...
changes in bottom trawling for flatfish species as a result of the replacement of mechanical stimulation by electric stimulation. Although fishing with electricity is illegal in the European Union, a number of temporary licences allowed converting beam trawlers to pulse trawling. To analyse how the adaption of pulse trawling changed this fishery, we studied fishing speeds and landings per unit effort as proxies for catch efficiencies for the main target species. Compared to conventional tickler chain beam trawls, pulse trawls were towed at lower speeds (small vessels À10%, large vessels À23%). Large vessels that switched from conventional beam trawls to pulse trawls at the end of 2009 gradually increased catch efficiency for sole over the period of almost 1 year. While pulse trawling was found to have higher catch rates (kg/h) for sole (small vessels þ74%, large vessels þ17%), lower catch rates were observed for plaice (small vessels À31%, large vessels À32%). Vessels that switched later achieved immediate gains in catch efficiency for sole. The change in catch efficiency is likely due to the difference in cramp response between the species.
... Since many questions remained on the impact of this electrical stimulation on the ecosystem, the alternative fishing technique will be banned in the EU in July 2021 (Sandra et al., 2019). Nonetheless, a lot of research has been performed to estimate this influence and explore the safety ranges of this alternative technique Soetaert et al., 2016). We will focus on the described direct effect of electrical pulse stimulation on fish and bacteria. ...
... The response of common dab, and other flatfish species, on the electrical stimulation is a contraction of the dorsal musculature leading to a muscular cramp, often described as a U-shape contraction with tail and head pointing upwards (van Stralen, 2005;de Haan et al., 2015). After exposure, common dab exhibits a startle or escape response (Soetaert et al., 2013;de Haan et al., 2015;Soetaert et al., 2016). Experiments using common dab showed that exposure to an electric stimulation did not result in the development of macroscopic skin injuries or increased mortalities that could be directly attributed to the exposure to an electric current (de Haan et al., 2015). ...
... Concerns are also increasing regarding the impacts of shrimp pulse trawls on neighboring nontarget organisms. Electric fields, as adopted in the shrimp pulse, seem to have only limited impact on Atlantic Cod Gadus morhua, Sole, and other adult stages of marine organisms (Polet et al. 2005a;Soetaert et al. 2014Soetaert et al. , 2016Desender et al. 2016). However, very little is known about possible negative effects of pulsed DC during different early life stages of bottom dwelling species. ...
... Adult Sole exhibited a cramp or escape response during exposure to a pulse of more than 40 Hz and below 20 Hz, respectively. No gross or microscopic lesions, mortality, nor deviant behavior were observed (Desender et al. 2016;Soetaert et al. 2016). Data on the effect of electric pulses on marine flatfishes during their early life stages, i.e., embryos or larvae, were fully lacking at the initiation of the present research. ...
Despite electric pulse fishing being regarded as a promising environmentally friendly fishing method, very little is known about possible negative effects on early life stages of bottom‐dwelling species such as Sole Solea solea. Electrotrawling for brown shrimp Crangon crangon is increasingly used in shallow coastal zones and estuaries of the southern North Sea. As these fishing grounds are often important nurseries and spawning areas for various marine species, electrotrawling could therefore harm inhabiting fishes during their early life stages. Hence, this research aims to investigate the effect of electric pulses used to catch brown shrimp on the survival and development of Sole embryos and larvae. Exposure of Sole embryos at 2 d postfertilization and larvae at 11 d posthatching to pulsed DC used to catch brown shrimp did not result in a lower survival 8 d after exposure. Additionally, no differences in yolk sac resorption and morphometric length measurements of the notochord, muscle, eye, and head were observed in the developing larvae.
... In order to provide sufficient basis for dispensing with the standing ban completely and implement this fishing technique on a broad commercial scale, one should clarify possible adverse ecosystem effects in accordance with the principles of the precautionary approach and responsible fishing (FAO, 2011). Despite the spinal injury encountered in cod (de Haan et al., 2008(de Haan et al., , 2011(de Haan et al., , 2016Rasenberg et al., 2013;van Marlen et al., 2014;), results of various studies substantiate a tentatively positive attitude towards electric fishing in terms of sustainability (Polet et al., 2005a;Smaal and Brummelhuis, 2005; van Marlen et al., 2009;Teal et al., 2014;Desender et al., 2016aDesender et al., , 2016bSoetaert et al., 2014Soetaert et al., , 2016de Haan et al., 2015). However, major gaps in knowledge on the impact of electric fishing still remain. ...
... Pulse parameter settings generated in the LPG were characterized by a bipolar square pulse shape and pulse duration of 250 μs generated at a frequency of 80 Hz. The applied voltage to the electrodes had a constant amplitude of 60 V (Soetaert et al., 2016). ...
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Pulse fishing may pose a promising alternative for diminishing the ecosystem effects of beam trawling. However, concerns about the impact on both target and non-target species still remain, amongst others the possible damage to the electro-receptor organs, the Ampullae of Lorenzini, of elasmobranchs. The current study aimed to examine the role of pulsed direct current (PDC) used in pulse trawls on the electro-detection ability of the small-spotted catshark, Scyliorhinus canicula. The electroresponse of the sharks to an artificially created prey-simulating electrical field was tested before and after exposure to the pulsed electrical field used to catch flatfish and shrimp. No statistically significant differences were noted between control and exposed animals, both in terms of the number of sharks exhibiting an electroresponse prior to and following exposure as well as regarding the timing between onset of searching behaviour and biting at the prey simulating dipole. These results indicate that, under the laboratory circumstances as adopted in this study, the small-spotted catshark are still able to detect the bioelectrical field of a prey following exposure to PDC used in pulse trawls. However, to fully grasp the impact of PDC on elasmobranchs, further studies are imperative, including examining the effect on reproduction and young life stages, the longer-term and indirect influences and experiments under field conditions.
... Nevertheless, in 2009 European Union member states were granted an exemption permitting 5% of the fleet to use pulse trawls in the southern North Sea, which was extended to 10% of the fleet in early 2014. By January 2016, 91 vessels had already adopted this technique commercially, of which 1, 3, 10, and 77 have Belgian, UK, German, and Dutch licenses, respectively (Soetaert et al. 2016). Although these vessels differ in rigging and the weight of fishing gear, their electrical parameters are similar and can be roughly divided into two pulse types as a function of the target species. ...
... The first type, constituting the vast majority of pulse vessels, targets flatfish, particularly Sole Solea solea (also known as Solea vulgaris). These electrotrawls use a bipolar cramp pulse with a frequency of around 80 Hz that elicits a cramp reaction in the fish's muscles which immobilizes it and causes it to bend into a U-shape (Soetaert et al. 2016). As a consequence, Sole cannot flee and are more easily scooped up by the foot rope of the fishing gear, which increases catch efficiency (Soetaert et al. 2015). ...
Pulse trawling is currently the best available alternative to beam trawling in the brown shrimp Crangon crangon and Sole Solea solea (also known as Solea vulgaris) fisheries. To evaluate the effect of repetitive exposure to electrical fields, brown shrimp were exposed to the commercial electrodes and pulse settings used to catch brown shrimp (shrimp startle pulse) or Sole (Sole cramp pulse) 20 times in 4 d and monitored for up to 14 d after the first exposure. Survival, egg loss, molting, and the degree of intranuclear bacilliform virus (IBV) infection were evaluated and compared with those in stressed but not electrically exposed (procedural control) and nonstressed, nonexposed (control) brown shrimp as well as brown shrimp exposed to mechanical stimuli. The lowest survival at 14 d (57.3%) occurred in the Sole cramp pulse treatment, and this was significantly lower than in the group with the highest survival, the procedural control (70.3%). No effect of electrical stimulation on the severity of IBV infection was found. The lowest percentage of molts occurred in the repetitive mechanical stimulation treatment (14.0%), and this was significantly lower than in the group with the highest percentage of molts, the procedural control (21.7%). Additionally, the mechanically stimulated brown shrimp that died during the experiment had a significantly larger size than the surviving individuals. Finally, no effect of the shrimp startle pulse was found. Therefore, it can be concluded that repetitive exposure to a cramp stimulus and mechanical stimulation may have negative effects on the growth and/or survival of brown shrimp. However, there is no evidence that electrical stimulation during electrotrawls would have a larger negative impact on brown shrimp stocks than mechanical stimulation during conventional beam trawling.
Received November 15, 2015; accepted May 16, 2016
... Although all organisms were still alive 14 days following exposure, the possibly elicited lesions were not evaluated, the behaviour pre-and post-exposure was not studied nor was Atlantic cod (Gadus morhua) included in the study. Since spinal injury occurs in 10% of the cod caught on-board of the flatfish pulse trawlers (Rasenberg et al., 2013; van Marlen et al., 2014), it is imperative to include this fish species in studies evaluating the safety of electric pulses (Soetaert et al., 2016). Snyder (2003) reported that, although often not externally obvious or fatal, spinal injuries and associated haemorrhages may be regularly present as a result of exposure to electricity, warranting the need for radiological and histological examination in safety assessment studies. ...
