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Relative efficiency and size selectivity of bottom-set gill-nets for dusky flathead, Platycephalus fuscus and other species in New South Wales, Australia
Abstract
The effects on catches and bycatches due to increases in mesh size were investigated in a bottom-set gillnet fishery for dusky flathead, Platycephalus fuscus in New South Wales, Australia. Sampling was done in 2 coastal lagoons during spring 2001 using a gillnet comprised of replicate panels of (i) the commercially-used mesh size (7.0 cm) and (ii) 3 larger mesh sizes (8.0, 8.9 and 9.5 cm). The targeted dusky flathead and 5 important bycatch species (bream, Acanthopagrus australis, luderick, Girella tri-cuspidata, sea mullet, Mugil cephalus, yellow-finned leatherjacket, Meuschenia trachylepis and blue swimmer crab, Portunus pelagicus) accounted for approx. 91 % of the total catch. Significant differences in the catch rates of these species were detected among the mesh sizes examined with trends for fewer numbers of total dusky flathead, total bycatch, sea mullet, yellow-finned leatherjacket and under-size bream and luderick evident in the larger mesh sizes. All species were characterised by a significant disequilibrium in the vertical distribution of catches, with the lower 50 % of the gillnet retaining between 56 and 89 % of individuals. Modal lengths of most species increased with mesh size and the selectivities of dusky flathead, bream and luderick were best described by binormal curves with well-separated modes. The results are discussed in terms of the probable capture processes of the main species and appropriate modifications to reduce their unwanted bycatches.
... It may be possible to reduce some bycatches and subsequent discarding in this fishery, especially for some of the important finfish species. Data presented in Gray (2002) and Broadhurst et al. (2003) indicate that increasing the minimum permitted mesh size to 95 mm would virtually eliminate bycatches of sea mullet and sand whiting and significantly decrease bycatches of undersize bream and luderick. Given that legal-size individuals of these species are not currently allowed to be retained in this fishery, this would reduce potential wastage. ...
... Apart from developing and testing alternate gillnets with more appropriate configurations (e.g. height and material of nets -see Hamley, 1975;Millar and Fryer, 1999;Broadhurst et al., 2003;Godoy et al., 2003) a further option is to investigate the effects of reducing the permitted maximum setting time (overnight) of nets during winter on catches. Discard levels may be lower and subsequent mortalities of fish may also be reduced because of the reduced soak times (Acosta, 1994;Chopin and Arimoto, 1995), potentially further reducing any potential negative ecological impacts of this fishery. ...
... Apart from developing and testing alternate gillnets with more appropriate configurations for use in these fisheries (e.g. height and material of nets -see Hamley, 1975;Millar and Fryer, 1999;Broadhurst et al., 2003;Godoy et al., 2003) a further option is to investigate the effects of reducing the permitted maximum setting time (overnight) of nets on catches. Discard levels may be lower and subsequent mortalities of fish may also be reduced because of the reduced soak times (Acosta, 1994;Chopin and Arimoto, 1995), potentially further reducing any potential negative ecological impacts of this fishery. ...
... Overall, relatively few (7% by number) dusky flathead below the current MLL were observed in catches and the levels of discarding of these fish (approximately 1 t across all three estuaries) were low, suggesting that the currently configured flathead nets may be relatively selective at catching legal-sized flathead. Independent experimental studies on the selectivity of flathead gillnets in NSW (Broadhurst, Gray, Young & Johnson 2003) confirmed this conclusion. The selectivity of nets, however, depended on several interacting factors including the abundance, availability and catchability of different-sized fish, as well as the efficiency of the fishing gear and thus levels of discarding of undersized dusky flathead could vary between years. ...
... About 41% of dusky flathead captured were smaller than 40 cm TL and this potential level of discarding would be considered highly wasteful if mortalities were high. An alternative management strategy, including an increase in the minimum mesh size to at least 80 mm (see Broadhurst et al. 2003), will need to be implemented into the fishery if the MLL for dusky flathead is further increased. The discarding of some other species from flathead gillnets could also potentially be of concern, particularly because many were juveniles of important species, including bream, luderick and sea mullet. ...
... It may be possible to reduce some bycatches and subsequent discarding in this fishery, especially for some of the important finfish species. Data presented in Gray (2002) and Broadhurst et al. (2003) indicate that increasing the minimum permitted mesh size to 95 mm would virtually eliminate bycatches of sea mullet and sand whiting and significantly decrease bycatches of undersize bream and luderick. Given that legal-size individuals of these species are not currently allowed to be retained in this fishery, this would reduce Table 4. ...
