No full-text available
To read the full-text of this research,
you can request a copy directly from the author.
Laboratory evidence for behavioural impairment of fish escaping trawls: A review
Abstract
It is now widely accepted that for some species a proportion of the undersized fish escaping trawl codends die as a direct result of stress, with 10% to 30% mortality commonly cited. It has also been suggested that there may be indirect or behaviourally mediated mortality; fish that encounter and escape the trawl, only to experience stress-induced behavioural deficits and succumb to predators in the hours or days afterwards. The goal of this review was to evaluate the plausibility of this behaviourally mediated, yet unobserved mortality. Three laboratory studies utilizing cod (Gadus morhua), walleye pollock (Theragra chalcogramma), and sablefish (Anoplopoma fimbria) have assayed for behavioural impairment in fish following application of stressors designed to simulate entrainment and escape from trawls. Where impairments in anti-predator capabilities occurred, it was determined that trawl-stressed fish exhibited reduced swimming speed, reduced shoal cohesion, and reduced predator vigilance compared to control fish. Although stressed fish appeared to rapidly recover their ability to avoid being eaten by predators, measurements of more subtle aspects of escapee behaviour suggest that impairments may persist for days after stressor application. Although these studies demonstrate that more investigation is required, when combined with a more extensive literature demonstrating that a variety of stressors can impair fish anti-predator behaviour, it is reasonable to conclude that many fish species escaping trawl codends will likely suffer behavioural deficits that subject them to elevated predation risk. As such, there is probably mortality associated with trawl fisheries that is generally unrecognized, unmeasured, and unaccounted for in current stock assessment models. Further, these studies demonstrate that behavioural competency needs to be considered in the design and implementation of by-catch reduction devises and strategies.
... We broadly use the term 'post-release' to include both animals landed and released and animals that encounter and subsequently escape fishing gear (e.g. Chopin and Arimoto 1995;Ryer 2004). Although it is an important and related topic, we do not include depredation, wherein predators remove or kill fish hooked on longlines or rod and reel, or entangled in gill nets prior to landing (e.g. ...
... As such, the paper is divided into four brief sections: (i) results of a literature survey, (ii) highlights of relevant findings and concepts, (iii) identification of research opportunities and (iv) synthesis and conclusions. We acknowledge and recommend the excellent review by Ryer (2004) on behavioural impairment of fish escaping trawls that focused on synthesizing three laboratory studies. Cooke and Cowx (2006) suggested there are similarities between commercial and recreational fishing sectors that provide opportunities for knowledge crossover and for addressing common issues, such as the fate of released animals. ...
... Post-release predation is thought to occur partly as a result of physiological and behavioural impairment in fish being released that renders them incapable of evading predators. Ten of the studies we found used metrics for assessing behavioural impairment (Olla et al. 1997;Jenkins and Brand 2001;Ryer 2002Ryer , 2004Cooke and Philipp 2004;Davis and Parker 2004;Ryer et al. 2004;Danylchuk et al. 2007a;Campbell et al. 2010a,b). For example, Olla et al. (1997) monitored swimming capacity and feeding behaviour subsequent to simulated capture in (in addition to measuring plasma physiology and predation rates). ...
The assumption that animals released from fishing gears survive has frequently been scrutinized by researchers in recent years. Mortality estimates from these research efforts can be incorporated into management models to ensure the sustainability of fisheries and the conservation of threatened species. Post-release mortality estimates are typically made by holding the catch in a tank, pen or cage for short-term monitoring (e.g. 48 h). These estimates may be inaccurate in some cases because they fail to integrate the challenges of the wild environment. Most obvious among these challenges is predator evasion. Stress and injury from a capture experience can temporarily impair physiological capacity and alter behaviour in released animals, a period during which predation risk is likely elevated. In large-scale commercial fisheries, predators have adapted their behaviour to capitalize on impaired fishes being discarded, while in recreational catch-and-release fisheries, exercise and air exposure can similarly impede the capacity for released fish to evade opportunistic predators. Owing to the indirect and often cryptic nature of this source of mortality, very few studies have attempted to document it. A survey of the literature demonstrated that <2% of the papers in the combined realms of bycatch and catch-and-release have directly addressed or considered post-release predation. Future research should combine field telemetry and laboratory studies using both natural and simulated predation encounters and incorporate physiological and behavioural endpoints. Quite simply, predation is an understudied and underappreciated contributor to the mortality of animals released from fishing gears.
... Marine animals that escape or are discarded from fisheries can be behaviourally impaired due to a spectrum of sublethal stressors (Chopin and Arimoto, 1995;Ryer, 2004). As such, discards may be unable to engage in normal swimming, feeding, and mating behaviours, imposing potential fitness costs. ...
... Some research has focused on linking stress-induced behavioural impairment to delayed (unobserved) mortality. Several studies on fishes (Olla et al., 1997;Morgan et al., 1999;Davis and Parker, 2004;Ryer, 2004) and invertebrates (Vermeer, 1987;Haupt et al., 2006) have shown that severe physiological exhaustion is likely the cause for impaired swimming performance and predator evasion behaviour. Recovery from behavioural impairment in finfish is likely dependent on the intensity and duration of the capture stressor and may contribute to the variability in reported recovery times (i.e., 90 min - Olla et al., 1992;days or weeks -Olla et al., 1995days or weeks -Olla et al., , 1997Morgan et al., 1999;Ryer et al., 2004;Ryer, 2004;Davis, 2005). ...
... Several studies on fishes (Olla et al., 1997;Morgan et al., 1999;Davis and Parker, 2004;Ryer, 2004) and invertebrates (Vermeer, 1987;Haupt et al., 2006) have shown that severe physiological exhaustion is likely the cause for impaired swimming performance and predator evasion behaviour. Recovery from behavioural impairment in finfish is likely dependent on the intensity and duration of the capture stressor and may contribute to the variability in reported recovery times (i.e., 90 min - Olla et al., 1992;days or weeks -Olla et al., 1995days or weeks -Olla et al., , 1997Morgan et al., 1999;Ryer et al., 2004;Ryer, 2004;Davis, 2005). We found no published field studies that investigated stress-induced behavioural consequences of commercial capture, other than in making links to predation (a lethal outcome; see Gitschlag and Renaud, 1994;Ross and Hokenson, 1997 for examples). ...
There is a widely recognized need to understand and reduce the incidental effects of marine fishing on non-target animals. Previous research on marine bycatch has largely focused on simply quantifying mortality. However, much less is known about the organism-level sublethal effects, including the potential for behavioural alterations, physiological and energetic costs, and associated reductions in feeding, growth, or reproduction (i.e., fitness) which can occur undetected following escape or release from fishing gear. We reviewed the literature and found 133 marine bycatch papers that included sublethal endpoints such as physiological disturbance, behavioural impairment, injury, reflex impairment, and effects on reproduction, feeding, and growth for animals that survived a fisheries interaction. Of the 133 identified articles, 22 documented sublethal effects of capture using metrics directly related to fitness, life history, or population-level processes. Sublethal effects were classified as either short-term (e.g., acute stress response), which could lead to long-term or delayed sublethal outcomes (e.g., growth, reproduction), which are directly fitness-relevant and could have had population-level effects. We recommend further investigation into the effects of injury on fitness, and the effects of capture stress on reproduction. It is completely unknown whether sublethal effects can have significant consequences at the population- or ecosystem-level. To date, the potential for discards to suffer from sublethal fitness effects has been almost entirely ignored, and added knowledge on the topic could benefit both conservation and management.
... Most studies have concentrated on evaluating factors that affect the severity and likelihood of injury, physiological disturbance and proximate causes of death related to fishing, such as extensive handling (Olla & Davis, 1989;Olla et al., 1992;Cooke et al., 2002), alteration of light conditions (Olla et al., 1997) and temperature or pressure shock (Olla et al., 1998;Davis & Parker, 2004). Other studies have drawn attention to another important component of unobserved mortality: sub-lethal effects that may indirectly cause additional mortality following modification of fish behaviour and subsequent increased vulnerability to predation (Jollie & Irby, 1979;Ryer, 2002Ryer, , 2004Cooke & Philipp, 2004;Ryer et al., 2004;Danylchuk et al., 2007). Effects such as reduced capacity for schooling, orientation, predator evasion or swimming after escape or release from fishing gears and during aquaculture manipulation have been observed in the laboratory for some species (Mesa, 1994;Olla et al., 1995;Domenici & Batty, 1997;Davis, 2002;Domenici, 2002;Ryer, 2002Ryer, , 2004. ...
... Other studies have drawn attention to another important component of unobserved mortality: sub-lethal effects that may indirectly cause additional mortality following modification of fish behaviour and subsequent increased vulnerability to predation (Jollie & Irby, 1979;Ryer, 2002Ryer, , 2004Cooke & Philipp, 2004;Ryer et al., 2004;Danylchuk et al., 2007). Effects such as reduced capacity for schooling, orientation, predator evasion or swimming after escape or release from fishing gears and during aquaculture manipulation have been observed in the laboratory for some species (Mesa, 1994;Olla et al., 1995;Domenici & Batty, 1997;Davis, 2002;Domenici, 2002;Ryer, 2002Ryer, , 2004. ...
... As a model predator, the D. labrax induced behavioural changes in S. pilchardus that can shed light on their interaction with predators or predation risk (Ryer, 2002(Ryer, , 2004Ryer & Olla, 1998a). Although the D. labrax used in the experiment were accustomed to feeding on artificial food and displayed no propensity to attack the S. pilchardus, D. labrax are known to prey on S. pilchardus (Costa, 1988), and live S. pilchardus are highly prized as bait for D. labrax in recreational fisheries (K. ...
The behavioural effects of confinement of sardine Sardina pilchardus in a purse seine were evaluated through three laboratory experiments simulating the final stages of purse seining; the process of slipping (deliberately allowing fishes to escape) and subsequent exposure to potential predators. Effects of holding time (the time S. pilchardus were held or entangled in the simulation apparatus) and S. pilchardus density were investigated. Experiment 1 compared the effect of a mild fishing stressor (20 min in the net and low S. pilchardus density) with a control (fishing not simulated) while the second and third experiments compared the mild stressor with a severe stressor (40 min in the net and high S. pilchardus density). In all cases, sea bass Dicentrarchus labrax were used as potential predators. Results indicated a significant effect of crowding time and density on the survival and behaviour of slipped S. pilchardus. After simulated fishing, S. pilchardus showed significant behavioural changes including lower swimming speed, closer approaches to predators and higher nearest-neighbour distances (wider school area) than controls, regardless of stressor severity. These results suggest that, in addition to the delayed and unobserved mortality caused by factors related to fishing operations, slipped pelagic fishes can suffer behavioural impairments that may increase vulnerability to predation. Possible sub-lethal effects of behavioural impairment on fitness are discussed, with suggestions on how stock assessment might be modified to account for both unobserved mortality and sub-lethal effects, and possible approaches to provide better estimates of unobserved mortality in the field are provided.
... Mortality studies of trawl-net escapees usually measure cumulative mortality rate after several days of holding, but quantitative information about stress levels or rates of recovery are rarely considered. Stress levels, however, may be so severe that they exert a direct lethal effect on the caught animals (Ryer, 2004;Broadhurst et al., 2006), and sub-lethal stress or exhaustion may have indirect lethal effects, such as greater vulnerability to predation. The magnitude of such effects is not accounted for in traditional survival experiments in which escapees are monitored in protective holding facilities where they are isolated from potential predators. ...
... Such an estimate includes the accumulated mortality of all involved stressors, including any injuries sustained during escape from the fishing gear or subsequently during collection, onboard retrieval, or transfer to monitoring facilities. The potential contribution from increased predation on escapees due to temporary physiological impairment is not included in this mortality estimate, but for some species it is considered to be substantial (Ryer, 2004). The degree of physiological impairment is likely proportional to the degree of exhaustion, thus the level of elevation of hemolymph lactate and the subsequent clearance time before lactate levels again normalise are useful proxies for degree of impairment and duration. ...
When caught in a trawl, some individuals interacting with the fishing gear may escape, but such interactions may lead to physiological trauma that causes direct delayed mortality and/or increased vulnerability to predation. Understanding fishery-induced stress levels and the recovery period of escapees is therefore crucial for predicting total fishing-induced mortality. Hemolymph lactate concentration is commonly used as an index of physiological stress in many crustacean species, and the clearing time of lactate back to normal levels indicates the ability to recover from stress. We measured the hemolymph lactate concentration in three groups of Antarctic krill (Euphausia superbaDana, 1850): Group 1, trawl escapees collected during fishing; Group 2, specimens subjected to simulated mesh penetration; and Group 3, an onboard acclimated control group. Individuals that had escaped the trawl during fishing had the highest concentrations of hemolymph lactate (mean > 6 mmol–l). Exposure to mesh penetration was in itself not stressful, as hemolymph lactate concentrations did not differ significantly between Group 2 and the control Group (mean 0.8 mmol–lversus 0.7 mmol–l, respectively). Additional stress factors during the capture and handling process likely added to the elevated lactate levels observed in Group 1. For the trawl escapees, the lactate clearance time during stress recovery was modeled as a function of exponential decay. Hemolymph lactate levels did not differ significantly among the three groups after 200 min, which suggested that Antarctic krill recovered from fishery-induced stress within this time period.