... During experiments performed by Haan et al. (2009 Haan et al. ( , 2011), spinal injury in cod exposed to the flatfish cramp pulse was observed in one quarter of the animals when stimulated close to the electrode. When exposed in a homogeneous electrical field, in one of the 39 cod a spinal column dislocation occurred (Soetaert et al., 2016). The finding in the present study that spinal injury was not encountered in cod, underscores the opportunity for developing and employing a startle pulse substituting the cramp pulse for flatfish leaving cod unscathed. ...
... Electrical parameters are mainly determined by the voltage, frequency, and pulse width (Holliman, 2011;Parker, Glover, Finney, Rogers, Stewart, Simmonds, & Rosenfeld, 2015;Soetaert et al., 2016). Because the current field gradually weakens outwards along the electrode in water, the high voltage of the bus can produce a current field that satisfies the anti-escape (Kim & Mandrak, 2019). ...
Because traditional barring facilities have difficulty in controlling the water surface and have low intelligence degree, a new type of intelligent electronic pulse barring facility was developed. The system established a dynamic charging and discharging model, an electric field strength calculation model, and a forward reverse rotation conduction model. The facility used a controller area network field bus to realize cooperative operation between one master machine and multiple slave machines that was suitable for fish blocking in various water areas. By using insulated gate bipolar transistors instead of thyristors, the electrode adopted an instantaneous rotational conduction mechanism that was efficient, safe, reliable, and resulted in low noise pollution. The system realized the application of the Internet of Things and an automatic fault phone alarm function in the aquaculture industry such that users could use phones or computers for real‐time remote monitoring. A failure of the fish barring facility would immediately trigger an alarm, such that corresponding measures could be taken to avoid the escape of many fish and complete the unattended aquaculture industry. After multiple freshwater fish arresting tests, the load capacity of the whole machine was 0.5 Ω, and the power consumption was 0.3–2 kW; this could save 65% more electricity than traditional thyristors and had a maximum interception area of 1200 m². This has significant practical application value in improving the digitalization, automation, and intelligence level of the aquaculture industry.
... For plaice and dab the catch numbers in the pulse net increased by a factor of 2.2 and 2.5, respectively. These results are consistent with the cramp response of fish to the electrical stimulation (van Stralen, 2005;de Haan et al., 2016;Soetaert et al., 2016) which immobilises the fish that will no longer be able to escape from the approaching gear by flight or burrowing behaviour. The effect is particularly strong in sole which curls up into a U-shape where the nose and tail may even touch each other (van Stralen, 2005). ...
The beam trawl fishery targeting sole is known for their substantial bycatch of flatfish below the minimum landing size. Pulse trawls were developed to improve the selectivity by replacing mechanical stimulation with electrical stimulation which immobilises fish in front of the footrope. Results are presented of an experiment on board of a commercial pulse trawler studying the effect of electrical stimulation on footrope and cod-end selectivity for three flatfish species - sole (Solea solea), plaice (Pleuronectes platessa) and dab (Limanda limanda) - in 29 paired hauls with electrical stimulation alternating between the starboard and portside gear. It was shown that electrical stimulation increased the footrope selection by a factor 2 in plaice and dab and a factor 7 in sole. The effect on sole is related to the specific response of sole which cramps into a U-shape. Footrope selection showed a small diurnal pattern with the highest selectivity of the pulse trawl during the day. Electrical stimulation and catch weight were shown to have a small effect on the cod-end selectivity retaining slightly more marketable sole. Cod-end mesh selection factors (SF) were estimated at 3.00 (se = 0.02), 2.11 (se = 0.02), and 2.3 (se = 0.1) for sole, plaice and dab, respectively. Selection ratio (SFA = selection range/mesh size) was estimated at 0.45 (se = 0.03), 0.23 (se = 0.02), and 0.41 (se = 0.04) for sole, plaice and dab, respectively. The SF and SFA of the pulse trawl is comparable to values reported for conventional beam or otter trawls.
... Electrical stimulation induces a cramp response in the fish that inhibits their ability to escape from the approaching trawl (Soetaert et al., 2015;de Haan et al., 2016). Pulse trawling proved to be particularly efficient for sole, which cramp in a U-shape making them easy to catch (Soetaert et al., 2016;Poos et al., 2020). Here we compare the sediment mobilization by the conventional tickler chain and chain mat beam trawls with the sediment mobilization by innovative pulse trawls as used in the Dutch fleet. ...
Bottom trawls impact the seafloor and benthic ecosystem. One of the direct physical impacts is the mobilization of sediment in the wake of trawl gear components that are in contact with or are close to the seabed. The quantity of sediment mobilized is related to the hydrodynamic drag of the gear components and the type of sediment over which they are trawled. Here we present a methodology to estimate the sediment mobilization from hydrodynamic drag. The hydrodynamic drag of individual gear components is estimated using empirical measurements of similarly shaped objects, including cylinders, cubes, and nets. The method is applied to beam trawls used in the Dutch North Sea flatfish fishery and validated using measurements of beam trawl drag from the literature. Netting contributes most to the hydrodynamic drag of pulse trawls, while the tickler chains and chain mat comprise most of the hydrodynamic drag of conventional beam trawls. Taking account of the silt content of the areas trawled and the number of different beam trawl types used by the fleet, sediment mobilization is estimated as 9.2 and 5.3 kg m−2 for conventional 12 m beam and pulse trawls, respectively, and 4.2 and 4.3 kg m−2 for conventional 4.5 m beam and pulse trawls.
... The pulse trawl has been subject to a considerable amount of scientific research, including questions on the selectivity of the gear [19,20], impacts on the behaviour and injuries to non-target species [21][22][23][24], and the impact on benthic habitat and fauna over both the short and long term (e.g. [23,25,26,27]). ...
The decision to ban the pulse trawl by the European Parliament in early 2019 was influenced by public debate over its scientific and political legitimacy. In their 2019 paper in Marine Policy, Le Manach et al. continued this debate by making three claims. First, that the pulse trawl has substantial negative social and environmental impacts. Second, that it received ‘illegal’ subsidies from the European Union amounting to €20.8 million. And third, that the Dutch government and industry did not provide adequate transparency in the allocation of subsidies. These claims are misleading and, in some instances, demonstrably false: the ongoing science on the effects of the pulse trawl shows relatively positive impacts; following the conventional definitions of “harmful” and “fisheries subsidies” in the economic literature, harmful subsidies provided to pulse trawling only amount to €0.3 million, or less than 2% of Le Manach et al.‘s estimate; and there is no evidence of intentional non-disclosure of information related to the distribution of subsidies by the Dutch government. Finally, we reflect on the consequences of the inaccuracies presented by Le Manach et al. in the governance of fishing gear innovation and their wider effect on the legitimacy of advocacy-based science.
... In the marketable sized cod, the fracture probability decreased with body size in marketable sized cod. In another experiments with cod with similar pulse settings and similar location of the cod next to the conductor, much fewer fractures were observed (0-5%), suggesting that body condition may influence the sensitivity for injuries (Soetaert et al., 2016a). Cod exposed to a homogeneous electric field with a range of pulse settings, including those of the commercial fisheries, did not show any abnormalities when examined histologically, except for 1 cod showing a spinal fracture (Soetaert et al., 2015). ...
WGELECTRA met three times (22–24/10/2014; 10–12/11/2015, and 17–19/01/2017) to discuss the ongoing research projects in Belgium, the Netherlands, and Germany and provide an overview of the state of the art knowledge of the ecological effects. Pulse trawls are used in the North Sea fishery for flatfish and brown shrimp. The shrimp pulse applies a low frequency pulse that invokes a startle response (tailflip) in shrimps. The sole pulse applies a higher frequency that invokes a cramp response that immobi-lise the fish species facilitating the catching process. The use of electricity in fishing has raised considerable concern among stakeholders which is mainly focused on the un-known effects on marine organisms and the functioning of the benthic ecosystem but also altered fishing efforts & catch efficiencies.