A scientific observer programme was used to quantify the composition and magnitude of discards in the gillnet fishery for dusky flathead, Platycephalus fuscus (Cuvier), in three barrier estuaries in New South Wales, Australia, during the 2001 fishing season. Regulations only permit the retention of legal-sized dusky flathead and legal-sized blue swimmer crab, Portunus pelagicus L., and mud crab, Scylla serrata (Forskål); all other organisms were discarded. Sampling was stratified into two time periods; before and after 1 July 2001 which coincided with the increase in the minimum legal length (MLL) of dusky flathead from 33 to 36 cm total length (TL). Eighty one catches were sampled, yielding 38 finfish species and two portunid crab species. Legal-sized dusky flathead were the most abundant organism captured, accounting for 23–47% by number and 34–54% by weight of the mean observed catch depending on the estuary and survey period, with a mean catch of 25–59 flathead weighing 13–25 kg per fishing-night. Species composition and relative abundance of catches differed among estuaries, but not between sampling periods. Predominant bycatch species included legal and undersize blue swimmer crab, sea mullet, Mugil cephalus L., luderick, Girella tricuspidata (Quoy & Gaimard), bream, Acanthopagrus australis (Günther) and yellowfin leatherjacket, Meuschenia trachylepis (Günther). These five species accounted for 82% of total bycatch by number and 71% by weight, pooled across the three estuaries. More crabs were retained than discarded, with retained legal-size crabs (byproduct) accounting for 16% of total bycatch by number and 13% by weight, with an average of 5–22 crabs weighing 1–6 kg being caught per fishing-night, depending on the estuary. Overall, 7% of dusky flathead captured (number) were below the MLL of 36 cm and discarded, suggesting the nets as currently configured may be relatively selective in catching legal-size flathead. However, 41% of dusky flathead were <40 cm TL, indicating that if the MLL for this species is increased to this length as proposed, new nets must be introduced into the fishery. The findings are discussed in terms of making the flathead fishery more sustainable, including alternative management strategies for the fishery.
... In recent years, concerns over the discarding of unwanted catches (termed 'bycatch') by NSW gillnetters, and the potential for associated negative impacts on the stocks of important species, resulted in fisherydependant surveys to firstly quantify bycatches (Gray 2002, Gray et al. 2004, 2005a, and then examine technical solutions to improve the selectivity of problematic gillnet configurations (Broadhurst et al. 2003, Gray et al. 2004. Because gillnets typically select fish across a narrow range of sizes (see Hamley 1975 for a review), much of the latter work simply involved increasing the existing minimum size of mesh (80 mm in most configurations) to correspond to the maximum girth of legalsized individuals of the key targets (Broadhurst et al. 2003). ...
... In recent years, concerns over the discarding of unwanted catches (termed 'bycatch') by NSW gillnetters, and the potential for associated negative impacts on the stocks of important species, resulted in fisherydependant surveys to firstly quantify bycatches (Gray 2002, Gray et al. 2004, 2005a, and then examine technical solutions to improve the selectivity of problematic gillnet configurations (Broadhurst et al. 2003, Gray et al. 2004. Because gillnets typically select fish across a narrow range of sizes (see Hamley 1975 for a review), much of the latter work simply involved increasing the existing minimum size of mesh (80 mm in most configurations) to correspond to the maximum girth of legalsized individuals of the key targets (Broadhurst et al. 2003). Based on a clear negative relationship between the catches of undersize Acanthopagrus australis and Girella tricuspidata (both < 25 cm total length [TL]) and the size of mesh, the minimum was increased to 95 mm for deep-walled configurations deployed for longer than 3 h (Gray et al. 2005a). ...
... There are few apparent gear-based solutions and/or spatial and temporal restrictions to the deployment of 80 mm gillnets in NSW which will allow catches of the various target species and their sizes to be maintained at current levels, while reducing bycatches (Broadhurst et al. 2003, Gray et al. 2005b. One remaining approach that may have some application is to examine changes to operational and/or post-capture handing procedures that minimise the mortality of bycatch, and therefore the negative impacts on their stocks due to discarding (Buchanan et al. 2002, Bettoli & Scholten 2006. ...