... Due to the extremely large size of the front meshes of the OBT nets, increased escape seems likely, but the combination of reasons is also possible. Any fish escape earlier in the fishing process would reduce the physical interactions and physiological fatigue which would likely reduce stress and mortality (Suuronen, 2005;Ryer, 2004). ...
... We surmise that either fewer smaller fish entered the OBT nets or escape of smaller fish occurred earlier in the OBT nets, through the extremely large meshes and before reaching the codends. Fish escape earlier in the fishing process would reduce the physical interactions and physiological fatigue which would likely reduce stress and mortality (Suuronen, 2005;Ryer, 2004). ...
We tested two off bottom trawls (OBTs: OBT1 and OBT2) to harvest Georges Bank haddock along with redfish and pollock, while simultaneously avoiding overexploited fish stocks, mainly Atlantic cod, yellowtail flounder (Limanda ferruginea), and windowpane flounder (Scophthalmus aquosus).
... Previous investigations have pointed out that the rate of escape underneath the trawl's fishing line can be high (Main and Sangster, 1981;Engås and Godø, small fish has larger space: Logically, smaller fish have more space to escape between the discs of the rockhopper gear than larger fish. However, fish escaping underneath the fishing line frequently incur injuries in the form of external scrape marks and internal ecchymosis (Ingólfsson and Jørgensen, 2006), which likely lead to increased mortality due to behavioral impairment, increased risk of predation, and disease susceptibility (Chopin and Arimoto, 1995;Ryer, 2004). Although Engås and Godø (1989) reported a reduction in escapement after the transition from steel bobbins gear to the rockhopper gear, escape rates of 33% underneath the rockhopper gear have been documented for cod (Ingólfsson and Jørgensen, 2006). ...
... Ingólfsson and Jørgensen (2006) reported an increased incidence of external and internal injuries on fish that were overrun by the rockhopper gear. The fate of injured escapees is not known, but it is generally believed that there will be some mortality due to behavioral impairment, increased risk of predation, and disease susceptibility caused by contact with the ground gear (Chopin and Arimoto, 1995;Ryer, 2004). Thus, it would be beneficial to the fishery to limit loss of cod below the fishing line during the capture process. ...
... Main and Sangster (1986) observed that different species react in predictable and different ways as they tire in towed fishing gear. For example, roundfish respond to an approaching trawl at a greater distance and have stronger burst and swimming capabilities compared to flatfish (Ryer, 2004). Behavioural studies in the trawl mouth have shown that haddock and, to a lesser extent whiting and saithe, rise up above the ground gear as they tire, whereas flatfish, cod, and Nephrops enter the trawl close to the sea bed (e.g., Sangster, 1981, 1982;1985a;1985b;). ...
... Experiments have shown that fish that escape late in the catching process are more exposed to the consequence of injuries and exhaustion (Ryer, 2004). A major advantage of the selective haddock trawl is that unwanted bycatch will escape the gear with little or no physical contact with the gear. ...
... While behavioural impairment (and other indicators of stress) associated with vitality may indicate an increased likelihood of predation (e.g. Ryer, 2004;Raby et al, 2013) or immuno-supression (e.g. Ellis, 1981;Lupes et al, 2006;Wedemeyer and Wood, 1974), measures of vitality (i.e. ...
... If reflex responses are impaired (e.g. reduced swimming speed, loss of orientation), then responsiveness will be reduced (Ryer, 2004;Raby et al., 2013). Injuries can affect not only a fish`s ability to evade predators (see below), but also shelter seeking and feeding abilities. ...
... Developing quantitative methods to accurately assess discard mortality requires an understanding of the factors that contribute to this mortality, how mortality is defined within each context, and how to measure variables under commercial fishing conditions (Davis 2002). Estimating discard mortality accurately is critical to both biomass estimates from stock assessment models and the development of successful management measures (Mesnil 1996;Chopin et al. 1997;Davis 2002;Ryer 2004). Unless the survival of bycatch is specifically quantified for a species, assessing the status of that stock, setting appropriate fishing levels, and developing an optimum yield may be problematic (Mesnil 1996;Chopin et al. 1997;Ryer 2004). ...
... Estimating discard mortality accurately is critical to both biomass estimates from stock assessment models and the development of successful management measures (Mesnil 1996;Chopin et al. 1997;Davis 2002;Ryer 2004). Unless the survival of bycatch is specifically quantified for a species, assessing the status of that stock, setting appropriate fishing levels, and developing an optimum yield may be problematic (Mesnil 1996;Chopin et al. 1997;Ryer 2004). ...
The survival of sublegal Atlantic cod Gadus morhua discarded in the U.S. Northwest Atlantic demersal longline fishery was examined for the effects that handling technique, sea surface temperature, and capture depth have on it. Longline-caught Atlantic cod were either removed from the hook by hand (unsnubbed) or by allowing the hydraulic hauler to pull the fish against the parallel steel cylinders placed vertically on the gunwale, causing the hook to pull through the jaw (snubbed). Jigged-caught fish served as an indicator of mortality occurring because of the experimental design in the survival experiments. Once caught, live fish were placed in underwater cages, and short-term survival was assessed after holding the fish for 3 d (approximately 72 h). Survival was analyzed with respect to three water depths and four sea surface temperature (SST) strata. Atlantic cod survival in these strata ranged from 31% to 100%. Depth and SST affected survival more than the dehooking technique; survival was higher in shallow depths and at lower temperatures. Unsnubbed fish had higher survival rates than snubbed fish.
... Burst-type swimming or struggling will cause anaerobic metabolism, a depletion of glycogen stores, a buildup of lactate, ion imbalance, and an overall homeostatic disturbance (Falco et al., 2022). Throughout recovery from these effects, individuals may have a reduced ability or motivation to forage, or a decreased capacity to escape from predators (Ryer, 2004). Therefore, the ability to recover may increase the chances of post-release survival and be potentially under selection. ...
... Firstly, there are the by-catches of unquantified sharks discarded at sea, or those landed and quantified, as well as mortality caused by accidental fishing. Secondly, overexploitation of certain shark stocks have affected the productivity of their populations and destroyed their ecosystems by promoting natural mortality (Rayer 2004;Dunn et al 2013). Many chondrichthyes are the top predators in their marine environment, occupying an important ecological niche. ...
Squaliform sharks are a relatively vulnerable bycatch in many deep-water fisheries. Diets of the leafscale gulper shark (Centrophorus squamosus) on the continental slope and continental shelf (200-1600 m) of Moroccan coasts (north-east Atlantic) were studied based on samples landed by 11 longliners in 8 Moroccan fishing ports. The samples were collected every month between February 2018 to March 2021. The diet of 713 individuals with sizes from 60 to 138 cm (total length), 362 females and 351 males, were determined by the examination of the stomach content. Macroscopic and microscopic observations were carried out and we found that the diet of C. squamosus consists of five groups of prey: teleostei, cephalopods, crustaceans, nematodes and chondrichthyes. The main species of prey consumed by the leafscale gulper shark are: Alepocephalus sp. and Aphanopus carbo (Teleostei), Sepia sp. and Octopus vulgaris (cephalopods), Aristeus antennatus and Nephrops norvegicus (crustaceans). In the diet of C. squamosus, chondrichthyes (Chimaera monstrosa and Dalatias licha). The Teleostei group comprised the preferential prey for leafscale gulper shark, with an index of relative importance of 174.18.
... We surmise that either fewer smaller fish entered the OBT net or escape of smaller fish occurred earlier in the OBT net, through the extremely large meshes and before reaching the codend. Fish escape earlier in the fishing process would reduce the physical interactions and physiological fatigue which would likely reduce stress and mortality (Ryer, 2004;Suuronen, 2005;Cook et al., 2019;Pol and Eayrs, 2021). ...
The massive biomass of Eastern Georges Bank haddock ( Melanogrammus aeglefinus ) is difficult to harvest without capturing less robust, but still valuable groundfish stocks like Atlantic cod ( Gadus morhua ) and some flatfish species. Specialized haddock trawls that raise the mouth of the nets off-bottom have reduced bycatch but the very poor status of Atlantic cod prioritizes even greater reduction to prevent exceeding regulatory fishing quotas. Raising the entire fishing gear off-bottom may further reduce bycatch while eliminating benthic impacts, expanding access to grounds previously off-limits to bottom-tending trawls. We evaluated an off-bottom trawl (OBT) to harvest Eastern Georges Bank haddock while reducing catches of overexploited stocks. The OBT net has very large meshes at the front end, made with innovative “helix” twine that produces lateral hydraulic forces while towing, resulting in self-spreading of the meshes. We established optimal gear configurations to achieve the target OBT net shape and distance to the seafloor by using an assortment of mensuration sensors/loggers and cameras. The OBT caught similar amounts of haddock and reduced some bycatch more than a standard bottom “Ruhle trawl”, but also caught fish of the same lengths despite the OBT using a smaller mesh-sized codend. The OBT also demonstrated similar requirements in vessel RPMs as the Ruhle trawl, despite having a larger swept area.
... 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. Broadhurst et al., 2006;Chopin and Arimoto, 1995;Gilman et al., 2013;Kaiser and Spencer, 1995;Raby et al., 2014;Ryer, 2004). Increased fishing mortality may lead to disturbed food web structures and population dynamics of other species Hiddink et al., 2011;Kaiser et al., 2002;van Denderen et al., 2013). ...
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.
... These findings contribute new data on the efficacy of artificial illumination to reduce Pacific halibut bycatch, but also their ability to reduce their bycatch before trawl capture. Capture-escape processes can lead to unobserved and unaccounted post-release mortality caused from physiological stress, fatigue, and injuries (Chopin and Arimoto, 1995;Davis andOlla, 2001, 2002;Ryer, 2004;Davis, 2005). Reducing Pacific halibut bycatch before trawl capture would likely have a positive effect on lowering this mortality. ...
In the U.S. West Coast groundfish bottom trawl fishery, Pacific halibut (Hippoglossus stenolepis) bycatch can impact some fishers' ability to fully utilize their quota shares of groundfishes. In this study, we compared the catch efficiency for Pacific halibut and four commercially important groundfish species between an illuminated and non-illuminated trawl. The illuminated trawl caught significantly fewer Pacific halibut and sablefish than the non-illuminated trawl. For Dover sole (Microstomus pacificus), petrale sole (Eopsetta jordani), and lingcod (Ophiodon elongatus), the illuminated trawl caught fewer individuals than the non-illuminated trawl. However, this catch difference was not statistically significant. Physiological data collected on Pacific halibut caught in illuminated and non-illuminated trawls show blood levels of cortisol, a stress hormone, were significantly higher in fish caught in the illuminated trawl than in the non-illuminated trawl in the absence of differences in condition factor or fat content. While our results have obvious implications for the West Coast groundfish bottom trawl fishery, our findings could also have potential applications in Alaska and British Columbia, Canada trawl fisheries where Pacific halibut bycatch occurs.
... These findings contribute new data on the efficacy of artificial illumination to reduce Pacific halibut bycatch, but also their ability to reduce their bycatch before trawl capture. Capture-escape processes can lead to unobserved and unaccounted post-release mortality caused from physiological stress, fatigue, and injuries (Chopin and Arimoto, 1995Davis and Olla, 2001Ryer, 2004Davis, 2005. Reducing Pacific halibut bycatch before trawl capture would likely have a positive effect on lowering this mortality. ...
In the U.S. West Coast groundfish bottom trawl fishery, Pacific halibut (Hippoglossus stenolepis) bycatch can impact some fishers' ability to fully utilize their quota shares of groundfishes. In this study, we compared the catch efficiency for Pacific halibut and four commercially important groundfish species between an illuminated and non-illuminated trawl. The illuminated trawl caught significantly fewer Pacific halibut and sablefish than the non-illuminated trawl. For Dover sole (Microstomus pacificus), petrale sole (Eopsetta jordani), and lingcod (Ophiodon elongatus), the illuminated trawl caught fewer individuals than the non-illuminated trawl. However, this catch difference was not statistically significant. Physiological data collected on Pacific halibut caught in illuminated and non-illuminated trawls show blood levels of cortisol, a stress hormone, were significantly higher in fish caught in the illuminated trawl than in the non-illuminated trawl in the absence of differences in condition factor or fat content. While our results have obvious implications for the West Coast groundfish bottom trawl fishery, our findings could also have potential applications in Alaska and British Columbia, Canada trawl fisheries where Pacific halibut bycatch occurs.