A number of laboratory experiments have been carried out in which a selection of fish species and invertebrate species have been exposed to electrical stimuli to study possi-ble adverse effects. The maximum pulse treatment applied exceeded the strength of the pulse used by the fishery. Electrical stimulation did not cause direct mortality dur-ing exposure. Exposure to the sole pulse stimuli invoked vertebral fractures and asso-ciated haemorrhages in roundfish species (cod), but not in flatfish species (sole, plaice, dab) or seabass. The results suggest that fractures are restricted to the larger size classes of cod that are retained in the net, whereas smaller cod that can escape through the 80 mm meshes did not develop fractures even when exposed to high field strength. The fracture incidence in cod increases with field strength and decreases with pulse frequency. Fracture incidence varied between experiments. Experimental induced frac-tures corresponded to fractures observed in cod and whiting sampled from commercial pulse trawls. Further studies are required to study the relationship between the frac-tures and the body size and determine the differences in fractures across fish species. Shrimp pulse exposure did not invoke fractures in roundfish or flatfish species.
Histological examination of fish exposed to pulse stimuli in laboratory experiments, did not reveal other abnormalities in species examined, except for a small haemorrhage in 2 of the 25 exposed plaice, and a significant increase in melanomacrophage centres in the spleen of cod exposed to the shrimp pulse 24 h after exposure. No adverse effect could be detected on the electro-sense organ used in food detection behaviour of small-spotted catshark. In an experiment exposing brown shrimp and ragworms to a sole pulse showed no consistent adverse effects, but shrimps that were exposed 20 times during a 4-day period to a sole pulse showed an increased mortality compared to one of the control treatments, but not compared to the 2nd control treatment or to mechani-cally stimulated shrimps.
Little is known on the effects of electrical stimulation on the development of eggs and larvae. One experiment exposing early life stages of cod (egg, larvae, early juveniles) to a pulse stimulus exceeding the pulse used in the fisheries did not find an increase in developmental abnormalities in exposed animals, but observed a reduced hatching rate and an increased mortality in 2 out of the 8 experiments. No adverse effects were observed in sole eggs and larvae.
No studies have been done on the effect of pulse stimulation on the functioning of the benthic ecosystem and nutrient dynamics. Although the laboratory experiments sug-gest that fish and invertebrates resume their normal behaviour after exposure, no in-formation is available on for instance the threshold levels at which the functioning of species is being adversely affected.
Electrical stimulation changes the species selectivity of the trawl. The catch efficiency of the pulse trawl for sole is higher, and the catch efficiency for plaice and other fish species is lower, when expressed in terms of the catch rate per swept area. It is uncer-tain whether the pulse trawl has a better size selectivity (reduced bycatch of undersized fish), but all experiments show that the bycatch of benthic invertebrates is substantially reduced. Applying electrical stimulation in the fishery for brown shrimp, offers a promising innovation to reduce the bycatch of fish and benthic invertebrates, while maintaining the catch rate of marketable sized shrimps. The reduction in bycatch de-pends on the design of the net, in particular the specifics of the groundrope.
In ecological terms, the replacement of the tickler chain beam trawl with pulse trawl with electrodes diminish the mechanical impact of trawling on the North Sea benthic ecosystem. Although the irreversible effects of electrical stimulation seem to be re-stricted to the vertebral fractures in cod and whiting, further research on the effects of electrical stimulation on marine organisms and ecosystem functioning is needed to as-sess the effects on the scale of the North Sea.
Beam trawl fisheries for sole are characterised by large amounts of unwanted bycatch (i.e. discards) consisting of fish below the Minimum Conservation Reference Size, unwanted fish due to low commercial value or lack of quota, and benthic invertebrates. In order to reduce the quantity of discards, a substantial part of the Dutch beam trawl fleet was allowed to replace the conventional tickler chain beam trawl (BT) with the pulse trawl (PT) on an experimental basis. The PT used electrical stimulation to immobilise and capture fish. Here we study whether pulse trawling reduced the amount of discards by comparing catch rates of landings and discards of BT and PT in the period 2009–2018 for a wide range of species. The PT caught (kg.km⁻²) significantly more marketable sized sole (Solea solea, 48 %), turbot (Psetta maxima, 8 %), brill (Scophthalmus rhombus, 28 %) and whiting (Merlangius merlangus, 95 %), and significantly less marketable sized plaice (Pleuronectes platessa, −16 %), cod (Gadus morhua, −32 %) and gurnards (−12 %). No significant difference was found for dab (Limanda limanda), gadoids, or rays and sharks. Among discards, the PT caught more undersized sole (27 %) and whiting (42 %) but less undersized plaice (−21 %), dab (−19 %) and grey gurnard (Eutrigla gurnardus, −31 %). The observed differences in species selectivity are discussed in relation to the response of fish to bottom trawl gear and the effects of pulse stimulation. For the benthic invertebrates Ophiuroidae, bivalves and crabs the PT caught fewer individuals (between −38 % and −57 %). No significant difference was observed in sea urchins and sea stars. Overall, this study shows that the transition from the BT to PT resulted in a 36 % decrease (95 % prediction interval: 31–42 %) in discards (kg.hour⁻¹).
Ecosystem effects of bottom trawl fisheries are of major concern. Although it is prohibited to catch fish using electricity in European Union waters, a number of beam trawlers obtained a derogation and switched to pulse trawling to explore the potential to reduce impacts. Here we analyse whether using electrical rather than mechanical stimulation results in an overall reduction in physical disturbance of the seafloor in the beam-trawl fishery for sole Solea solea. We extend and apply a recently developed assessment framework to the Dutch beam-trawl fleet and show that the switch to pulse trawling substantially reduced benthic impacts when exploiting the total allowable catch of sole in the North Sea. Using Vessel Monitoring by Satellite and logbook data from 2009 to 2017, we estimate that the trawling footprint decreased by 23%, the precautionary impact indicator of the benthic community decreased by 39%, the impact on median longevity of the benthic community decreased by 20%, the impact on benthic biomass decreased by 61%, and the amount of sediment mobilised decreased by 39%. The decrease in impact is due to the replacement of tickler chains by electrode arrays, a lower towing speed and higher catch efficiency for sole. The effort and benthic physical disturbance of the beam-trawl fishery targeting plaice Pleuronectes platessa in the central North Sea increased with the recovery of the plaice stock. Our study illustrates the utility of a standardized methodological framework to assess the differences in time trends and physical disturbance between gears.
Ecosystem effects of bottom trawl fisheries are a major concern. We analysed whether the replacement of mechanical stimulation by electrical stimulation may reduce the adverse impacts on the benthic ecosystem in the beam trawl fishery for sole. Although the use of electricity is not allowed to catch fish in European Union waters, a number of beam trawlers got derogation and switched to pulse trawling to explore the potential to reduce impacts. We extended a recently developed assessment framework and showed that the switch to pulse trawling substantially reduced benthic impacts when exploiting the Total Allowable Catch of sole in the North Sea. We applied the framework to Dutch beam trawl logbook data from 2009 to 2017 and estimated that the trawling footprint decreased by 23%; the precautionary impact indicator of the benthic community decreased by 39%; the impact on median longevity decreased by 20%; the impact on benthic biomass decreased by 61%; the amount of sediment mobilised decreased by 39%. The decrease is due to the replacement of tickler chains by electrode arrays, a lower towing speed and higher catch efficiency for sole. The effort and benthic impact of the beam trawl fishery targeting plaice Pleuronectes platessa in the central North Sea increased with the recovery of the plaice stock. This study illustrates the usefulness of a standardized methodological framework to assess the differences in time trends and trawling impact between gears.
In pulse fishing, electric pulses are used to startle the target species from the seabed, after which they end up in thenet. The technique can only be used by fishers with a derogation. Pulse fishing is subject of various debates during the run-up to the European decision-making process on the possible lift of the derogation. Some stakeholders have concerns about the potential impact of pulse fishing on the ecosystem and about the extra competition between different métiers as a result of the introduction of pulse fishing. It is essential to take those concerns seriously, as they impact the outcome of the pulse fishing debate.
In this report we made an inventory of stakeholders’ concerns and propose research questions to address these. Most concerns have already been covered by completed research or are part of ongoing research. The concerns that are not yet fully covered are: 1) visualization of direct effects of pulse fishing, by fishing in the wake of a pulse fishing vessel; 2) effects of pulse fishing on reproduction; 3) possible disadvantages for local fishers due to the arrival of pulse fishing vessels in ‘their’ areas; 4) local depletion of target species sole due to increased fishing pressure in an area; 5) risks of overfishing of shrimp due to increased fishing efficiency.
We recommend putting more effort into visualization of results and effects, in order to reach a larger audience.
In this study, we investigate whether the neoclassical economic theory, capital market frictions or preference-based theory can explain investment behaviour in the Dutch beam trawler fishery in the North Sea. By presenting a number of estimations, we show that vessels conducting pulse fishing invest substantially more than similar vessels undertaking conventional fishing, even after controlling for differences in such variables as capital stock, prices, profits and quotas. One possible explanation for this result is that vessel owners may obtain a separate benefit from investing in pulse fishing.