The mortalities and contributing parameters were estimated for key species discarded during commercial gillnetting (80 mm mesh) targeting dusky flathead Platycephalus fuscus in a southeastern Australian estuary. Bycatches (1470 individuals from 16 species over 11 deployments) were assessed for their immediate mortalities onboard the gillnetter, before subsamples (570 individuals from 11 species) were discarded into cages and monitored for their short-term fate over 4 d. Appropriate controls were concurrently caged and monitored. Blood samples were taken from some live meshed-and-discarded yellowfin bream Acanthopagrus australis and luderick Girella tricuspidata and analysed for plasma cortisol and glucose. Concomitantly angled fish were similarly sampled (to provide baseline estimates of blood physiology). The immediate mortalities of the abundant species ranged between 0 (undersize blue swimmer crab Portunus pelagicus <6 cm carapace length) and 70% (undersize P. fuscus <36 cm total length [TL]). Water temperature had a statistically significant positive relationship with the immediate mortality of G. tricuspidata and large-tooth flounder Pseudorhombus arsius, and TL had a significant negative relationship with the immediate mortality of black sole Synaptura nigra. Compared to baseline estimates, mean plasma cortsiol concentrations in meshed-and-discarded G. tricuspidata and A. australis were significantly greater, and approached levels comparable to most teleosts after peak stress. Mean glucose concentrations were not concomitantly elevated, possibly reflecting limited time between stress and sampling for some individuals. Short-term mortalities occurred throughout the entire 4 d monitoring period for most species and ranged from 0 (yellowfin leatherjacket Meuschenia trachylepis) to 29% (A. australis). Water temperature and TL were identified as having significant impacts similar to those described above on the delayed fate of A. australis and G. tricuspidata. The partitioned mortalities were combined to provide estimates of overall mortality (+/- SE) for the main species that ranged between 5.9 +/- 3.3% (P. pelagicus) and 76.9 +/- 7.8% (undersize P. fuscus). Discard mortality in this fishery could be mitigated by allowing fishers to retain a small percentage of undersize P. fuscus, restricting the deployment of nets in water temperatures >16 to 17 degrees C, and encouraging the careful removal of catches from meshes.
... Unlike mullet, flathead are viewed as a primary target for recreational fishers and harvest rates in this sector are roughly equal to that reported from the commercial net fishery (Department of Agriculture and Fisheries, 2019e; 2021; Leigh et al., 2019;McGilvray et al., 2018a;Teixeira et al., 2021;Webley et al., 2015). Flathead stocks are highly accessible (Broadhurst et al., 2003;Gray & Barnes, 2015) and these species are at higher risk of experiencing a regional overfishing event due to cumulative fishing pressures. This is more likely to occur in areas with higher populations including in south-east ...
... Given their accessibility across environments, dusky flathead remain a top targeted teleost (Broadhurst et al., 2003;Gray & Barnes, 2015). This popularity is reflected in the large catches and sustained targeting across survey periods. ...
The Queensland Ecological Risk Assessment Guideline (the Guideline) was released in March 2018 as part of the Queensland Sustainable Fisheries Strategy 2017–2027. This Guideline provides an overview of the strategy being employed to develop Ecological Risk Assessments (ERAs) for Queensland’s fisheries. The Guideline describes a four-stage framework consisting of a Scoping Study; a Level 1, whole of fishery qualitative assessment; a Level 2, species-specific semi-quantitative or low-data quantitative assessment and; a Level 3 quantitative assessment (if applicable).
A Scoping Study and Level 1 ERA for the East Coast Inshore Fishery was released in May 2019 (Jacobsen et al., 2019). The Level 1 assessment identified ecological components at higher risk from net fishing activities, and these were progressed to a Level 2 assessment. Level 2 ERAs are focused at the species level with risk evaluations based on a Productivity & Susceptibility Analysis (PSA). The PSA evaluates risk for each species through an assessment of seven biological attributes and up to seven fisheries-specific attributes.
Based on the outputs of the Level 1 ERA and following a species prioritisation process, the Tunnel Net Fishery Level 2 ERA assessed risk for 16 target & byproduct species, and seven Species of Conservation Concern (SOCC). All target & byproduct species were found to be at low to medium risk, while the SOCC were found to be at medium to high risk from tunnel net fishing activities. The risk profiles for SOCC were heavily influenced by the biological attributes (productivity); particularly those relating to their longevity and reproductive outputs.