... The stressed and weakened state of released animals is likely to lead to behavioural impairment, which may decrease the likelihood of them being able to avoid predators Ryer 2002Ryer , 2004Ryer et al. 2004;Marçalo et al. 2013;Raby et al. 2014). Seabirds are likely to be one of the most prevalent predators of discarded animals because they forage at or just below the sea surface, are known to follow fishing boats to specifically prey on discards, and therefore present one of the first predatory threats to discarded animals at the point when most will be at their most vulnerable (e.g. ...
The introduction of catch welfare to commercial wild-capture fisheries will be challenging. In this chapter, we discuss how taking a science-based approach to understanding catch welfare in commercial fisheries could lead to practical solutions to improving welfare that will not only have ethical benefits, but may also have tangible benefits for the fishery, including improved sustainability, product quality and shelf life, and hence profitability. There has been little research to date specifically directed at the development of catch welfare in commercial fisheries. However, there is a substantial and growing body of literature on the fate and vitality of released animals from commercial fisheries—most recently catalysed by the introduction of the Landing Obligation in the EU. Furthermore, there is much to be learned from the aquaculture industry with regard to good welfare practices and product quality, particularly regarding catch handling and slaughter. This chapter utilises this available knowledge to develop a risk assessment-based framework for identifying capture-related stressors and suggests ways of mitigating their impact on the welfare of the catch, as well as on product quality. This framework is developed in context with four contrasting case study capture methods: trawl, purse seine, gill/trammel nets and pots. Finally, it concludes with a summary of the current research priorities and significant strategic challenges for developing welfare-conscious practices in commercial fisheries.
... observed in CBA. It has been shown that fish that escape from trawl codends often display impaired swimming and behavioural deficits that leave them vulnerable to elevated predation risk and reduced feeding success (Ryer 2004). Such fish are likely to have poorer swimming capacity, as respiratory substances in white muscle tissue are depleted (Breen et al. 2004;Wood et al. 1983). ...
Capture-based aquaculture (CBA) combines aquaculture practices with capture fisheries to keep the catch alive for either short or long periods of time, for feeding or for live storage. CBA enables us to market numerous species ranging from molluscs, scallops and crustaceans to fish such as tuna, cod, eel and groupers. In CBA, handling and adaptation to new environments have an additional influence upon the stressors to which fish are exposed during capture, and the duration of this impact increases dramatically from minutes and hours in traditional fishing to days and months in CBA. We show how a strong focus on welfare is already present in cod CBA fisheries and the rationale behind this focus. We present a case study on CBA of Atlantic cod (Gadus morhua) as a robust example and model species for detecting welfare risks and mitigating against them. We discuss the main welfare issues in relation to the three broad phases of capture, transport and live storage, and identify common current fish welfare challenges in CBA. We highlight the advantages of pursuing this approach using lessons learnt from an industry in which fisheries and aquaculture meet and where an existing and successful knowledge transfer process between fisheries and aquaculture is already under way.
... The stressed and weakened state of discarded animals is likely to lead to behavioural impairment, which may decrease the likelihood of them being able to avoid predators (Olla et al. 1997;Ryer, 2002Ryer, , 2004Ryer et al. 2004;Raby et al, 2013). Furthermore, predators are known to preferentially target sick or weakened individuals (Miller et al. 2014). ...
D2-15-Guidance-on-Promoting-Survival-of-Discarded-Fish
... The clear reduction in eulachon bycatch before trawl capture in trawls outfitted with LEDs translates to substantially fewer fish exposed to capture-escape processes within the trawl. These processes can cause physiological stress, fatigue, injuries (from contact with sorting grids, webbing, and (or) other fishes, etc.) and lead to unobserved and unaccounted postrelease mortality (Chopin and Arimoto 1995;Davis andOlla 2001, 2002;Ryer 2004;Davis 2005). Depending on its magnitude, a reduction in eulachon bycatch mortality could have important conservation benefits. ...
This study examined the extent that eulachon (Thaleichthys pacificus) and groundfishes escape trawl entrainment in response to artificial illumination along an ocean shrimp (Pandalus jordani) trawl fishing line. Using a double-rigged trawler, we compared the catch efficiencies for ocean shrimp, eulachon, and groundfishes between an unilluminated trawl and a trawl illuminated with 5 green LEDs along its fishing line. Results showed a significant reduction in the bycatch of eulachon and yellowtail rockfish (Sebastes flavidus) in the presence of LED illumination. As eulachon are an Endangered Species Act listed species, this finding provides valuable information for fishery managers implementing recovery plans and evaluating potential fishery impacts on their recovery and conservation. For other rockfishes (Sebastes spp.) and flatfishes, however, we did not see the same effect as the illuminated trawl caught similarly or significantly more fishes than the unilluminated trawl. Prior to this research, the extent that eulachon and groundfishes escape trawl capture in response to illumination along an ocean shrimp trawl fishing line was unclear. Our study has provided results to fill that data gap.
... However, if seeking towards the lower part of the gear to avoid the light source corresponds to an anti-predator behaviour, it seems counterproductive to approach the light source when attached to the lower net panel as have been seen in cod, whiting and plaice (Melli et al., 2018b). Ryer (2004) suggested that the innate behavioural repertoire of fishes displaced into an inappropriate habitat may be ineffective at countering threat, and this may hold the key to understanding the response of fishes to the VISIONET as well as the LED lights. The toolbox of behavioural responses contains the responses driven by adaptive mechanisms (Marchesan et al. 2005), i.e. responses used in their habitat, such as predator avoidance. ...
Fish are believed to rely largely on visual cues when orientating relative to fishing gears, artificial light are increasingly used to influence fish behavior, for example by making escape routes more visible when ambient light levels are low. We tested if a newly developed luminous netting, VISIONET, could change the vertical behavior of six commercially important species. We inserted an ascending V-shaped piece of VI-SIONET in the tapered section in front of the entrance of a divided codend. The inten-tion was to trigger the optomotor response and guide fish into the upper compartment so they can escape through large meshes. However, results from experimental fishing showed that fish entered the lower compartment more frequently as a response to the presence of VISIONET. The behavioral reaction to VISIONET were largely similar to what has been observed when using green LED-lights, except for haddock, for which the response was opposite. Large Nephrops significantly increased their preference for the lower compartment when using VISIONET. Our results show that low intensity light is sufficient to alter the vertical behavior of both fish and Nephrops. Although opto-motor response appear to be well functioning in parts of the catching process, it was not triggered by the VISIONET design. We need to improve our understanding of the mechanisms underlying the responses of fish to different light characteristics to use VISIONET more efficiently for bycatch mitigation in trawl fisheries.
... Ing olfsson and Jørgensen (2006) reported a high incidence of external and internal injuries to fish that were overrun by the rockhopper gear. The fate of injured escapees is not known, but it is generally believed that there will be some mortality due to behavioral impairment, increased risk of predation, and disease susceptibility caused by contact with the groundgear (Chopin and Arimoto 1995;Ryer 2004). It would therefore be beneficial for fishery management to limit the loss of Haddock below the fishing line during the capture process, thereby reducing potential mortality. ...
The catch efficiency of two types of groundgear—a conventional rockhopper and a new type of groundgear called the semicircular spreading gear (SCSG)—was investigated through experimental fishing for Haddock Melanogrammus aeglefinus conducted in the Barents Sea. A retainer bag was attached behind the footrope of the trawl, and the number of fish that were overrun by the trawl was compared with the catch in the trawl cod end. The catch efficiency increased slightly for larger Haddock for both groundgears. The SCSG was found to have a significantly higher catch efficiency than the conventional rockhopper groundgear. The estimated improvement in catch efficiency varied between 4.5% and 12.3%, with an equivalent reduction in escape rate underneath the groundgear of more than 70%. The rockhopper groundgear can have a catch efficiency as low as 76%, corresponding with values reported in previous studies. Average catch efficiency for the rockhopper gear was significantly lower during the night in comparison with the daytime. No such difference was found with the SCSG. The SCSG is more efficient for catching Haddock, and it is lighter than the rockhopper groundgear. Both are important factors in reducing seabed impact and fuel consumption. When compared to results of a similar study on Atlantic Cod Gadus morhua, we found that in general, both groundgears had a greater catch efficiency for Haddock, which accords with differences in behavior between the two species.
... Predators may become concentrated around trawls, resulting in intensive predation pressure relative to other fisheries (Ramsay, Kaiser, & Hughes, 1998). Fish are also typically released in a state of reduced vitality (e.g., unable to maintain equilibrium, achieve neutral buoyancy), and are therefore less likely to escape predators (Ryer, 2002(Ryer, , 2004). ...
Discarding non‐target fish from commercial fisheries is controversial and has been a persistent concern for fisheries managers globally. Discard management strategies typically begin by understanding mortality rates among discarded fish, a challenging task given the dynamic, highly context‐specific nature of fisheries. An alternative is to develop our knowledge of how stressors operate by first understanding the causes of mortality that drive this context dependence. Particularly relevant to mitigation efforts is an understanding of how fish respond to the physical factors of fishing, such as the gear itself and methods of fishing and handling the gear. We provide a synthesis of how commercial fishing methods may influence discard mortality and outline means by which capture‐induced stress and injury can be mitigated for common commercial gear types, emphasizing method variants or alternatives during capture, handling, and release that could improve survival. This synthesis identifies exhaustion and injury as the most detrimental and ubiquitous stressors experienced by discarded fish, with few options for mitigating their effects. Trawls and hanging net fisheries are identified as the most harmful gears for by‐catch, characterized by high stress regardless of method variants and limited options for mitigation. Irrespective of gear type and type of stressor, minimizing durations of capture and handling and encouragement of good handling behaviour (e.g., during landing and sorting) will reduce the magnitude of stress and injury in fish, and ultimately increase survival.
... K.V. Cook et al. Fisheries Research 206 (2018) 96-108 or escape predation (Ryer, 2004). If impairment causes migratory delays (as in Donaldson et al., 2012;Cook et al., 2018), there is also a greater likelihood that fish will be captured again, intensifying physiological responses and potential for mortality with every release. ...
Recommendations and regulations regarding handling of non-target fish (i.e. bycatch) are often vague and subjective in commercial fisheries. Identifying how different components of capture influence the condition of discarded fish can help develop specific guidelines and best handling practices. Using an experimental approach, we modified the severity of capture stressors in commercial purse seine fisheries for Pacific salmon and monitored indices of injury and reflex impairment in chum salmon (Oncorhynchus keta), a species commonly discarded from these fisheries. Study fish were held for 5 or 10 days. Modeling of changes in injury and impairment sought to disentangle the latent effects of capture stressors and the role of sex and maturity. Thresholds in physiological responses to times (i) pursed in the net and (ii) air exposed on deck were also evaluated. Injury progressed throughout holding, was more extensive in females, and accelerated faster in less mature fish. Both crowding severity and set size (i.e. estimated number of fish caught) increased injury and impairment, effects that were exacerbated with time pursed. Physiological indicators of exhaustion also increased with time pursed and 15 min was identified as an important transition point, potentially representing the temporal limit to anaerobic exercise. The time between 1 and 3 min of air exposure was identified as being important to survival, and after 6 min of air exposure, endogenous energy stores may have become exhausted. Resulting recommendations include keeping nets loose during sorting, releasing fish prior to 15 min of being pursed, and keeping air exposure within the range of 1–2 min, or less. Additionally, females and less mature fish appear to be more susceptible to the injurious effects of capture.
... Íàèáîëåå âàaeíûé âîïðîñ -çíàíèå ñåëåêòèâíîñòè îðóäèÿ ëîâà â êîíêðåòíûõ óñëîâèÿõ ðàáîòû äëÿ êîððåêòíîãî ñîïîñòàâëåíèÿ ðàçìåðíîé ñòðóêòóðû ðûá ïî èõòèîëîãè÷åñêèì è àêóñòè÷åñêèì äàííûì è êîððåêòíûõ ðàñ÷åòîâ çàïàñà [ICES, 2004[ICES, , 2005. Îäíàêî îïðåäåëåíèå ýôôåêòèâíîñòè è ñåëåêòèâíîñòè îðóäèÿ ëîâà âñòðå÷àåò áîëüøèå òðóäíîñòè, îñîáåííî â ñëó÷àå ãëóáîêîâîäíûõ è ïðèäîííûõ òðàëåíèé, à òàêaeå äëÿ ìíîãîâèäîâûõ ñêîïëåíèé ðûá è òðóäíî îòëàâëèâàåìûõ íåáîëüøèõ êîñÿêîâ [Ìàðêèí, 1984;Òåî-ðèÿ…, 1985;Êàäèëüíèêîâ, 1997;Êîðîòêîâ, 1998;Êàñàòêèíà, 1999;Áåëîâ, 2000;Ìèçþðêèí è äð., 2004;Arreguin-Sanchez, 1996;Shevelev et al., 1998;Engas et al., 2000;Krieger et al., 2001;ICES, 2004ICES, , 2005Ryer, 2004;Mason et al., 2005;Stockwell et al., 2007].  ýòèõ ñëó÷àÿõ âîçëàãàåòñÿ áîëüøàÿ íàäåaeäà íà òåõíè÷åñêîå óñîâåðøåíñòâîâàíèå òðàëîâ [Êóäðÿâöåâ, 2000;Êàäèëüíèêîâ, 2001;Engas et al., 2000;è äð.] è äðóãèõ îðóäèé ëîâà [Kurkilanti, 1999], à òàêaeå íà âêëþ÷åíèå â ïðîöåññ òðàëåíèÿ ìíîãîëó÷åâûõ ðàçíî÷àñòîòíûõ àêóñòè÷åñêèõ ñèñòåì [Êóäðÿâöåâ, 2008;Russel et al., 2003], âèäåîàêóñòè÷åñêèõ òåõíîëîãèé [Åðìîëü÷åâ è äð., 1999;Godo et al., 1999;Ermolchev, Zaferman, 2003;Rose et al., 2005] è ïîäâîäíûõ àïïàðàòîâ . ...