WGELECTRA chaired by Adriaan Rijnsdorp (the Netherlands) and Maarten Soetaert (Belgium) met from 17-19 April 2018 at Wageningen Marine Research, Haringkade 1, IJmuiden, the Netherlands. The working group was attended by 17 participants from five countries to address the request for advice from the Netherlands to compare the ecological and environmental effects of using traditional beam trawls or pulse trawls when exploiting the TAC of North Sea sole, on (i) the sustainable exploitation of the target species (species and size selectivity); (ii) target and non-target species that are exposed to the gear but are not retained (injuries and mortality); (iii) the mechanical disturbance of the seabed; (iv) the structure and functioning of the benthic ecosystem; and to assess (v) the impact of repetitive exposure to the two gear types on marine organisms. This report does not consider the pulse fisheries on shrimp or on razorclam.
In order to provide advice, WGELECTRA developed an assessment framework to evaluate the ecological and environmental effects of traditional beam trawls and of pulse trawls. The assessment is based on (i) a description of the changes in the beam trawl fleet targeting sole and plaice in the North Sea during the introduction of pulse trawls; (ii) a review of the scientific information on the effects of electrical stimulation on marine organisms; (iii) results of on-going research projects. In preparation for the working group meeting, the chairs circulated a work plan to the participants, including a draft table of content of this report and an outline of the assessment framework. The bulk of the information included in this report was made available to the participants prior to the meeting. The working group meeting was focussed on an in-depth discussion of the scientific evidence and the assessment. As several research projects are still on-going, part of the evidence being used in the assessment is in the preparation phase and has not yet been peer-reviewed.
At present about 89 mainly Dutch owned vessels operate under an exemption from the EU-legislation to catch sole using pulse trawls in the North Sea. In addition, 7 vessels deploy pulse trawls to catch brown shrimp during part of the year. In Scotland, 26 vessels have been granted licences to deploy an electrotrawl to catch razorclams as part of a trial fishery. The stimulus in the razorclam fishery is very different from that in the sole fishery. The current report is focussed on the pulse trawl fishery on sole. Unless specifically stated, where ”typical or commercial” stimulus is stated in this document it refers to the sole pulse.
Pulse trawls for sole were introduced in the Dutch flatfish fishery to reduce the high fuel cost and substantial environmental damage of the traditional beam trawl fishery with tickler chains. The fleet of today’s pulse licence holders land about 95% of the Dutch landings of sole. The fleet comprises two vessel types. The smaller Euro cutters (<= 221 kW) alternate pulse trawling for sole with the fishery for brown shrimps and the otter (twin) trawl fishery for other demersal fish or Nephrops. The larger vessels (>221 kW) use the pulse trawl to fish for sole throughout the year. Some vessels alternate pulse fishing for sole with traditional beam trawl fishing for plaice.
The total fleet directed sole fishing effort of today’s pulse licence holders (beam trawl and pulse trawl) has slightly decreased during the transition to pulse trawling between 2009 – 2017 while their contribution to the Dutch sole landings increased by 20% (from 75% to 95%). During the transition phase, pulse trawlers have shifted their distribution pattern in the southern North Sea. On local fishing grounds off the Thames and along the Belgian coast, fishing effort has increased. In other areas, fishing effort was either stable or has decreased.
Pulse trawls are more selective than traditional beam trawls when catching sole. The landing efficiency estimated from catch and effort data of the Dutch beam and pulse trawl fleet is 30% higher for sole and 40% lower for plaice. The improved species selectivity is also reflected in the 16% (small vessels) and 24% (large vessels) lower catch rate of discarded fish in the pulse trawl as observed in the discard monitoring programme. It is uncertain whether the pulse trawl has improved the size selectivity, e.g. catching fewer undersized fish relative to larger sized classes of the same species.
Pulse trawls are deployed at a lower towing speed than traditional beam trawls. Average towing speed is reduced by 22% from 6.3 to 4.9 knots in large vessels and by 15% from 5.4 to 4.6 in small vessels. The replacement of mechanical stimulation by electrical stimulation has reduced the physical disturbance of the seafloor. The average disturbance depth of an experimentally trawled study site was reduced from 4.0 cm with the traditional beam trawl to 1.8 cm in the pulse trawl (-55%). The lower towing speed and cleaner catch are expected to improve the survival of discarded flatfish.
The available literature on the potential negative effects of electrical stimulation of pulse trawling was reviewed. The impact of exposure to electrical pulses is determined by the frequency of exposure and the interval between successive exposures, as well as the sensitivity of the animal. Due to the reduced towing speed and slight reduction in fishing effort in the pulse fishery for sole, the overall exposure probability is reduced. Due to the heterogeneity of trawling, only 17% of the grid cells (1x1 minute) trawled have a trawling intensity of more than one time per year.
A number of laboratory experiments were carried out in which a selection of fish species were exposed to electrical stimuli to study possible adverse effects. These studies indicate that pulse stimulation used in the fishery for sole did not cause direct mortality during exposure but may cause spinal fractures and associated haemorrhages in gadoid round fish species (in particular cod), but not in flatfish species (sole, plaice, dab) or seabass. Preliminary results from an on-going project showed that 18% of 362 cod sampled from nine fishing trips of six pulse vessels showed a spinal fracture and/or full dislocation, while 24% showed smaller spinal abnormalities. Results suggest that the sensitivity is size dependent with lower incidence rate in small (<18 cm) and large (>65 cm) cod. Further studies are required to study the relationship between spinal fractures and body size and determine the differences in sensitivity towards spinal injuries across fish species. Data on sub-lethal effects and/or long-term effects are scarce and inconclusive. Small-spotted catshark Scyliorhinus canicula were still able to detect the bioelectric field of a prey following exposure.
Preliminary experiments with a range of benthic invertebrates generated variable results due to the low number of animals tested. More elaborate experiments with brown shrimp and ragworms did not find evidence for increased mortality when exposed to pulses similar to those used in the sole fisheries. However, when exposed 20 times during a 4-day period, an increased mortality was noted for brown shrimp compared to one of two control treatments, but not to mechanically stimulated shrimps.
Little is known on the effects of electrical stimulation on the development of eggs and larvae. One experiment exposing 8 early life stages of cod (embryos, larvae, early juveniles) to a very strong shrimp pulse stimulus, (a strength which only occurs very close to a commercial electrode), did not find differences in morphometrics between exposed and control animals, but observed a reduced developmental rate in one embryonic stage and an increased mortality in 2 larval stages following exposure.. No adverse effects were noted following exposure of two embryonic, two larval and one juvenile stage(s) in sole. Both experiments only studied possible short-term effects of the pulse and included a limited set of parameters to evaluate the sub-lethal effects. The effects of the sole pulse on reproduction have not been studied yet.
In contrast to the mechanical disturbance of the traditional beam trawl, preliminary results of recent studies on the effect of pulse stimulation on the biogeochemical functioning of the benthic ecosystem have not provided evidence that the electrical pulses used in the fishery for sole result in changes in sediment oxygen consumption, oxygen micro-profiles or surface chlorophyll levels. Effects on benthic ecological functioning has not yet been investigated.
Summarising the available evidence shows that the replacement of the tickler chain beam trawl with pulse trawl with electrodes to exploit sole results in a reduction of the environmental impacts: catch rate of fish discards (-16% to -24%), catch rate of benthos (-62% in large vessels and +6% in small vessels), trawling footprint (-18%), mechanical impact on seafloor and benthos (–50%) and CO2 emissions (-46%). There is insufficient evidence to fully understand the impact of electrical pulse on marine organisms and the benthic ecosystems across the North Sea. The possible adverse effects of electrical pulses on marine organisms and the benthic ecosystem are still being investigated. The available evidence so far suggests that the spinal fractures induced by the cramp response to the sole pulse are observed in two roundfish species, but not in flatfish which comprise more than 80% of the catch. Various gaps in knowledge on the effects of electrical stimulation on marine organisms and ecosystem functioning still exist. The ongoing research on the effects of electrical stimulation on marine organisms and ecosystem functioning will improve the scientific basis to assess the ecological effects on the scale of the North Sea.
Pulse trawling is currently the most promising alternative for conventional beam trawls targeting sole and shrimp, meeting both the fisher's aspirations and the need for more environmentally friendly fishing techniques. Before electrotrawling can be further developed and implemented on a wider scale, however, more information is needed about the effects of electrical pulses on marine organisms. The organisms used in the present experiments were brown shrimp (Crangon crangon L.) and king ragworm (Alita virens S.) as model species for crustaceans and polychaetes, respectively. These animals were exposed to a homogeneously distributed electrical field with varying values of the following parameters: frequency (5-200 Hz), electrical field strength (150-200 V m−1), pulse polarity, pulse shape, pulse duration (0.25-1 ms), and exposure time (1-5 s). The goal of this study was to determine the range of safe pulses and thereby also to evaluate the effect of the pulses already being used on commercial electrotrawls. Behaviour during and shortly after exposure, 14-d mortality rates, and gross and histological examination were used to evaluate possible effects. The vast majority of shrimp demonstrated a tail flip response when exposed to electric pulses depending on the frequency, whereas ragworm demonstrated a squirming reaction, independent of the frequency. No significant increase in mortality or injuries was encountered for either species within the range of pulse parameters tested. Examination of the hepatopancreas of shrimp exposed to 200 V m−1 revealed a significantly higher severity of an intranuclear baculoform virus infection. These data reveal a lack of irreversible lesions in ragworm and shrimp as a direct consequence of exposure to electric pulses administered in the laboratory. Despite these promising results, other indirect effects cannot be ruled out and further research hence is warranted.
In trawl fisheries, beam trawls with tickler chains, chain mats or bobbin ropes are used to target flatfish or shrimp. High fuel consumption, seabed disturbance and high discard rates are well-known disadvantages of this fishing technique. These shortcomings are increasingly gaining international public and political attention, especially with the upcoming discard ban in Europe. The most promising alternative fishing technique meeting both the fisherman's aspirations, and the need for ecological progress is pulse fishing with electrotrawls. Here, the mechanical stimulation by tickler chains or bobbins is replaced by electrical stimulation resulting in reduced bottom contact, fuel costs and discards. Although a significant amount of research has been done on electrotrawls and their impact on marine organisms, most data were published in very diverse sources ranging from local non-peer-reviewed reports with a limited distribution to highly consulted international peer-reviewed journals. Therefore, there is a clear need for a comprehensive yet concise and critical overview, covering and summarizing all these data and making these available for the scientific community. This article aims to meet the above goals by discussing the working principle of electric fields, the history of electrotrawls and their current application in the North Sea and impact on marine organisms. It is concluded by elaborating on the opportunities and challenges for the further implementation of this alternative fishing technique.
We evaluated electricity as a stunning method before slaughter of Atlantic salmon Salmo salar by assessing both stunning effectiveness and injuries. About 300 salmon (1.2–6.6 kg) were exposed to one-phase, sinusoidal, 50-Hz AC in seawater; electrical field strengths ranged from 15 to 250 V/m and current durations from 0.2 to 12 s. We measured the duration of the epileptic-like seizures after stunning and the degree of unconsciousness based on behavioral responses. Fish were killed, bled, and gutted 10 min after stunning and then analyzed for injuries. The proportion of fish sufficiently stunned, the duration of an unconscious condition, and the occurrence of broken vertebrae and hemorrhages were all dependent on electric field strength and current duration. The electric field strength required to stun the fish was inversely proportional to the current duration, dropping from 200 V/m at 0.8 s to 25 V/m at 6–12 s. Electricity proved to be efficient in stunning fish, but to avoid injuries in market-sized Atlantic salmon the current duration should be less than 1.5 s at field strengths ranging from 125 to 150 V/m.
We conducted three sets of experiments to determine aspects of pulsed direct current responsible for high incidence of spinal injury in electrofished rainbow trout Oncorhynchus mykiss and to test the electrotaxis efficiency of a new complex pulse pattern (low-frequency bursts of high-frequency pulses). In the first set of experiments, we energized three types of anodes with current pulsed at 60 pulses per second (pps) to produce fields with relatively low, intermediate, or high voltage gradients near the anode. Results showed no direct relationship between voltage gradient and injury rate. In the second set of experiments, we tested the complex pulse pattern and four other pulse frequencies using a spherical anode. The results demonstrated a low injury rate with the complex pulse pattern and a curvilinear increase in spinal injuries with rising pulse frequency. Moderate to high pulse frequencies, therefore, rather than high voltage gradients, appear to be the primary cause ofspinal injury. We suggest that more fish injuries are seen at higher pulse frequencies because these injuries are caused by myoclonic jerks associated with shock-induced epileptic seizures, and such seizures develop more rapidly at higher frequencies than at lower frequencies. In the third set of experiments, we found the electrotaxis efficiency of the complex pulse pattern was similar to those of regular 60 pps and 30 pps patterns. The complex pattern, therefore, combined low incidence of injury with good electrotaxis and narcosis.
High rates of spinal injury from electrofishing have been reported for rainbow trout Oncorhynchus mykiss, but little is known about the fate and performance of injured fish. We conducted a long-term experiment to evaluate how incidence and severity of electrofishing-induced spinal injury affects growth and survival. We electroshocked 866 wild rainbow trout from the Gallatin River, Montana, using one of three different DC pulse shapes (smooth, half pulse, and full pulse), X-rayed the fish to determine degree of spinal injury, and compared short-term (100-d) growth and long-term (335-d) growth and survival after transplanting the fish to a 0.6-ha pond (stocking biomass, 255 kg/ha). Rainbow trout shocked with pulsed DC had significantly higher (P = 0.0001) incidence (40–54%) of spinal injury than those shocked with smooth DC (12%); injuries were also more severe among fish captured with pulsed DC (P < 0.01). Incidence and severity of injury were positively correlated with fish length (r = 0.79–0.83, P < 0.02). Few surviving fish (7 of 418) at the end of the study exhibited outwardly visible spinal deformities. Healing of spinal injuries was readily apparent on radiographs as evidenced by calcification and fusion of damaged vertebrae. Long-term survival of rainbow trout was not affected by pulse shape used during capture or by severity of electrofishing-induced injury. However, fish with moderate to severe injury (spinal misalignment and fracture), representing 28% of the total number shocked, had markedly lower growth and condition after 335 d than fish with no or low spinal injury.
These conclusions are extracted from a published review and synthesis of literature on electrofishing and its harmful effects on fish. Although a valuable sampling technique for over half a century, electrofishing, which involves a very dynamic and complex mix of physics, physiology, and behavior, remains poorly understood. New hypotheses have been advanced regarding power transfer to fish and the epileptic nature of their responses to electric fields, but these too need to be more fully explored and validated. Fishery researchers and managers are particularly concerned about the harmful effects of electrofishing on fish, especially endangered species. Although often not externally obvious or fatal, spinal injuries and associated hemorrhages sometimes have been documented in over 50% of fish examined internally. Such injuries can occur anywhere in the electrofishing field at or above the intensity threshold for the twitch response. These injuries are believed to result from powerful convulsions of body musculature (possibly epileptic seizures) caused mostly by sudden changes in voltage as when electricity is pulsed or switched on or off. Significantly fewer spinal injuries are reported when direct current, low-frequency pulsed direct current ( 30 Hz), or specially designed pulse trains are used. Salmoninae are especially susceptible. Other harmful effects, such as bleeding at gills or vent and excessive physiological stress, are also of concern. Mortality, usually by asphyxiation, is a common result of excessive exposure to tetanizing intensities near electrodes or poor handling of captured specimens. Reported effects on reproduction are contradictory, but electrofishing over spawning grounds can harm embryos. Electrofishing is often considered the most effective and benign technique for capturing moderate to large-size fish, but when adverse effects are problematic and cannot be sufficiently reduced, its use should be severely restricted.
The aim of this study was to investigate the effects of electro-fishing in sea water. We evaluated the feasibility of an electro-fishing system using numerical simulations for laboratory tanks and the open sea and performing a laboratory experiment. A non-homogeneous bi-dimensional electric-field model for marine water and fish based on discrete formulation of electro-magnetic field equations was developed using GAME (geometric approach for Maxwell equations) software. Voltage gradients inside the fish and close to the body were determined. Re- sults showed that fish in the open sea and in groups had greater internal voltage differences than did fish in tanks and single fish. Sea bass (length:10 and 30 cm) were exposed in laboratory tanks to pulsed direct current (PDC), 25-125 Hz and duty cycle (5-40%). We measured the electro-taxis and tetanus thresholds after electrical exposure. It is significant that these values decreased with increasing the size of fish. No differences were found after electro-fishing on overall appearance, internal and external haemorrhage, standard freshness scoring techniques and carcass quality characteristics
A histological study was conducted in Mullus barbatus captured from two different zones of the Catalan coast (western Mediterranean): 1. a polluted site (near Barcelona) and 2. a less contaminated zone (Creus Cape). No histopathological alterations were found in any of the organs studied. An image analysis study was carried out on splenic sections in order to quantify the number and area of melano-macrophage centres (MMC). The area of MMC was significantly greater in fish sampled from the polluted area indicating that MMC centres in Mullus barbatus could be useful to monitor environmental degradation in NW Mediterranean.