This book provides an overview of the world practice in hydroacoustic survey of sea and lake fish, outlines principal approaches adjusting this method to Lake Baikal environments, details the trawl and acoustic methods in Baikal omul studies, as well as biological sampling and data processing procedures. The distribution of Baikal omul during its feeding migrations in various years and the abundance, biomass and size characteristics are analyzed. The authors describe the sound scattering layers of Lake Baikal as a possible acoustic background, their experience in creating a hardware-software field complex to assess total biomass of omul and other biological resources, as well as a computer data base. Recommendations on monitoring omul by gydroacoustical means are included.
The book is intended for a wide range of experts in ecology, hydrobiology, hydrophysics, limnology, nature management and fishery.
... In general, fish sustain various types of injuries during the catching or escape process, such as stress, behavioural impairment, scale damage with possible subsequent osmotic disturbances or infections, barotrauma-related injuries, or other types of injuries. These factors are known to cause long-term delayed mortality due to the elevated risk of predation and susceptibility to disease (Chopin and Arimoto, 1995;Davis, 2002;Ryer, 2002Ryer, , 2004Ryer et al., 2004). It is likely that buffer towing increases the risk of the above mentioned injuries and it is therefore highly probable that buffer towing contributes to unaccounted fishing mortality. ...
The high abundances of Northeast Arctic cod (Gadus morhua) in the Barents Sea have led to the development of a new fishing tactic called buffer towing. On factory trawlers, the trawl is deployed immediately after taking the catch onboard, a tactic used to ensure a continuous supply of fish is being processed. If the desired amount of fish is caught before the catch from the previous haul has been fully processed, the trawl is lifted off the seabed and towed at a given depth at low speed. This is called buffer towing. Cod that escape from the codend when the trawl is shallower than the initial fishing depth are exposed to an increased likelihood of barotrauma-related injuries, increased disease susceptibility, and predation, which could be lethal, or affect growth and reproduction capability. Therefore, this study quantified the escape rate and size selectivity during buffer towing of cod. A new analytical method was applied that allows using the same trawl configuration as applied during commercial fishing and avoids potential bias in the assessment of buffer towing size selection. Our results demonstrated a significant size selection for cod during buffer towing where cod measuring up to at least 42 cm in length were proven to escape. In particular, at least 60% of cod measuring 20 cm were estimated to escape during buffer towing. For cod measuring 30 and 40 cm, at least 53 and 45% were estimated to escape during buffer towing, respectively.
... In fact, in Mauritanian waters the main fishing areas are located in the northern area, where a fleet targeting cephalopods operates alongside demersal and large pelagic trawlers (Inejih 2000;García˗Isarch and Sobrino 2013;FAO 2007). The huge amount of discards generated by these fleets could be altering the natural food web of demersal elasmobranches, increasing the availability of scavenged prey as well as benefiting them from predating behaviourally impaired fish that have escaped trawl nets (Dunn et al. 2013;Ryer 2004). These sharks are highly opportunistic scavenger species (Martin and Treberg 2002;Carrassón et al. 1992), actively taking advantage of any extra food supply. ...
This study analyses the sampling of 22,118 individuals and 24 tons of demersal elasmobranches caught at 243 trawling stations during four Maurit surveys, which sampled Mauritanian waters, from 84 to 1835 m depth. We identified a total of 48 different species belonging to 17 families. Rajidae was the most diverse family (10 species), followed by the family Centrophoridae (6 species). Other diversefamilies were Etmopteridae, Scyliorhinidae and Somniosidae, with five species each. This chapter analyses biodiversity, community assemblages, distribution patterns and the demographic structure of the Mauritanian deepwater elasmobranches. We discuss the bathymetric ranges of the different species of the communities identified, from the shelf to the deepest part of the slope. For each community we present the geographic and bathymetric patterns of the following descriptors: biomass, abundance, richness and diversity indices. We also present spatial patterns of abundance, average sizes, length distribution and sex ratio by bathymetric range of the most abundant species (Centroscymnuscoelolepis, Centroselachus crepidater, Deania calcea and Centroscyllium fabricii). Ecological constraints, environmental influence, and the direct and indirect fishing impact that shape the community structure of demersal elasmobranches are also discussed, since they play a key role in the balance and dynamics of the Mauritanian deep-sea ecosystem.
... Lactate accumulation as a result of exhaustive exercise (Skomal & Bernal 2010) can contribute to high levels of stress, blood acidosis, and/or mortality (Frick et al. 2010, Danylchuk et al. 2014, Gallagher et al. 2014b, Hutchinson et al. 2015, while elevated blood glucose is a common response to capture stress as hormones mobilize hepatic glycogen to fuel active muscle tissue (Hoffmayer & Parsons 2001, Mandelman & Farrington 2007, Skomal & Bernal 2010. These disturbances can be related to long fight times and on-hook fight behaviors which can affect post-release recovery and predator evasion (Olla et al. 1992, 1995, Ryer 2004, Wilson et al. 2014, as evidenced by the behavioral impairments seen here. ...
Bycatch interactions with deep-sea elasmobranchs are increasingly common and can lead to dramatic declines in abundance over short time scales. Sharks hooked in the deep sea could face a higher likelihood of severe physiological disturbance, at-vessel mortality, and postrelease mortality (PRM) than their shallower counterparts. Unfortunately, robust PRM rates have not yet been estimated for longline-caught deep-sea sharks, and as such are not currently incorporated into total fishery mortality estimates or bycatch assessments, limiting the effectiveness of current conservation or management initiatives. We empirically estimated PRM for 2 focal taxa of deep-sea shark, the Cuban dogfish Squalus cubensis and the gulper shark Centrophorus sp., using post-release enclosures deployed at-depth. We calculated 24 h PRM rates of 49.7 ± 8.5% (mean ± SE) for S. cubensis and 83 ± 16% for Centrophorus sp. and identified blood lactate, total length, glucose, and vitality scores as predictors of PRM in S. cubensis. We also observed all 24 h PRM within 11 h post-capture and demonstrated the effects of recovery depth and at-vessel blood chemistry metrics on post-release behavior. Our results suggest that PRM rates of deep-sea sharks are high and highlight the need for filling in this gap in fishery mortality estimates for other common discards in the future.
... The field results obtained in this study can be used in future laboratory experiments to guarantee that the simulated fishing impact is realistic. Laboratory experiments can then be used to explore the links between magnitude of stressor, stress reactions and subsequent fish survival and to evaluate behavioral impairments related to the stress caused by purse seine fishing (Olla et al. 1997;Ryer 2004;Davis 2005). ...
Observations from the purse seine fishery off northern Portugal are used to describe the early dynamics of sardine (Sardina pilchardus) stress reactions and identify likely stressors during the commercial fishing operation. Sardine blood and muscle were sampled from the onset of fishing (school identification and encircling) to the end of fish transfer onboard (90–120min later). The evolution of haematocrit, haemoglobin, cortisol, glucose, ionic concentrations, ATP and its catabolites were modelled using linear mixed models as a function of time spent in the net, biological (sex, reproductive state and condition) and operational variables (catch, light level and phase of fishing operation). Significant linear trends with time were detected for most stress variables and mean concentrations after 2h in the net were similar to literature values corresponding to acute stress reactions for teleosts. Biological variables were rarely significant and explained a small proportion of variation, while operational variables were never significant. For each stress variable, levels varied considerably between trips but the temporal evolution was common across trips. Random trip effects were uncorrelated among most biochemical variables, suggesting that distinct factors affected each stress variable during the sampled trips. However, the linear trend with time spent in the net observed for most stress variables indicates that the duration of the fishing operation is an important stressor in purse seine fishing due to the progressive water volume restriction, crowding and manipulation associated to the fishing method.
... Key factors that have been difficult to study include the additive effects of enhanced predation susceptibility resulting from reflex impairment (e.g. Ryer, 2004) and of release of discards into different environmental conditions compared to where they were caught. A comparative approach such as used here holds some promise in incorporating such effects in assessments and decisionmaking. ...
... The tertiary response involves changes in whole-organism behaviour, performance, and disease resistance, including fitness and survivorship (Mazeaud et al. 1977). Capture and release may induce a suite of behaviours ranging from minimal changes (Whoriskey et al. 2000) to considerable departures from behavioural norms (Ryer 2004;Arlinghaus et al. 2007). Capture and release may either increase (e.g., Mäkinen et al. 2000) or decrease swimming activity (Thompson et al. 2008). ...
... It was possible that, for both predators, some of the other fish identified to species, or unidentified, had also been discarded and scavenged, and as a result the discarded fish component of the diet may have been underestimated. Scavenging of commercial discards and predation of wounded crustacean and fish escapees from trawl nets could provide a positive feedback from the commercial fishery to P. bachus and H. percoides populations (Groenewold & Fonds 2000;Ryer 2004). Both predator species are frequently caught on baited longlines (Horn 2004), confirming scavenging behaviour. ...
The diets of red cod Pseudophycis bachus and sea perch Helicolenus percoides were determined from examination of stomach contents of 246 and 494 specimens, respectively, sampled by bottom trawl on Chatham Rise to the east of South Island, New Zealand. The distributions of both species on Chatham Rise overlapped geographically and by depth; the entire P. bachus distribution was encompassed by the H. percoides distribution. The diets of both species were dominated by crustaceans, but fish was also an important dietary component for P. bachus, while pelagic tunicates were important for H. percoides. Despite the overlap in spatial distribution, P. bachus and H. percoides had distinct diets with non-significant overlap, which will markedly reduce resource competition between these two species. The most important crustacean prey were Galatheidae (Munida sp.) and the pandalid shrimp Notopandalus magnoculus in P. bachus, and two-spined crab (Pycnoplax victoriensis) and isopods in H. percoides. Within-species variation in diet was influenced by predator location and ontogeny. The main ontogenetic shifts in diet were from small shrimps to larger fish prey for P. bachus, and from small crustaceans (mysids and galatheids) to larger crustaceans (scampi and crabs) for H. percoides.
... Having escaped the trawl, these species may still suVer from post-exercise mortality or increased predation risk Winger et al., 1999;see also Wordle, 1993;Godø, 1994.) Ryer, 2004). Post-escapement survival can vary widely among species, and smaller escaped individuals sometimes sustain higher injury than larger ones (Soldal et al., 1993). ...
Fishing gear technology and fish passage and protection constitute two discrete applications of conservation engineering with a common underlying biological basis. Much of what we know today about fish locomotion and behavior has its roots in one or the other of these applications. These parallel tracks are particularly evident in the study of swimming performance, and widely applied models of swimming ability were derived from efforts to join these two bodies of research. Those models, however, have recently been discredited, and emerging evidence suggests substantial errors in both estimates of swimming ability and the appropriate application and interpretation of available data. This has serious implications, both for criteria used for developing fish passage solutions (e.g. at fishways and road crossings) and also for our understanding of the mechanics of fish capture with commercial fishing gear.
... Caging may protect escaping sh from predation, and potentially increase post-escape survival rates. Injuries and/or exhaustion caused by the capture and escape process have been shown to result in behaviour impairments in sablesh (Anoplopoma mbria) and walleye pollock (Theragra chalcogramma), making individuals more vulnerable to predation (Ryer, 2004;Ryer et al., 2004). The predation mortality may therefore add to the overall mortality but remains unknown. ...
Survival rates and injuries of haddock (Melanogrammus aeglenus), cod (Gadus morhua) and saithe (Pollachius virens) were studied after they escaped from codends and grids in full-scale trials in the Barents Sea. Escaped sh were collected in a cage connected to a hooped codend cover for the codend escapees, or a grid cover for the grid escapees. Trawl-caught controls were sampled by removing the codend and attaching the cage to the trawl extension. Acoustic release devices were used to time the sampling. Due to technical problems, the replicates were fewer than planned. Control sh were also sampled in sh traps. Survival rates of cod and saithe were 100%. Haddock survival was lower (5098%) and in some cases related to sh length. Haddock survival could not be shown to depend upon the selectivity device, but the number of replicates does not allow us to draw a rm conclusion. Scale loss of haddock decreased as sh length increased in all experimental groups. Cod and saithe suered fewer skin and n injuries than haddock.