Onderzoek naar het welzijn van hondshaaien bij de vangst. Er is onderzocht of de methode van het gebruik van stroom wel diervriendelijk is. De conclusie is dat dit geen beletsel voor de diervriendelijkheid oplevert
Cardiac output (CO), heart rate (HR) and stroke volume (SV) were monitored in rainbow trout Oncorhynchus mykiss electroshocked (pulsed DC) with various voltage, frequency, pulse width and shock duration settings. Additionally, behavioural recovery times and internal haemorrhaging were examined. During electroshock, heart function became erratic and typically ceased for much of the event (cardiac arrest). Following electroshock, CO increased almost entirely due to an increase in SV. Cardiac function generally returned to resting levels within 2–3 h. Behaviourally, recovery was much more rapid, typically taking only a few minutes for fish to regain equilibrium and to begin swimming normally, and only rarely was >1 h. Internal injury ranged from 0 to 7 cm2 of haemorrhaging along the spine and surrounding musculature, although only 4% of the fish had corresponding damage to the vertebrae. Comparisons across the various electrofisher settings indicate that response and injury are highly variable, but generally increased with more intense settings. Higher frequencies and voltages appear to most negatively affect behavioural recovery and injury while longer shock durations increased the length of cardiac arrest and the duration of cardiac recovery. Because of the variability in results and small sample sizes, however, the strongest conclusion that can be drawn from this work is that electrofishing, regardless of the settings, has a considerable negative impact on rainbow trout that is often not apparent externally. Additionally, this study has shown how electrofishing directly effects fish physiologically through impairment of cardiac function. Studies examining the physiological and behavioural response of fishes and subsequent recovery will be necessary for the development of electrofishing guidelines which minimize the disturbance to fishes.
Pulse trawling is used to a growing extent in the Dutch flatfish beam trawl fleet, and deemed as a promising alternative to tickler chain beam trawling. A comparative fishing experiment was carried out with one vessel using conventional beam trawls, and the other two using flatfish pulse trawls supplied by two different companies. Pulse trawl landings were lower both expressed in kg h−1 (67% based on auction data) or baskets per hectare (81%).
The pulse trawls had fewer fish discards (57%, p < 0.0001), including 62% undersized plaice (Pleuronectes platessa L.) (p < 0.0001), and 80% discarded weight of benthic invertebrates (p = 0.0198) per hectare. The pulse fishing technique resulted in a lower fuel consumption (37–49%), and consequently in spite of lower landings net revenues were higher. A downside of using pulse trawls is the possible spinal damage of marketable cod (Gadus morhua L.), but because total cod landings by beam trawls are low (4–5%), the implication will likely be limited.
The fishery for brown shrimp (Crangon crangon) in the North Sea is carried out by more than 600 vessels with total annual brown shrimp landings of around 20,000 t. Due to the small mesh size used, the catches also contain large amounts of unwanted by-catch. To find ways of reducing this by-catch, experiments were carried out with electric pulses. The basic idea was to selectively invoke a startle response with shrimp without stimulating any by-catch species. A selective groundrope could then be used in combination with electric pulses to obtain catch separation.As a preparation for sea trials, laboratory experiments were carried out. The pulse generators were tested for their basic characteristics. Experiments were carried out with fish and invertebrate species that are frequently caught in the brown shrimp fishery. The effect of pulse amplitude and frequency in relation to ambient parameters on the response of these animals was tested. To assess the effect of the pulses on these animals, survival experiments were carried out. The main conclusion was that shrimps react strongly to the pulses and most of the other species regularly caught in shrimp trawls do not, so selective electro-fishing has potential. The survival tests indicated that the pulses have no effect on the survival and general behaviour of the animals that have been in the electric field.
Fisheries scientists have long studied the behavior of fish in electrified water. However, the science of electrofishing (the use of electricity to capture fish) evolved independently of other fields of science whose knowledge could have explained the behaviors of fish to electric shock. Theories to explain the behavior of organisms in electric fields developed out of two paradigms: Classical stimulus–response (S–R) theory, and “local action” of electrical energy on nerves and muscle fibers. Stimulus–response theories dominated in the late 19th Century; although they were abandoned by the early 20th Century by animal behaviorists they persisted in work with fish. An alternative theory, the “local action” paradigm, arose soon after 1900. Attributing galvanotropisms to the action of electricity on local nerves and muscles fibers, the local action theory remains in one form or another the main explanation for galvanotropisms today. However, neurologists have recognized for more than 100 years that electric stimulation of vertebrates causes epileptic seizures. Similar epileptic seizures are produced by alternating, direct, and pulsed currents of any shape or frequency. The observed behaviors result from stimulation of the central nervous system, not from local nerve and muscle responses. Spike–wave patterns of neural discharge on electroencephalograms, which are diagnostic of epilepsy, have been recorded in fish. Current electrofishing terms can be matched up with epileptic terminology: Twitching, orientation, taxis, and the turn or escape maneuver are automatisms; narcosis is a petit mal seizure; pseudo-forced swimming is due to tonic–clonic contractions, and tetany is a grand mal seizure. Spinal injuries are due to myoclonic jerks and happen early in the seizure when automatisms occur. Also, patchy discoloration of the skin due to chromatophore activity is explainable as a result of sympathetic discharge during grand mal seizure. Epilepsy explains all of the phenomena seen in electroshocked fish.
The effects of electricity as a stunning method in the slaughtering process of Atlantic salmon Salmo salar and pollock Pollachius virens were studied. About 330 slaughter-sized Atlantic salmon were exposed to sinusoidal alternating current (AC) in seawater. Electrical field strength ranged from 25 to 100 V/m and current duration from 1 to 10 s at frequencies between 30 and 2,000 Hz. In addition, 122 pollock and 25 salmon were stunned by applying square wave AC in the range of 50–1,000 Hz. After stunning, subsequent unconsciousness was evaluated by means of behavioral responses, and injuries were observed by examining fillets. For sinusoidal AC, the proportion of Atlantic salmon sufficiently stunned and the occurrences of obviously broken spinal columns and hemorrhages were dependent on the electrical frequency used. The AC frequency was the most predominant factor in both stunning and inflicted injuries. The proportion of injured salmon exhibited a unimodal response to sinusoidal AC frequency, increasing from 30 Hz to 50– 80 Hz and declining with higher frequencies. There was not a consistent injury-versus-frequency pattern for pollock exposed to square wave AC. For Atlantic salmon, use of square wave AC did inflict a higher rate of injuries than that of sinusoidal AC. For minimizing the occurrence of injuries while sustaining sufficient stunning before slaughter, sinusoidal AC frequencies between 500 and 1,000 Hz are recommended at field strengths exceeding 50 V/m and current durations of 10 s.
Electric fields affect large fish more strongly than small fish. Electrical fishing systems should preferentially capture
the larger members of a fish stock, but in practice their selectivity is controlled by the uniformity of the electric field
distribution and the particular fish reaction to electrical stimulation being exploited. From published data, the selectivity
of a practical system is evaluated for various electric field distributions and types offish reaction. It is found that the
probability of capture by the system increases with fish size, but with a high electric field strength the system can be non-selective.
Direct observations on the reactions of flatfish of the Pleuronectidae to electrical stimulation were made by towing a manned
sledge supporting an energised electrode array over the sea bed. The electrical stimulus was pulsed DC, ranging in frequency
from 4 to 40 Hz. Involuntary muscular contractions were induced by this stimulus which caused the majority of fish to flee
from the electrified zone. Reactions were classified into a few broad categories and the approximate fish size in each observed
event was recorded. The results suggested that the most efficient frequency for inducing flatfish to leave the bottom is around
20 Hz, and that large fish are more strongly stimulated by an electric field than small fish; the latter being a significant
demonstration under natural conditions of a theory based on aquarium experiments. Observations were made on the reactions
of flatfish to a towed chain to assess its comparative efficiency in forcing fish to leave the sea bed.
The first pilot study on the effects of electric pulse stimulation on larger cod carried out in 2008 was based on a single nominal setting of the Verburg-Holland UK153 pulse system with the intention to determine the range of pulse characteristics with which injuries to the fish occurred. This study was designed to obtain a more detailed view on the effects of the pulse characteristics and to investigate if a specific pulse parameter has a key role in the effects. Lower pulse amplitudes with longer pulse width and higher frequency could induce different effects than pulses with higher amplitudes, shorter pulse width and lower frequency. In this way the effects can be related to a specific pulse parameter and its threshold value. Another aim of major importance is that this research also carried out on the electrically exposed fish that would normally escape through the cod-end meshes with unknown longer term effects. This was already recommended after the first experiment in 2008.
Electrical brain stimulation is used in a variety of clinical situations, including cortical mapping for epilepsy surgery, cortical stimulation therapy to terminate seizure activity in the cortex, and in deep brain stimulation therapy. However, the effects of stimulus parameters are not fully understood. In this study, we systematically tested the impact of various stimulation parameters on the generation of motor symptoms and afterdischarges (ADs). Focal electrical stimulation was delivered at subdural cortical, intracortical, and hippocampal sites in a rat model. The effects of stimulus parameter on the generation of motor symptoms and on the occurrence of ADs were examined. The effect of stimulus irregularity was tested using random or regular 50Hz stimulation through subdural electrodes. Hippocampal stimulation produced ADs at lower thresholds than neocortical stimulation. Hippocampal stimulation also produced significantly longer ADs. Both in hippocampal and cortical stimulation, when the total current was kept constant with changing pulse width, the threshold for motor symptom or AD was lowest between 50 and 100Hz and higher at both low and high frequencies. However, if the pulse width was fixed, the threshold did not increase above 100Hz and it apparently continued to decrease through 800Hz even if the difference did not reach statistical significance. There was no significant difference between random and regular stimulation. Overall, these results indicate that electrode location and several stimulus parameters including frequency, pulse width, and total electricity are important in electrical stimulation to produce motor symptoms and ADs.
Continuous direct current (DC) and pulsed DC (PDC) of varying frequency and pulse period are commonly used to immobilize and collect crappies Pomoxis spp. in freshwater. However, little information is available about the minimum electrical-setting thresholds required for immobilization or how the settings relate to incidence of injury. We investigated the effect of increasing power densities on the immobilization and injury of black crappies P. nigromaculatus (average total length = 154 mm) treated with DC and various PDC settings. Forced swimming toward the electrodes was observed in black crappies exposed to DC, but that was less apparent for PDC. The minimum peak power densities required to immobilize black crappies ranged from 0.10 to 6.5 mW/cm3 and depended on pulse frequency and period. The incidence of hemorrhaging ranged from 0% to 50% and that of spinal damage from 9% to 45%. However, the severity of injury also depended on pulse frequency and period. No fish suffered mortality at or below the immobilization thresholds, but mortality ranged from 0% to 15% at settings above the thresholds. Mortality was observed with PDC settings of 15 Hz only. Fish that were tetanized following electrical treatment were more prone to injury than those that exhibited narcosis.
Adult rainbow trout Salmo gairdneri captured by electrofishing were analyzed for spinal injury by X-ray photography and autopsies. The effects of three electrical pulse shapes were compared. Of 209 fish captured, 50% suffered spinal injuries involving an average of eight vertebrae that were dislocated, splintered, or both. One-quarter-sine wave pulses injured a significantly higher proportion of fish (67%) than either exponential pulses (44%) or square wave pulses (44%; P < 0.05). Quarter-sine waves also damaged significantly more vertebrae per fish (average, 9.5) than did exponential pulses (6.6); the average number damaged by square waves (8.2) did not differ significantly from either of the other means. Electrofishing could bias mark–recapture studies of large rainbow trout. Electrofishing in waters containing endangered or threatened species should be considered with great caution.
Most studies of injury associated with electrofishing have focused on salmonids; few have given attention to warmwater fishes. Under controlled laboratory conditions, we treated bluegill Lepomis macrochirus, channel catfish Ictalurus punctatus, and largemouth bass Micropterus salmoides of various sizes to duty cycles ranging from 1.5% to 100%. This range of duty cycles represented continuous DC and pulsed-DC frequencies ranging from 15 to 110 Hz and pulse durations of 1 to 6 ms. At each duty cycle, fish were exposed to power densities in excess of those required to immobilize them within 3 s, and we subsequently determined the incidence of hemorrhage, spinal injury, and mortality. Incidence of hemorrhage averaged 3% (range, 0–25%), differed among species, and was not related to duty cycle or fish size. Incidence of spinal injury averaged 3% (range, 0–22%) and mortality averaged 10% (range, 0–75%); both differed among species and were related to duty cycle, fish size, and interactions among these variables. Largemouth bass was the species most vulnerable to hemorrhage, spinal injury, and mortality, channel catfish the least vulnerable; bluegills exhibited effects that were intermediate. Small centrarchids were especially susceptible to mortality. Fish tetanized by the electrical treatment were more likely to experience injury and mortality than fish that were only narcotized. However, mortality was not related to the injuries studied because hemorrhage and spinal injuries were similar in fish that survived electroshock and in those that died. We suggest that electrofishing with intermediate to high duty cycles could reduce electrofishing-induced injury and mortality to warmwater fish. Additionally, the power output and electrode system should be managed to induce narcosis and prevent tetany and to avoid the large peak powers required to immobilize small individuals.
Most studies of injury associated with electrofishing have focused on game fishes, but few have given attention to cohabiting small nongame species. Under controlled laboratory conditions, we subjected small nongame cyprinids, ictalurids, and percids to a wide range of voltages and waveforms to examine potential harmful effects. Fish were treated with power levels distributed uniformly between the thresholds required to immobilize game fish and also were subjected multiple times to those thresholds to simulate the range of conditions that might exist in a heterogeneous electrical field formed during electrofishing in field situations. Across waveforms and species, the incidence of hemorrhages averaged 2% (range = 0–20%), the incidence of spinal injuries averaged 6% (range = 0–30%), and mortality averaged 16% (range = 0–90%). Continuous DC was generally less harmful than pulsed-DC waveforms; hemorrhages and spinal injuries tended to increase with high pulse frequencies, and mortalities tended to increase with low pulse frequencies. Ambiguities in the results were apparent, suggesting that some species may experience extensive harm, whereas others may not. Given the potential to harm numerically small populations and populations of imperiled species, we suggest (1) expanded efforts to overcome the power limitations that prevent effective use of continuous-DC electrofishing in many field situations and (2) pilot studies at geographic locations where numerically small populations of nongame species may be a concern.
Fish size and electrical waveforms have frequently been associated with variation in electrofishing effectiveness. Under controlled laboratory conditions, we measured the electrical power required by five electrical waveforms to immobilize eight fish species of diverse sizes and shapes. Fish size was indexed by total body length, surface area, volume, and weight; shape was indexed by the ratio of body length to body depth. Our objectives were to identify immobilization thresholds, elucidate the descriptors of fish size that were best associated with those immobilization thresholds, and determine whether the vulnerability of a species relative to other species remained constant across electrical treatments. The results confirmed that fish size is a key variable controlling the immobilization threshold and further suggested that the size descriptor best related to immobilization is fish volume. The peak power needed to immobilize fish decreased rapidly with increasing fish volume in small fish but decreased slowly for fish larger than 75–100 cm. Furthermore, when we controlled for size and shape, different waveforms did not favor particular species, possibly because of the overwhelming effect of body size. Many of the immobilization inconsistencies previously attributed to species might simply represent the effect of disparities in body size.
This book brings to date the reports and conclusions from the Montreal Neurological Institute's clinical, physiological, and neuro-surgical studies of epilepsy, and is, in a sense, a sequal to "Epilepsy and cerebral localization," published in 1941. There is extensive addition of new material on subcortical mechanisms, functional cortical localization, surgical and medical treatment and electroencephalography. The book is illustrated with 8 color plates and 314 black and white illustrations. (PsycINFO Database Record (c) 2012 APA, all rights reserved)
The behavioural responses of rainbow trout Oncorhynchus mykiss Richardson and crucian carp Carassius auratus (L.) to homogeneous electric fields were determined. A range of voltage gradients to simulate responses of fish in electric fields at various distances from the anode was tested. The subsequent recovery of stunned carp was also examined. For both species, the frequency and duty cycle of rectangular pulsed DC waveforms had strong, interactive effects. Threshold field strengths for perception and attraction responses increased with frequency at low (10%) duty cycles. Fish were immobilized more effectively at intermediate frequencies (ca. 100 Hz). Recovery periods were also longest suggesting that hyper-reflexivity and tetanus effects were prevalent. Responses were weaker at high frequencies (>300 Hz), whilst immobilization at low frequencies (<40 Hz) only occurred at field strengths representing a zone close to the anode. Carp exposed to pulsed currents had substantially lower immobilization thresholds and longer recovery times compared with continuous DC. Exponential and sine-wave outputs were found to produce significantly higher attraction thresholds and longer recovery times respectively compared with rectangular pulse shapes. The field implications of these results are discussed.