... Having escaped the trawl, these species may still suVer from post-exercise mortality or increased predation risk Winger et al., 1999;see also Wordle, 1993;Godø, 1994.) Ryer, 2004). Post-escapement survival can vary widely among species, and smaller escaped individuals sometimes sustain higher injury than larger ones (Soldal et al., 1993). ...
... The dynamic of the stress is simulated using the same method as for effort (rule set 'Stress'). As pointed out by Ryer (2004), there are many factors influencing fish stress in fishing gear and especially prior and during escape. In this study, we considered four stressing situations: ...
... Key factors that have been difficult to study include the additive effects of enhanced predation susceptibility resulting from reflex impairment (e.g. Ryer, 2004) and of release of discards into different environmental conditions compared to where they were caught. A comparative approach such as used here holds some promise in incorporating such effects in assessments and decisionmaking. ...
Benoît, H. P., Plante, S., Kroiz, M., and Hurlbut, T. 2013. A comparative analysis of marine fish species susceptibilities to discard mortality: effects of environmental factors, individual traits, and phylogeny. – ICES Journal of Marine Science, 70:99–113.
Determining the sustainability of fishing mortality for discards requires information on discard amounts as well as capture and release mortality rates. Formal estimates of these rates are costly and only available for a limited number of species and fisheries. In their absence, proxies for discard mortality could inform risk assessments of fishing mortality sustainability for discarded species. Here, time-to-mortality (TM) was assessed for 48 marine fish species exposed to air following capture during an annual multi-species bottom-trawl survey. Species-specific estimates of TM were related qualitatively to more formal estimates of discard mortality from commercial fisheries, confirming the use of TM as a proxy. The effects on TM of species and individual traits, phylogenetic similarity (proxy for traits not explicitly included in the analysis) and environmental factors related to capture were also assessed. Much of the observed individual variability was explained by intraspecific and interspecific positive relationships between body size and TM. Sedentary species and those lacking a gas bladder or deciduous scales had greater TM. Effects of phylogeny and capture depth and temperature were also found. This study demonstrates how reliable proxies of discard mortality rate can be readily obtained in the field or estimated from relevant covariates.
... Thus, fishing induces changes in behavior (or promotes expression or success of extreme behaviors) through alterations on the population level (i.e., population parameters, life history traits; see Tables 19.5, 19.6). However, bottom trawling is also likely to cause indirect changes in behavior at the ecosystem level, through habitat destruction (Watling and Norse 1998, Jennings et al. 2001b, Kaiser et al. 2002, and direct changes in behavior at the individual level by eliciting strong avoidance behavior as has been observed in fishes (Ryer 2004). ...
... These estimates, which ideally should be validated by an independent study of discard survival using fish tagging, likely represent a maximum estimate of survival because they do not include factors such as the effect of enhanced predation risk resulting from the temporary impairment of reflexes due to capture, handling and release in the fishery (e.g. Ryer, 2004; see discussion in Benoît et al., 2012a). The estimates, nonetheless, help establish a general scale for possible discard-related losses, and can serve to highlight how conditions in the fisheries catching skates may be influencing these losses. ...
Benoît, H. P. 2013. Two decades of annual landed and discarded catches of three southern Gulf of St Lawrence skate species estimated under multiple sources of uncertainty. – ICES Journal of Marine Science, 70: 554–563.
Estimating fishery impacts on commercially unimportant species is often hindered by limited and possibly biased data for landed and discarded catch, and poor information on discard mortality. The three skate (family Rajidae) species occurring in southern Gulf of St Lawrence (Canada) exemplify this problem. Assessing the contribution of fishing to important declines in their adult abundance has been complicated by catch data that are not disaggregated by species, concerns about the reliability of discard amounts estimated from fisheries observer surveys, and unknown discard mortality rates. An approach is presented for producing annual estimates of landed and discarded catch, as well as discard mortality rates, for the three species for the period 1991–2011. The approach used data from landing statistics and from observer surveys, and models for disaggregating mixed fishery catches into their constituent species and for estimating minimum discard mortalities. Bootstrapping was used to propagate errors associated with different components of the estimation process. The estimation was partly validated by comparing recorded landings with landings estimated from fisheries observer surveys. This paper demonstrates how multiple sources of uncertainty in discard loss estimation can be addressed by dividing the estimation process into linked components that can be individually addressed and ideally validated.
... The likelihood that a fish survives capture and discarding may be more complicated than described above in some circumstances, due to indirect mortality. For example, behaviourally impaired discarded individuals may face enhanced predation risk (Ryer, 2004), which would not be integrated into estimates of survival based exclusively on fish holding. Injured fish may also develop secondary infections that eventually induce mortality not observed during holding. ...
Obtaining a representative estimate of discard mortality for population and ecosystem assessments is very challenging. This can only rarely be done directly by recovering tagged discarded individuals. Instead, semi-quantitative measures of individual fish vitality or physical condition, obtained by onboard observers prior to discarding, can be used. Such vitality measures can be a good indicator of discard mortality, and by virtue of the data collection method, should also reflect the condition of discards throughout the fishery. Furthermore, vitality can be predicted using covariates known to affect discard mortality, allowing for a more general assessment. We argue that a representative mortality rate can be estimated using the product of at least two probabilities: that of belonging to a particular vitality class, conditional on the factors experienced during capture and catch handling; and the probability of surviving the event, conditional on pre-release vitality. Here we estimate mortalities for five fish taxa captured in southern Gulf of St. Lawrence fisheries. The conditional survival probabilities were obtained using survival analysis of data from experiments in which fish were captured using commercial fishing methods and held to assess short-term mortality (2–3 days). The analysis included a mixture model with a fraction of unaffected individuals, which appears appropriate for data from bycatch mortality studies. Based on this study and the mechanistic interpretation of the mixture model, short-term monitoring of discard mortality may be sufficient to characterize longer term impacts in a number of taxa.
... Once a predator is detected, behavior that ensures survival typically takes priority (Pitcher, 1986). Among fish, common antipredator behaviors include decreased activity (Gotceitas and Colgan, 1990;Magnhagen, 1988), reduced feeding or travel distance to food (Dill and Fraser, 1984;Fraser and Gilliam, 1987;Huntingford et al., 1988;Metcalfe et al., 1987), increased time until resumption of feeding (Gotceitas and Godin, 1991), increased school/shoal cohesion (Ryer, 2004), and altered habitat preference . Flatfish utilize a detection minimization strategy, which incorporates the easily observable flatfish morphological adaptations , as well as more subtle behavioral tactics that vary in their degree of expression between species Lemke and Ryer, 2006a,b). ...
We tested if a newly developed luminous netting, VISIONET, could change the vertical behaviour of six commercially important species in a Nephrops (Nephrops norvegicus) trawl fishery. We inserted a V-shaped piece of VISIONET ascending on each side of the tapered section just ahead of a divided codend. The length-based effect on the vertical separation of fishes and Nephrops was quantified, and we evaluated if the presence of VISIONET had the potential to increase the fish capture in the upper compartment.
Contrary to our expectation, gadoids entered the lower compartment more frequently than in the control trawl. This was similar to that previously found when applying green LED lights in the tapered section, however opposite for haddock (Melanogrammus aeglefinus). The flatfishes did not respond. Large Nephrops significantly increased their preference for the lower compartment.
Our results show that low intensity light is sufficient to alter the vertical distribution of both fishes and Nephrops. Responses of fishes to different variables, including light intensities, need to be studied in more detail to understand the underlying mechanisms and to ultimately reduce unwanted catch more efficiently. Luminous netting can be integrated in any given trawl design and does not require batteries or electronics.
This thesis provides an overview of recent selectivity studies conducted in eastern North Pacific trawl fisheries (e.g., West Coast groundfish bottom trawl fishery, Pacific hake [Merluccius productus] fishery, and ocean shrimp [Pandalus jordani] fishery). Collectively, these fisheries play a significant role in supporting fishing jobs, income, and coastal communities. However, bycatch can impact fishers ability to fully utilize the fisheries resource. Thus, developing gear modifications to reduce bycatch are increasingly important. In this thesis, results from VIII selectivity research papers addressing bycatch issues in eastern North Pacific trawl fisheries are presented.
In the U.S. West Coast groundfish bottom trawl fishery, constraining species such as darkblotched rockfish (Sebastes crameri), sablefish (Anoplopoma fimbria), and Pacific halibut (Hippoglossus stenolepis) bycatch can impact fishers ability to maximize their quota shares of healthier groundfish stocks. In Papers I-III, results from sea trials evaluating sorting grid bycatch reduction devices (BRDs) to reduce catches of these species are presented. Results from these papers demonstrate the ability of sorting grid devices to reduce bycatch while retaining a relatively high proportion of the targeted species. In Paper IV, the efficacy of T90 mesh codends to improve catch composition in the Dover sole-thornyhead-sablefish complex fishery were examined. In this fishery, where catches of juvenile and sub-adult sablefish are affecting fishers ability to achieve a higher ex-vessel value (e.g., landed value) of the sablefish resource, and higher attainment rates of Dover sole (Microstomus pacificus), results presented in Paper IV demonstrates that T90 mesh codends have potential to increase fishers opportunities to capitalize on their Dover sole individual fishing quota and enhance their net economic benefits while more effectively attaining their quota shares of sablefish.
In Papers V-VIII, results are presented from studies testing the efficacy of artificial illumination (e.g., light-emitting diodes [LEDs]) to reduce fish bycatch. In Paper V, research tested if simple enhancements to the visibility of a low-rise selective flatfish trawl headrope could improve bycatch reduction for darkblotched rockfish, sablefish and Pacific halibut. Findings from Paper V suggest that use of illumination could have potential applications for reducing bycatch under particular situations. For example, fishers seeking to reduce sablefish catches and/or Pacific halibut bycatch when targeting English sole (Parophrys vetulus) and petrale sole (Eopsetta jordani) could potentially benefit from illuminating the trawl headrope, whereas fishers seeking to target Dover sole and/or sablefish but avoid darkblotched rockfish, would likely not benefit from using illumination. In Papers VI-VII, studies evaluating the efficacy of LEDs to reduce eulachon (Thaleichthys pacificus) and groundfish bycatch were examined. For eulachon, an anadromous smelt species endemic to the eastern North Pacific, their bycatch is an issue facing the ocean shrimp fishery as the species’ southern Distinct Population Segment was listed as “threatened” under the U.S. Endangered Species Act (ESA) in 2010. Results presented in Papers VI and VII continue to support the hypothesis that there is a significant reduction in eulachon bycatch when artificial illumination is present. For rockfishes and flatfishes, findings suggest their ability to escape trawl entrainment in response to illumination along the fishing line is not as strong as previously indicated. As conservation of ESA-listed eulachon is an ongoing management priority, Papers VI and VII contribute new data on the efficacy of footrope illumination to reduce their bycatch. Lastly, Paper VIII conducted two separate experiments evaluating the influence of artificial illumination on Chinook salmon (Oncorhynchus tshawytscha, a species with ESA listings) behavior and escapement out of a BRD in a Pacific hake midwater trawl. Findings from Paper VIII demonstrate that artificial illumination can influence where Chinook salmon exit out the BRD tested, but also that illumination can be used to enhance their escapement overall. Because ocean distributions of Chinook salmon and Pacific hake often overlap, interactions between Pacific hake trawl gear and Chinook salmon are likely to continue to be an issue facing the fishery. Findings from Paper VIII provides data on a gear modification that can minimize Chinook salmon bycatch.
Lastly, the collective work presented within this thesis has contributed substantially to the development and advancements of gear modifications for reducing bycatch in eastern North Pacific trawl fisheries and the conservation of ESA-listed species.
Papers I, II, and VIII are published in Fisheries Research, Papers III, IV, and V are published in Marine and Coastal Fisheries, Paper VI is published in the International Council for the Exploration of the Sea Journal of Marine Science, and Paper VII is published in the Canadian Journal of Fisheries and Aquatic Sciences.
The hypothalamic‐pituitary‐interrenal (HPI) or stress axis in teleost fishes produces their primary glucocorticoid, cortisol. Although generally an adaptive response, prolonged HPI axis stimulation can impair organismal performance. Previous work has shown that stressed teleosts have higher mortality to predation than unstressed conspecifics, suggesting a role for HPI axis in modulating predator–prey interactions. Our current study investigated whether elevated cortisol levels altered the predation rate of a wild teleost fish, the bluegill sunfish (Lepomis macrochirus). Wild juvenile bluegill were given intraperitoneal implants of cocoa butter (i.e., sham), or cocoa butter containing cortisol or cortisol and the glucocorticoid receptor antagonist RU486. After 24 hr, fish were tethered along the bottom of the lake and their survival under natural predation was recorded following 24 hr. A subset of fish was used to validate the efficacy of cortisol implants in this setting. No treatment effect on survival was observed, suggesting that elevated cortisol has minimal involvement in mediating predator–prey interactions in this context. However, experimental fish may have demonstrated resiliency to physiological perturbations owing to the relatively acute duration of our experimental series, and negative effects might be manifested over a more chronic period. Graphical summary of the effects of cortisol‐treatment on predator mortality of tethered bluegill (Lepomis macrochirus) in a natural lentic system.
Using an iterative approach and extensive field testing we developed a new video trawl survey system that used a live-feed video camera mounted in the cod end of a demersal trawl to record, identify, and quantify fish as they pass through the net. The majority of tows are made with an open cod end, allowing the fish to escape the net after being recorded by the camera. Periodically, closed cod end tows are made to collect biological samples and validate the video data. Eight field trials were conducted on Georges Bank and in the Gulf of Maine, and 229 h of video were recorded. The in-trawl camera system performed reliably under harsh conditions in the field. Preliminary data analysis showed that the in-trawl camera system can be used to count and identify roundfishes, such as Atlantic Cod Gadus morhua and Haddock Melanogrammus aeglefinus with a high degree of accuracy (97%), and the video can be used to calculate absolute abundance estimates. However, identifying flatfishes to the species level in the video was challenging, although improvements in identification rates were realized by modifying our camera system and lighting. This approach provides an alternative methodology to acquire abundance and distribution data for groundfish stocks. We described the iterative approach used to develop the video trawl system and discussed the future direction of the video processing and analysis.
Received August 15, 2016; accepted January 9, 2017
Four categories of anthropogenic stressors affect the behavior and vital functions of crustaceans in aquatic ecosystems: pollution, introduction of non-indigenous species, stock manipulation, and habitat destruction. The most affected vital functions are, in descending order of importance, reproduction, nutrition and growth, and habitat selection. Two principal groups of stressors were distinguished: (i) pollution and species introduction, for which most of the effects on crustacean behavior were observed directly; and (ii) fishing and habitat destruction, for which most of the evidence was circumstantial. In many cases, pollution and species introduction affect individuals directly, whereas fishing and habitat destruction affect crustacean behaviors indirectly, primarily by changes in important population (e.g., sex ratio and population density) or ecosystem (e.g., habitat, physicochemical, species dominance, species composition) parameters. The information yielded by this review and that of future studies may prove useful in designing efficient conservation plans.
The development, implementation, and testing of a stereo-optical imaging system suitable for environmental monitoring of a tidal turbine is described. This monitoring system is intended to provide real-time stereographic imagery in the near-field (<10 m) of tidal turbines proposed for deployment in Admiralty Inlet, Puget Sound, Washington. Postdeployment observations will provide the necessary information about the frequency and type of interactions between marine animals and the turbine. A method for optimizing the stereo camera arrangement is given, along with a quantitative assessment of the system's ability to measure and track targets in three-dimensional space. Optical camera effectiveness is qualitatively evaluated under realistic field conditions to determine the range within which detection, discrimination, and classification of targets is possible. These field evaluations inform optimal system placement relative to the turbine rotor. Tests suggest that the stereographic cameras will likely be able to discriminate and classify targets at ranges up to 3.5 m and detect targets at ranges up to, and potentially beyond, 4.5 m. Future system testing will include the use of an imaging sonar ("acoustical camera") to evaluate behavioral disturbances associated with artificial lighting.
The field of biomechanics tends focuses on suborganismal processes. Muscle kinetics, body and fin morphologies, and sensory systems characterize the mechanical interactions between fishes and their habitat. These in turn define the fishes' ability to exploit their environment, influencing not only home range, but also reproductive success, energetics, trophic interactions, and the scope of available habitats. Fisheries managers must take into account more than ecology and life history theory when developing policy: physiology, energetics, behavior, and stochastic habitat and ecosystem‐level processes must all be considered. The chapter discusses the current state of the field of fish biomechanics in a management context, identifying advances and gaps in knowledge that may both aid the development of sound management policies and provide helpful directions for future research. When developing conservation policy, managers often focus on critical life history stages: those parts of the life cycle that determine population-level processes. Many factors influence whether and in what condition fishes progress through these life stages. The chapter also presents a summary of gaps in current knowledge, with suggestions to how future biomechanics research might help to improve practices in both fisheries management and conservation.
During a fishing trip to record video footage of fish escaping from a by-catch reducing device located in a commercial prawn trawl, two bottlenose dolphins, Tursiops truncatus, were observed to actively manipulate the codend at the seabed, removing and consuming components of catch (mostly juvenile whiting, Sillago spp.). The observed feeding pattern suggests a well established behavioral response to trawling activities and is discussed with respect to (1) the potential nutritional benefit that dolphins may derive from such activities and (2) the effects that scavenging may have on selectivity of the gear.
Two experiments were done in a flume tank to quantify the effects of codend mesh circumference and weight of catch on water flow. The mesh circumference of the posterior section of prawn-trawl codends (with and without bycatch reducing square-mesh panels) were assessed with three weights of catch (30 kg, 50 kg, and 70 kg). Compared with a codend with a circumference of 100 meshes throughout its entire length, a codend with an anterior section of 100 meshes and a posterior section of 200 meshes in circumference significantly increased the displacement of water forwards (up to 1120 mm from the end of the codend). This result varied with the weight of catch in the codend. The second experiment involved placing composite panels of square-shaped mesh (bycatch reducing devices) into the tops of the anterior sections of two codends with the same configurations as those above (termed the '100 panel codend' and '200 panel codend,' respectively). There was a displacement of water forwards immediately under the square-mesh panel in the 200 panel codend (by up to 2200 mm from the end of the codend). The results are discussed in terms of 1) the probable effects that codend mesh circumference and water displacement in codends have on fish behavior, and 2) implications for the future development of bycatch reducing devices like square-mesh panels in prawn-trawls.
Skin injury and mortality of Baltic cod (Gadus morhua L.) escaping from trawl codends equipped with two different types of 95 mm exit window were investigated between May and June 1994 in the southern Baltic Sea (ICES Subdivision 25). Fish escaping through the end of an open codend extension (i.e. the trawl was towed without the codend) were also studied. Escaping fish were collected in a small mesh cage that was released from the codend cover, closed and then slowly towed to the cage site. In total, nine valid hauls were completed, and 261 cod (24 to 50cm long) were held in cages for a period of 10 to 14 days. Only two escapees (34 and 36 cm) died during the experiment; both fish died during the first caging day. The degree of skin injury of fish was examined after cagings. Scale loss was the predominant visible skin injury; the main area of injury for most specimens was the posterior half of the body. Other skin injuries included lesions around the flank and snout, and dark 'net marks' behind the head. Scale loss was observed in 29 of 109 (27 %) cod that had escaped through exit windows codends. The average injured area was 2.5 % of the total skin area. For the open codend escapees, 48 of 139 (35 %) fish examined exhibited scale loss, and the average injured area was 2.3 % of the total skin area. No clear relationship existed between the degree of skin injury and fish size. Most of the observed skin injuries were probably caused by mechanical abrasion while fish were inside the trawl.
Juvenile flatfish habitat is usually modeled on the basis of sediment grain-size, depth and temperature. Recent evidence indicates that some juvenile flatfishes associate with emergent structures such as sponge, shell and other biogenic and bed-form features of otherwise low-relief shelf habitats. In laboratory experiments we examined the habitat preference and effects of habitat structure upon predation vulnerability of sub-yearling (Age-0) Pacific halibut Hippoglossus stenolepis and northern rock sole Lepidopsetta polyxystra. When given the choice between bare sand or sand with 16% sponge coverage, halibut demonstrated strong preference for sponge, while rock sole showed no preference. Larger Age-2 halibut (used as predators in the subsequent experiment) also preferred sponge, but this preference declined with increasing hunger. When allowed to forage for Age-0 flatfishes in either bare sand or sponge, predators consumed more prey in sand and consumed more Age-0 halibut than rock sole. We were able to determine which behavioral processes in the predator-prey interaction were modified by the presence of habitat structure. Predator-prey encounter rates decreased in the sponge habitat as predator search was impeded: predators paused more frequently and swam more slowly to maneuver through the sponges. Sponges also tended to hinder the pursuit of prey. Rock sole utilized stereotypic flatfish defense-mechanisms, relying upon immobility, burial and crypsis, and were less likely to flush at a predator's approach than halibut. Halibut have a less developed ability to mimic sediments, but a deeper/narrower body that confers greater swimming speed, and were more likely to flush as a predator approached. Once they had flushed and were pursued by a predator, halibut were more likely to escape than were rock sole. These experiments support an accumulating body of evidence that emergent structure, in otherwise low-relief benthic habitats, may play an important role in the ecology of some juvenile flatfishes. Removal of emergent structure by towed fishing gear and other anthropogenic and/or natural disturbance may influence patterns of distribution for juvenile halibut, as fish redistribute to less preferred habitat, and may decrease survival rates through increased losses to predation.
Small-scale biogenic or physical features, such as depressions, burrows, shells, boulders, cobbles and sand waves, dominate soft-sediment coastal and shelf environments. Such structures may provide refuge from predation and feeding areas for juvenile fish. We investigated the potential for juvenile snapper Pagrus auratus to utilise areas characterised by small-scale structures by sampling soft-sediment habitats that varied in both sediment characteristics and biogenic structure. Juvenile snapper abundance was estimated at each site using opera house traps, while variations in small-scale benthic habitat structure were quantified from video transects of the seafloor. The captured fish had 172 +/- 43 mm fork length, indicating that they were 1 to 2 yr old. Statistical modelling revealed that complex habitat structure had a positive influence on the abundance of juvenile snapper, suggesting that these areas are preferentially utilised. This finding highlights the potential for disturbance of such structures (by, for example, trawling or dredging) to affect the abundance of juvenile snapper.
Survival of 2- to 4-month-old vendace (5-10cm) after escaping from a 24mm square mesh trawl cod end was studied in Lake Puulavesi, Finland, during July-August 1993. On average, 50% of the escaped vendace died, although mortality varied considerably. Most mortality occurred during the first day after escaping. Hauls conducted in the late evening and at night were accompanied by the highest mortality (60-80%). Scale loss and exhaustion experienced by vendace during trawl capture may have caused the rapid death of escapees through loss of osmoregulatory control. -from Authors
Mortality of Baltic herring (Clupea harengus L.) escaping through the meshes of 26 mm and 36 mm diamond mesh (stretched mesh length) codends attached to a pelagic trawl was studied. After 30 min of towing, a cage that encircled the codend (and therefore captured codend escapees) was released from the trawl and anchored at depths of 7–17 m for periods of 1.5–9 days. Fish escaping through the end of an open codend extension were also collected and held in the same manner as fish that escaped through the codend meshes. No difference between mortality of herring escapees from 26 mm and 36 mm codends was observed, and herring that escaped through the open codend extension suffered the same level of mortality as fish that passed through codend meshes. Significant differences in mortality of escapees were observed between small (<12 cm) and large (12–17 cm) body size categories (P ≤ 0.05). For all three codend types combined, the 7-day post-capture mortality estimate for small herring was 72% (95% confidence intervals = 47–88%), whereas the estimate for large herring was 30% (95% confidence intervals = 11–60%). The predicted 14-day post-capture mortalities were 91% and 62% for small and large escapees, respectively. Mortality of 12–17 cm herring captured by seine-net and handline (i.e. controls) and held 9–16 days in cages (9–13%) was substantially lower than that of trawl escapees. Codend escapees sustained more skin damage than herring caught by seine and handline. Liver glycogen stores of small (7–11 cm) codend escapees were nearly depleted during the first 2 days after capture, and remained low during the 5-day observation period. In contrast, seine-caught herring showed significantly higher and increasing glycogen content after 3 days of recovery. Results of this study indicate that factor(s) other than passing through codend meshes caused most of the observed mortality for herring escapees. We suggest that skin injuries and exhaustion that occur while fish are inside the funnelling rear part and the codend of the trawl are the most likely causes of escapee mortality. On the basis of these results, the justification and usefulness of codend mesh size management in the herring trawl fishery is questionable.