Histopathological examination of liver, pancreas, spleen and gonads was conducted in Atlantic tomcod (Microgadus tomcod) collected during September–November 1994 from estuarine sites receiving pulp and paper mill effluents in the Restigouche Estuary, the Miramichi Estuary and Pictou Harbour, and reference sites in the Kouchibouguac and Margaree Estuaries. This study was part of a larger investigation into the environmental health of estuaries and the coastal environment of the southern Gulf of St. Lawrence. Density and surface area of pigmented macrophage aggregates (PMA), and presence or absence of inflammatory, preneoplastic and neoplastic changes were evaluated in spleen and liver. Stages of maturity of the gonads were determined. In the Miramichi Estuary only, 28% of the fish had extensive multifocal granulomatous lesions centered on PMA in the spleen, likely caused by an infectious agent. In these affected fish, the density of PMA was slightly increased, and the surface area of PMA was markedly increased (10×) compared to normal fish from the same site. Both the density and the surface area of PMA were lower in spleens of fish from the Restigouche Estuary than at other sites. Condition factor and stage of maturity of the gonads were higher at this site. In the Restigouche Estuary only, more than 90% of the fish had zones of vacuolation in the pancreas. In the liver, density of PMA was generally low and did not differ significantly among sites. No preneoplastic or neoplastic lesions were observed. Further studies are needed to identify the causes and pathogenesis of the two lesions that were observed only at contaminated sites: the multifocal granulomatous lesions in the spleen and the zones of vacuolation in the pancreas. A larger sample of older fish should be examined to evaluate prevalence of preneoplastic and neoplastic changes. Histopathological markers have successfully identified potential effects of pulp and paper effluents on fish health, but the observed lesions cannot, at this stage, be attributed specifically to toxic exposure.
In response to questions asked by ICES on the effects of pulse stimulation in commercial beam trawling on components of the marine ecosystem a number of preliminary studies were undertaken in the period between 1 September 2008 and 1 March 2009. These activities involved the exposure of cod to a simulated electric pulse under laboratory conditions and observation of behaviour, including the foraging response, and monitoring mortality and possible internal injuries such as vertebral damage by X-ray photography.
The aim of this study was to evaluate the effect of electric field strength and current duration on wild-caught Atlantic herring Clupea harengus stunned with sinusoidal 50-Hz AC in seawater. The fish were exposed to electric field strengths ranging from 16 to 142 V/m and current durations from 1 to 12 s. We recorded the elapsed times between the point at which each fish became unconscious and the points at which it resumed normal behavioral functions. We also investigated injuries such as broken spinal columns and hematomas after the fish were filleted. The threshold electric field strength required to stun all of the fish to unconsciousness was 33 V/m for 1 s. The duration of the unconscious condition increased as both electric field strength and current duration increased. Of a total of 260 Atlantic herring, 60% had broken spinal columns. The proportion of fish with fractured spines was independent of field strength and duration, but the number of fractures per fish increased with field strength. We conclude that electrical stunning would promote the welfare of Atlantic herring that are landed alive but negatively affect fillet quality owing to hematomas associated with the fractures.
A description is given for the design of an isolated output, servo-controlled voltage, electroconvulsive therapy (ECT) machine for the safe, controlled application of 60-Hz alternating current (ac) signals between 0 and 230 V rms for controlled periods ranging from 0.1 to 2 sec. Added safety is provided by current limiting in the range of 360 to 770 mA rms for patient impedances ranging from 500 to 50 omega. The use of this equipment has permitted a detailed description of the impedances characteristic of unilateral ECT.
The influence of electrical stimulus parameters on focal seizure production was studied. Stimulations and recordings were carried out with bipolar electrodes stereotactically positioned in the ventral hippocampus of kindled rats. After discharge thresholds were determined for stimulus trains with different combinations of train durations (0.5, 1, 2, 5, 10 s), intratrain frequencies (10, 20, 50, 60, 100 Hz), and pulse widths (0.5 and 1.0 ms). For a given combination of pulse width and intratrain frequency, thresholds decreased as train duration increased; the effect was profound with shorter durations but markedly attenuated with train durations > or = 5 s. For a particular train duration and pulse width, thresholds varied inversely with intratrain frequency; the degree of variation in the thresholds with frequency was greater the shorter the train. For the train durations studied, thresholds were lower with 1.0-ms pulses for intratrain frequencies > or = 20 Hz. However, for 10 Hz, the 0.5-ms pulses were more effective for 2-, 5- and 10-s trains. After discharge thresholds were found to also vary with the number of pulses in the stimulus trains. For stimuli with fewer pulses, thresholds were higher and showed marked variation with train duration, intratrain frequency, and pulse width. For stimuli with more pulses, thresholds fell to a lower limit and the influence of train duration was substantially lessened. This study provides a systematic examination of the influence of stimulus parameters on generation of focal seizures and should prove useful in designing and interpreting future experiments studying the neurobiology of seizures in the hippocampus and related structures and helpful in antiepileptic drug testing.
In the forebrain of the fish (Tinca tinca L.) it is possible to induce an epileptic focus using a local application of penicillin. A secondary focus arises in the symmetrical point of the contralateral forebrain hemisphere. The secondary discharge follows the primary one with an average latency of 52–85.5 msec. The length of this latency depends on body temperature. Independent secondary discharges have been observed.
Electrographic seizures originating in the primary focus and irradiating into the other hemisphere usually have the pattern of spike and wave activity. The paroxysmal activity starts suddenly, may last several minutes and longer, and does not stop abruptly, being gradually transformed into frequent bursts of focal discharges.
The only pathways for the propagation of focal and paroxysmal activity are the forebrain commissural connections. Their transection interrupts interhemispheral propagation completely.
The isolation of the forebrain from lower diencephalic and mesencephalic brain structures does not significantly influence either the genesis of epileptic activity, of its transfer between forebrain hemispheres. No modifications of the resting EEG of the telencephalon were observed.
Possible causes of the observed deficiency of inhibitory mechanisms in terminating the self-sustained epileptic afterdischarge are discussed in terms of phylogeny of brain structures.
The earliest electroconvulsive therapy (ECT) equipment was surprisingly advanced in its characteristics. Designed by some of the leading clinical scientists of the day in conjunction with creative engineering entrepreneurs, many devices contained features as advanced as are now in contemporary devices. A review of the development of ECT devices is of historical interest and it offers a fresh glimpse into technical issues that occupy recent interest.
Melano-macrophage centres, also known as macrophage aggregates, are distinctive groupings of pigment-containing cells within the tissues of heterothermic vertebrates. In fish they are normally located in the stroma of the haemopoietic tissue of the spleen and the kidney, although in amphibians and reptiles, and some fish, they are also found in the liver. They may also develop in association with chronic inflammatory lesions elsewhere in the body and during ovarian atresia. In higher teleosts, they often exist as complex discrete centres, containing lymphocytes and macrophages, and may be primitive analogues of the germinal centres of lymph nodes. Melano-macrophage centres usually contain a variety of pigments, including melanins, and these increase in range and volume in older fish or in the presence of cachectic disease. Melano-macrophage centres act as focal depositories for resistant intracellular bacteria, from which chronic infections may develop. Iron capture and storage in haemolytic diseases appears to be a primary function, but antigen trapping and presentation to lymphocytes, sequestration of products of cellular degradation and potentially toxic tissue materials, such as melanins, free radicals and catabolic breakdown products are among other functions that have been ascribed. Recent work suggests that they are a site of primary melanogenesis rather than mere storage. Melano-macrophage centres increase in size or frequency in conditions of environmental stress and have been suggested as reliable biomarkers for water quality in terms of both deoxygenation and iatragenic chemical pollution.
Specimens of farmed European sea bass (Dicentrarchus labrax L., 1758) were exposed to different cadmium (Cd) concentrations (4.47, 5.63, 7.08 and 8.91 mg l(-1)) for 24 and 48 h. The effects of Cd on numbers of some cell types and structures (i.e., chloride cells, CCs; macrophage aggregates, MAs; rodlet cells, RCs) and on structure and ultrastructure of the main organs (gill, liver, intestine, kidney) were studied with routine process for light and transmission electron microscopy. Following cadmium exposure, the numbers of branchial CCs as well as intestinal and renal RCs increased significantly within 24h. Increase in metal concentration did not affect the magnitude of the numerical increment of the aforementioned cells. Moreover, in treated fish (24 and 48 h) the numbers of MAs in both head kidney and spleen were significantly higher than in control conspecifics, whilst the global area of MAs was less influenced by the acute treatment. In exposed sea bass, all the examined organs exhibited cellular modifications which appeared time- and dose-dependent. The gills showed telangectasia, lamellar fusion, oedema, epithelial lifting and leukocyte infiltration. In the liver, kidney and intestine acute cell swelling and vacuolization were common. Ultrastructurally the alterations observed frequently in hepatocytes, tubular epithelial cells and enterocytes included presence of numerous myelinoid bodies, damaged mitochondria, dilatation of endoplasmic reticulum, high number of lysosomes and autophagolysosomes. In intestinal and kidney tubular epithelia of treated fish, rodlet cells displayed some anomalies like dilatation of nuclear envelope, cytoplasmic vacuolization, presence of myelinoid bodies, rodlets degeneration and extensive discharge activity.
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