The mortality of discarded fish bycatch is an important issue in fisheries management and, because it is generally unmeasured, represents a large source of uncertainty in estimates of fishing mortality worldwide. Development of accurate measures of discard mortality requires fundamental knowledge, based on principles of bycatch stressor action, of why discarded fish die. To date, discard mortality studies in the field have focused on capture stressors. Recent laboratory discard experiments have demonstrated the significant role of environmental factors, size- and species-related sensitivity to stressors, and interactions of stressors, which increase mortality. In addition, delayed mortality was an important consideration in experimental design. The discard mortality problem is best addressed through a combination of laboratory investigation of classes of bycatch stressors to develop knowledge of key principles of bycatch stressor action and field experiments under realistic fishing conditions to verify our understanding and make predictions of discard mortality. This article makes the case for a broader ecological perspective on discard mortality that includes a suite of environmental and biological factors that may interact with capture stressors to increase stress and mortality.La mortalité des prises accessoires rejetées à l'eau est une question d'importance dans la gestion des pêches; parce qu'elle est rarement mesurée, elle représente une source considérable d'incertitude dans les estimations de la mortalité due à la pêche à l'échelle mondiale. L'élaboration de mesures précises de la mortalité des prises accessoires nécessite des connaissances fondamentales sur les causes de cette mortalité basées sur les modes d'action des facteurs de stress. À ce jour, les études se sont concentrées sur les stress reliés à la capture. Des études récentes en laboratoire ont souligné le rôle significatif des facteurs de l'environnement, de la sensibilité au stress spécifique à la taille et à l'espèce et des interactions entre les facteurs de stress, qui accroissent tous la mortalité. De plus, la mortalité retardée est un élément important du plan d'expériences. La meilleure façon d'aborder le problème de la mortalité des prises accessoires est par une combinaison d'études de laboratoire des différentes classes de facteurs de stress pour obtenir des connaissances sur les principes fondamentaux de leur mode d'action et par des expériences sur le terrain dans des conditions de pêche réalistes pour vérifier ces connaissances et faire des prédictions sur la moralité. Il faut donc utiliser dans l'étude de la mortalité des prises accessoires une perspective écologique élargie qui considère une série de facteurs environnementaux et biologiques qui peuvent interagir avec les facteurs de stress lors de la capture pour accroître le stress et la mortalité.[Traduit par la Rédaction]
Recent studies suggest that mortality of undersized fishes escaping through trawl codends may range from 0 to 100%, with mortalities of 10 to 30% being common. These values may be low, as they do not account for fishes which become behaviorally compromised by their passage through the trawl and ultimately succumb to predators. The goal of this study was to simulate in the laboratory the stressors associated with trawl passage and determine if they degrade the behavioral capabilities of juvenile walleye pollock Theragra chalcogramma to avoid predation. In the first of 2 experiments, groups of Age 1 yr+ walleye pollock were subjected to 3 treatments: (1) controls: no stressor; (2) swim/escape: forced swimming for 90 min at 0.33 m s(-1) in a towed net, followed by escape through 8 cm square mesh; (3) swim/crowd/escape: forced swimming followed by 3 min of crowding, followed by escape. To evaluate the effect of these treatments on pollock behavior, a sable-fish Anoplopoma fimbria (48 to 53 cm) was placed in an observation arena with the group and pollock anti-predator behavior was quantified. Beginning immediately after simulated trawling and for up to 24 h afterwards, pollock exposed to both trawl-stressor treatments were less likely to avoid the predator than controls, allowing it to approach closer. They were also less able to form a cohesive shoal, and in the case of the swim/crowd/escape treatment, swam more slowly than control fish. To determine if trawl-stressed fish are more vulnerable to predation, in a second experiment I mixed control and swim/crowd/escape pollock together and then subjected them to predation by a 48 to 60 cm lingcod Ophiodon elongatus, observing the behavior and enumerating the number of pollock consumed in each treatment, Lingcod concentrated attacks upon solitary individuals or those straggling behind the shoal, were more likely to lunge at pollock that did not move away when approached, and were more successful the closer the pollock at lunge initiation. As a result, trawl-stressed pollock were consumed in greater numbers than controls. On the basis of these results, it is reasonable to expect that juvenile walleye pollock passing through trawls suffer behavioral deficits, subjecting them to elevated predation risk, If this is a generic effect, these results suggest that there may be a significant bycatch associated with many commercial trawl fisheries which is generally unrecognized, unmeasured, and unaccounted for in current stock-assessment models.
A broad-based public consensus has emerged that bycatch should be minimized to levels approaching insignificance. This view, as reflected in U.S. and worldwide legislation and agreements, demonstrates the widely held belief that discarded portions of fishery catches (including economic resources, protected species, and unobserved mortalities of animals not caught) represent an unacceptable waste of natural resources. Bycatches in their various forms can have significant consequences for populations, food webs, and ecosystems. The economic effects of bycatches can influence not only the levels of yields to individual fisheries, but also may have major effects on allocations among competing fisheries. The lack of comprehensive monitoring programs in most areas to assess bycatches and integrate them into population and multispecies models seriously impedes a full understanding of bycatch consequences and the efficacy of measures for their amelioration. Nevertheless, where evidence for significant bycatches exists, a risk-averse and perhaps adaptive management philosophy is clearly warranted. Establishing the benefits and costs associated with bycatch management is a priority as managers attempt to define the practicality of bycatches approaching zero given the institutional, scientific, and industry resources necessary to accomplish the job.
The relationship between sea state induced vessel motion and cod-end selection is investigated. The paper is divided into three parts. Part 1 reports on sea trials to investigate the relationship between vessel motion and cod-end dynamics. By comparing the average period of longitudinal cod-end pulsing to the most important cyclic component of the tension in the trawl warps and the most important cyclic component of the linear accelerations of the fishing vessel, it is established that the pulsing of the cod-end is a response to sea state induced vessel motion.
Field studies indicate great variability between fish species in their susceptibility to direct mortality resulting from stress incurred during entrainment and escape through trawl codends. Moreover, stressors that do not directly kill fish may still cause indirect mortality, such as behavioral impairment leading to predation. However, it is unknown whether resistance to direct mortality also imparts resistance to behavioral impairment. Juvenile sablefish Anoplopoma fimbria are more resistant to direct mortality resulting from physical damage and stress than are walleye pollock Theragra chalcogramma. We measured juvenile sablefish resistance to behavioral impairment resulting from simulated trawl passage and compared results to those for walleye pollock, a species already studied. Juvenile sablefish (18–27cm) were subjected to three levels of simulated trawl/escape stressors in the laboratory: (1) control: no stressors; (2) swim: forced swimming for 90min at 0.33ms−1 in a towed net, followed by escape through 8cm square mesh; and (3) swim/pin: forced swimming for 60min, then pinning against net meshes for 30min, followed by 3min crowding prior to escape. Subsequently, we examined sablefish behavior in the presence of a threatening but non-lethal predator (38–49cm sablefish). In a second experiment, equal numbers of trawl-stressed and control fish were mixed and exposed to predation by a lingcod Ophiodon elongatus (62–87cm). The first experiment demonstrated that sablefish suffered the same behavioral impairments as walleye pollock: stressed fish swam slower, shoaled less cohesively and allowed the predator to approach closer than did controls. In the second experiment all three levels of trawl stress caused fish to be consumed in greater numbers (by lingcod) than control fish, again, like walleye pollock. Therefore, although differing in susceptibility to potentially lethal stressors, both species exhibited similar impairments in response to sub-lethal stressors. This suggests that for numerous fish species, behaviorally impaired individuals escaping codends may be consumed by predators, contributing to unobserved mortality.
Fisheries management strategies often rely upon regulation of mesh sizes or the use of exclusion/escape structures to reduce the bycatch of undersize fish or non-target species. Design and evaluation of fishing gears to reduce bycatch should be based upon knowledge of fish behavior and sensory capabilities. The response of juvenile walleye pollock Theragra chalcogramma to an approaching panel made of 8 cm square mesh netting was compared under a range of laboratory illuminations, from <1 x 10-8 (darkness) to 1.7 x 10-3 μmoles photons m-2 s-1. Fish in darkness struck the net more frequently and swam closer to the net than fish at the highest illumination. These results, along with those of other laboratory and field studies, suggest that trawl bycatch reduction strategies which rely upon undersize or non-target fish using vision to guide them out of the net may be less effective at night or at depths where ambient light levels fall below critical levels.
The success of modifications to fishing gear in reducing or eliminating bycatch mortality requires in many cases that escaping fish be able to use vision for volitional guiding out of gear. Light adequate for vision is often not present as fishing operations are commonly conducted at depths or at night when light quantity is below visual capability. Previous observations of walleye pollock behavior in the laboratory indicated that darkness inhibited their ability to respond and orient to trawling gear. The goal of the present study was to test this result in situ by comparing swimming and orientation capabilities of walleye pollock under light and dark conditions in commercial type trawl gear using infrared illumination and video cameras to monitor these behaviors. Under light conditions, fish swam actively and were oriented parallel to the principle axis of the trawl. In contrast, under dark conditions there was little or no active swimming and the fish showed a significant degree of deviation from the principle axis of the trawl. These results showed that vision plays a major role in trawl gear interactions for walleye pollock and points out the importance of measuring light quantity in field studies of bycatch processes and mortality.
The mortality of discarded fish bycatch is an important issue in fisheries management and, because it is generally unmeasured, represents a large source of uncertainty in estimates of fishing mortality worldwide. Development of accurate measures of discard mortality requires fundamental knowledge, based on principles of bycatch stressor action, of why discarded fish die. To date, discard mortality studies in the field have focused on capture stressors. Recent laboratory discard experiments have demonstrated the significant role of environmental factors, size- and species-related sensitivity to stressors, and interactions of stressors, which increase mortality. In addition, delayed mortality was an important consideration in experimental design. The discard mortality problem is best addressed through a combination of laboratory investigation of classes of bycatch stressors to develop knowledge of key principles of bycatch stressor action and field experiments under realistic fishing conditions to verify our understanding and make predictions of discard mortality. This article makes the case for a broader ecological perspective on discard mortality that includes a suite of environmental and biological factors that may interact with capture stressors to increase stress and mortality.
Predators and food are the keys to understanding fish shoals; synchronised co-operation defeats predators, and optimal food gathering in shoals reflects a shifting balance between joining, competing in, or leaving the group. In the wild, predators may arrive while shoaling fish are feeding, and so vigilance is a crucial behaviour. Once detected, predator defence takes precedence over feeding, since an animal’s life is worth more than today’s dinner.
We measured reaction distance, escape velocity, and the apparent looming threshold (ALT) of heat-shocked goldfish (Carassius auratus) attacked by trout (Oncorhynchus mykiss). We tested fish at the acclimation temperature of 15 °C after heat-shocking prey for 2 min at temperatures ranging from 34 to 39 °C. Escape speeds were unaffected by heat shock. Reaction distance decreased from about 21 cm for fish shocked at 35 °C to about 6 cm for those shocked at 39 °C. ALT increased from 0.2 rad∙s−1 for controls to 0.4 rad∙s−1 for goldfish heat-shocked at 39 °C. The elusiveness of prey, E, was measured as the number of attacks required per prey capture. E was related to ALT as: E = 1.29 (±0.47)∙ALT−0.82(±0.25) (mean (±2 SE)). Factors that decrease responsiveness of prey have large effects on the ability of prey to avoid predators.
The effects of pentachlorophenol on the predator-avoidance behaviour of the guppy (Poecilia reticulate) in response to largemouth bass (Micropterus salmoides) predation was investigated. There were no consistent effects of pentachlorophenol on habitat use or general behaviour of the guppies. In the presence of predators, all guppies occurred significantly more often alone, motionless, and in the top third of the water column in the nonopen areas. Nine variables associated with predator efficiency were monitored to determine which treatment groups of guppies were easiest to capture. The bass had significantly lower capture success, performed more strikes and chases, and spent more time chasing guppies from the untreated and 100 μg/L groups than those from the 500 and 700 μg/L groups. This suggests that the guppies from the two high treatment groups had a slower response to predator attack and could not maintain a prolonged escape burst of speed.
We experimentally infected juvenile chinook salmon (Oncorhynchus tshawytscha) with Renibacterium salmoninarum (Rs), the causative agent of bacterial kidney disease (BKD), to examine the vulnerability to predation of fish with differing levels of Rs infection and assess physiological change during progression of the disease. Immersion challenges conducted during 1992 and 1994 produced fish with either a low to moderate (1992) or high (1994) infection level of Rs during the 14-week postchallenge rearing period. When equal numbers of treatment and unchallenged control fish were subjected to predation by either northern squawfish (Ptychocheilus oregonensis) or smallmouth bass (Micropterus dolomieui), Rs-challenged fish were eaten in significantly greater numbers than controls by nearly two to one. In 1994, we also sampled fish every 2 weeks after the challenge to determine some stressful effects of Rs infection. During disease progression in fish, plasma cortisol and lactate increased significantly whereas glucose decreased significantly. Our results indicate the role that BKD may play in predator-prey interactions, thus ascribing some ecological significance to this disease beyond that of direct pathogen-related mortality. In addition, the physiological changes observed in our fish during the chronic progression of BKD indicate that this disease is stressful, particularly during the later stages.
Thermally shocked juvenile rainbow trout and chinook salmon were selectively preyed upon by larger trout in the laboratory when exposure times to elevated temperatures exceeded a minimum duration. This duration was 10% (chinook) and 20% (rainbow) of the exposure duration that caused obvious loss of equilibrium (complete body inversion) of half a test population at that temperature. Longer exposures increased vulnerability to predation relative to controls almost exponentially. Shorter exposures made shocked fish less susceptible to predation. Shocked fish showed some recovery from heat effects when returned to the initial holding temperature for or 1 hr prior to predation. Susceptibility of rainbows to predation (relative to controls) appears to follow the time- and temperature-dependant response pattern previously shown by others for death and visible equilibrium loss. Selective predation began at exposures about 10% of the median death time in the range 26 to 30 C.
Summary The authors estimate that between 17.9 and 39.5 million tons (average 27.0 million) of fish are discarded each year in commercial fisheries. These estimates are based on a review of over 800 papers. The highest quantities of discards are from the Northwest ...
The experiments investigated the effects of a school of live prey fish on the hunting behaviour of squid, cuttlefish and pike (ambush predators) and perch (a chasing predator). The hunting behaviour of all species is described, including some previously unrecorded actions.For all species the increasing shoal size from one fish to six, to 20, decreased the success of the predators' attacks per encounter with a prey. This was partly because attacks on larger shoals lasted longer, and fish became increasingly difficult to catch as a hunt went on. However, for some of the species there was a clear effect of the shoals at the start of the hunt. For other species it was less conclusive.The disruptive effect of the shoals was shown to act for the ambush predators by interfering with the “optimal” sequence of an attack and causing avoidance actions together with so-called irrelevant behaviour. For the perch, shoals seemed to disrupt by forcing the predator continually to switch targets during his pursuit.The artificiality of the experiments is discussed in relation to field observations. The relatively high incidence of avoidance and irrelevant behaviour seems to be an artefact, but the main results are quite compatible with field studies. In nature fish predators may be able to get sufficient food by restricting their attacks to individuals which are conspicuous either by an abnormal appearance or by becoming separated from the school. An individual prey fish no doubt gains a considerable advantage from the school by burying itself in the crowd. Our experiments show that a shoal can provide further protection in hampering the attack of a predator.
The relationship between predator avoidance deficits induced by a simple handling stress and the level of plasma corticosteroids was determined for juvenile coho salmon Oncorhynchus kisutch. Groups of fish were held out of water for 1 min, then allowed to recover from this stress for 1, 90, or 240 min. After each recovery period, some unstressed and stressed fish were sampled for plasma cortisol, and others were subjected to predation by lingcod Ophiodon elongatus. Levels of corticosteroids in stressed fish remained high throughout the 240-min period of recovery, but predator avoidance returned to control levels in less than 90 min. Results suggest that juvenile coho salmon quickly recover basic survival skills of predator avoidance after mild stress, even though cortisol levels continue to indicate a stressed condition.
Two consequences of a recently published method of calculating fish school volumes are examined in the light of ecological population data. First, minnow and roach schools occupy only about 0.01% of the volume of water in which they live, graphically illustrating the vagaries of field sampling, and providing reference values for foraging studies. Secondly, I show that roach schools in one river are constantly within attack range of pike predators. If this situation is general, theories which assume that schools function strategically merely to reduce the probability of encountering a predator, break down, whereas my predictions suggest that there is strong selection for direct predator evasion tactics in schools.
Our understanding of predator-prey interactions in fishes has been influenced largely by research assuming that the condition of the participants is normal. However, fish populations today often reside in anthropogenically altered environments and are subjected to many kinds of stressors, which may reduce their ecological performance by adversely affecting their morphology, physiology, or behaviour. One consequence is that either the predator or prey, or both, may be in a substandard condition at the time of an interaction. We reviewed the literature on predator-prey interactions in fishes where substandard prey were used as experimental groups. Although most of this research indicates that such prey are significantly more vulnerable to predation, prey condition has rarely been considered in ecological theory regarding predator-prey interactions. The causal mechanisms for increased vulnerability of substandard prey to predation include a failure to detect predators, lapses in decision-making, poor fast-start performance, inability to shoal effectively, and increased prey conspicuousness. Despite some problems associated with empirical predator-prey studies using substandard prey, their results can have theoretical and applied uses, such as in ecological modelling or justification of corrective measures to be implemented in the wild. There is a need for more corroborative field experimentation, a better understanding of the causal mechanisms behind differential predation, and increased incorporation of prey condition into the research of predator-prey modellers and theoreticians. If the concept of prey condition is considered in predator-prey interactions, our understanding of how such interactions influence the structure and dynamics of fish communities is likely to change, which should prove beneficial to aquatic ecosystems.
This study examined whether rapid recovery from stress-induced impairment in predator evasion, observed in previous studies on coho salmon, Oncorhynchus kisutch (Walbaum), was a general characteristic of different stocks of the same species and different species of Pacific salmon. We monitored stress-induced non-predator mortality, predator evasion and Cortisol concentrations of smolts of coho and spring Chinook, O. tshawytscha (Walbaum), after administration of standardized single and multiple handling stresses. Marked differences in the response to handling stress among stocks of coho and spring chinook smolts were evident, with recovery from impaired predator evasion occurring within 24 h of a 30-s handling stress for coho smolts and 24 h of a 1-min handling stress for spring chinook smolts. Differences in stress-induced non-predator mortality among stocks were also observed. The results point to the importance of screening hatchery salmonid stocks to assess differing capabilities of dealing with stress.
A behavioural bioassay was used to determine the response threshold to squid extract of sablefish, Anoplopoma fimbria, held at three different feeding regimens. Sablefish responded to the odour of bait by changing swimming activity and turning behaviour. The response threshold to bait odour was influenced by both the amount of food eaten and the duration of food deprivation. The total concentration of amino acids in the bait extract was assumed to determine the response threshold as chemical fractionation studies have shown that this class of compounds is essential for the stimulatory capacities of food extracts. When fed to satiation (9.4% wet body weight) and tested after one day of food deprivation, the mean response threshold to total dissolved free amino acids was 4.4 × 10−8m (range=7.6 × 10−8 to 3.6 × 10−8m). When fed at 1.6-2.3% wet body weight, the threshold sensitivity had increased to a mean value of 1.8 × 10−10m (range=8.4 × 10−10 to 7.0 × 10−11m) after one day of food deprivation; after four days of deprivation, the sensitivity had increased even further to a mean value of 1.4 × 10−11m (range=1.6 × 10−10 to 1.4 × 10−12m). It was also apparent that the intensity of behavioural responses to the bait odour increased with both stimulus concentration and duration of food deprivation. These results suggest that sablefish intensify their search for prey under increased feeding motivation. The active space of a bait source was estimated from the threshold values obtained. Depending on state of food deprivation, rate of chemical release from the bait and the current velocity, maximum lengths of active space within which sablefish would exhibit food searching responses vary from 10 m to several km. Stock assessment based on catch data from baited gear will need techniques that take into account those factors influencing active space for food searching.
Laboratory apparatus which simulated capture of fish in the cod-end of a towed trawl was used to induce post-capture stress as measured by alterations in behavioural, physiological and mortality indices in juvenile walleye pollock Theragra chalcogramma and juvenile and adult sablefish Anoplopoma fimbria. Differences in resistance to net entrainment varied between species with the severity of stress and the potential for recovery depending on light intensity, net velocity and towing duration. At a light intensity which simulated daylight at depth in clear ocean water (0.5 μmol photons m−2 s−1), walleye pollock juveniles were able to maintain swimming in nets towed at 0.65 m s−1 for 3h with no discernible effects on behaviour or mortality. However, when net velocity was increased to >0.75m s−1 or light intensity was decreased to <0.002 μmol photons m−2 s−1, fish became entrained in the meshes of the net and exhibited significant alterations in feeding behaviour, predator evasion and increases in plasma cortisol concentrations. Marked increases in stress-induced mortality also occurred, in some cases after a delay of 6 days and eventually reaching 100%. In comparison with walleye pollock, sablefish juveniles became entrained in the meshes of the net at higher velocities (>0.92m s−1) or lower light intensities (<0.0004 μmol photons m−2 s−1) and were much more resistant to post-capture stress. Towing of net-entrained fish for 15 min caused no detectable changes in feeding and cortisol and for 2 h, no changes in feeding although mortality increased from 0% for 15-min tows to 19% for 2-h tows. Towing for 4 h caused significant alterations in feeding and cortisol with feeding recovering to control levels by 6 days and cortisol by 3 days; mortality was 25%. When adult sablefish were towed for 4 h followed by 15-min exposure to air, feeding was inhibited 6 days after towing, but recovered within 30 days with no mortality observed after 30 days. The results demonstrate the value of using laboratory-based behavioural and biochemical indices to identify factors that may potentially affect post-capture survival among different species of fish.
Sea temperature around the coast of Scotland rises from 7 °C in late winter to 12 °C in late summer. Recently, it was shown that the selectivity of trawl codend was poorer in late winter than it was in late summer. A change in water temperature is expected to affect the escape speed of fish, and therefore the selectivity of the gear. Four experiments carried out in this study showed the effect of this seasonal temperature change on the ability of haddock, Melanogrammus aeglefinus, to escape from a codend. A temperature increase from 7 to 12 °C changed the minimum twitch contraction time of the lateral muscle from 38.8 to 27.6 ms. This gives a maximum tail beat frequency of 12.78 Hz at 7 °C rising to 19.12 Hz at 12 °C. Escape reflexes of the fish were significantly slower at 7 °C than at 12 °C (P<0.001). The shortest time to complete first body bend (stage 1) was 40 ms at 7 °C and 20 ms at 12 °C. The mean time taken to complete stage 1 at 7 °C (63.9 ms, S.E. 1.2) was significantly (t-test, P<0.001) longer than at 12 °C (40.1 ms, S.E. 0.92). The mean time taken to complete the propulsive stroke (stage 2) of the fast start was significantly (t-test, P<0.001) longer at 7 °C (108.5 ms, S.E. 5.3) than at 12 °C (72.6 ms, S.E. 2.6). The maximum speed recorded while competitively swimming for food reward was 7.9 L s−1 at 7 °C and 12.5 L s−1 at 12 °C. The observed maximum tail beat frequency used by haddock when leaving the mesh of a codend escape panel changed from 12 to 25 Hz. The general underlying physiological effect of a temperature increase of 10 °C was to double the speed of the maximum swimming ability . The effect of a change of only 5 °C on the ability to manoeuvre out of codend selection devices is discussed.
and Summary
The aim of this work was to investigate the relationship between shoal size and vigilance. The behaviour of minnows ( Phoxinus phoxinus ) foraging on an artificial food patch during the simulated stalking approach of a model predator (pike: Esox lucius ) was recorded for shoals of 20, 12, 6 and 3 fish. Minnows in large shoals reduced their foraging sooner but remained feeding on the patch longer than in small shoals. The relatively late reaction of small shoals to the model and the rapid cessation of feeding once the predator was detected, indicates that small shoals were less vigilant than large shoals. The gradual reduction of foraging in large shoals was accompanied by an increasing number of investigative approaches in which individuals monitored the model's approach. This enabled minnows in larger shoals to balance more efficiently the conflicting demands of feeding and watching for predators.
The incidence of diurnal predation (0800–1600h) on schools of several hundred thousand adult flat-iron herring,Harengula thrissina, was quantified. Nine species of piscine predators perpetrated 653 attacks and 158 captures during 42.5 h of observation. Four predators: cabrilla,Mycteroperca rosacea, cornetfish,Fistularia commersonii, green jack,Caranx caballus, and black skipjack,Euthynnus lineatus, were responsible for 92% of the attacks and 96% of the successes. Among these predators attack rates (attacks time–1) and success rates (successes attacks–1) varied with respect to time of day, although all predators were active throughout the day. As a consequence, the herring school experienced fairly homogeneous attack and success rates. These rates were estimated as roughly half the crepuscular attack rates but they were sustained over a much longer period. Piscivorous predators, particularly those species frequently found in association with schooling prey, may be focusing their attacks on opportunities, not specific times of day and/or light levels. More direct information needs to be gathered on diurnal as well as crepuscular attack and success rates in order to assess the impact that piscine predators have on schooling prey.
The main objective of this study was to assess how and to what extent Baltic herring avoid and escape midwater trawls under commercial fishing conditions. Information was obtained by acoustic, visual and direct sampling techniques. The majority of herring schools in the path of an approaching trawl were observed to enter the trawl. When they did avoid the trawl it was by swimming downwards. The frequency of strong avoidance reactions was significantly higher during daytime than at night. Herring entering the trawl passed quickly into the funnelling rear part of the trawl, where they swam vigorously, trying to maintain their position in relation to the trawl. Fish often struck against the netting and their scales were seen passing through the meshes. Some herring escaped through the meshes of the upper rear panels of the trawl body, especially in the dark. Escaping herring were significantly smaller than those caught in the codends. Nevertheless, most herring ended up in the codend. We conclude that, although herring occasionally avoid and escape midwater trawls, the majority of fish in the trawl path are caught in the codend.