The Level 2 ERA made a list of recommendations to assist in the management and mitigation of risk in the Tunnel Net Fishery. A number of these measures are already being discussed and considered as part of the Queensland Sustainable Fisheries Strategy 2017–2027 and will be progressed through the East Coast Inshore Fishery Working Group.
... In Australia, however, several studies have focussed on commercial gill net fisheries in tropical (Halliday et al. 1997) and sub-tropical estuaries (Gray 2002;Gray et al. 2003;Gray et al. 2005) and temperate marine environments (Fowler et al. 2009). These studies have investigated species and size composition of catches, and discard rates in commercial fisheries (Gray 2002;Gray et al. 2003;Broadhurst et al. 2004;Gray et al. 2004;Fowler et al. 2009) but little is known about recreational gill net fisheries (Lenanton et al. 1996;Henry and Lyle 2003;Jones 2009). Several studies have investigated survival of fish that have been caught and released (Broadhurst and Barker 2000;Broadhurst et al. 2008) while others have examined the potential for reducing discards or mitigating the effects of discarding (Gray et al. 2000;Broadhurst et al. 2004;Gray et al. 2005;Broadhurst et al. 2009). ...
... These studies have investigated species and size composition of catches, and discard rates in commercial fisheries (Gray 2002;Gray et al. 2003;Broadhurst et al. 2004;Gray et al. 2004;Fowler et al. 2009) but little is known about recreational gill net fisheries (Lenanton et al. 1996;Henry and Lyle 2003;Jones 2009). Several studies have investigated survival of fish that have been caught and released (Broadhurst and Barker 2000;Broadhurst et al. 2008) while others have examined the potential for reducing discards or mitigating the effects of discarding (Gray et al. 2000;Broadhurst et al. 2004;Gray et al. 2005;Broadhurst et al. 2009). ...
... The larger mesh should alleviate some of the fishing mortality of conspecifics and some small individuals comprising by-catch, but because nearly all of the key unwanted species are represented by wide ranges of sizes across the same areas as P. fuscus, their absolute numbers in by-catches will remain similar. 10,36 Alterations to other technical aspects of the gears are a simple option that might facilitate a reduction in their fishing mortality on stocks. Therefore, in this study, the effects of altering the hanging ratio, fishing height and twine material on catches and by-catches in this fishery were tested. ...
... Equally importantly, this configuration had no effect on the catches of the targeted P. fuscus or the legally retained byproduct, P. pelagicus, both of which are benthic-dwelling. These observations are similar to those made during an earlier study in the same fishery, 36 and can be attributed to species-specific differences in vertical distributions, and the lower probability of fish encountering the net. 26,27 The fishing depth of pelagic gillnets (drift-net) impact on catches of tunas, with nets set lower in the water column away from surface, catching less fish. ...
Three experiments were done to test for the influences of different (i) hanging ratios (E = 0.5, 0.65 and 0.8); (ii) fishing heights (25 and 12 meshes); and (iii) twine diameters (0.41, 0.56 and 0.62 mm) and materials (multifilament nylon and multimonofilament nylon polyamide) on catches and by-catches in the estuarine gillnet fishery for Platycephalus fuscus in New South Wales, Australia. In each experiment, the various 100-m treatment panels comprising 80-mm (nominal) mesh rigged according to the different configurations being examined were configured in a single gang between 1090 and 1310 m in length, and fished according to commercial practices. The results showed no significant differences between different hanging ratios or twine diameters for the numbers, weights and size compositions of catches and by-catches. Twine material had an effect on only one key by-catch species (Acanthopagrus australis), with fewer caught in panels made from multifilament nylon compared with multimonofilament nylon. In contrast to the above modifications, lowering the fishing height of the floatline significantly reduced total by-catch by up to 46% and the individuals of key by-catch species (Mugil cephalus, A. australis and Girella tricuspidata) by between 60 and 85% with no effect on catches of targeted P. fuscus, or legally retained byproduct, Portunus pelagicus. The results are used to provide directions for the future management of this fishery and have relevance to other similar bottom-set gillnet fisheries.
... Although this result may have been partly due to a greater prevalence of larger fish in the Clarence River, more fishers in this river used gillnets with larger mesh (95 and 100 mm) compared with the standard 80 and 83 mm mesh most commonly used in the other estuaries. Previous studies have shown that the mean L F of G. tricuspidata is generally larger when caught in gillnets with 100 mm mesh compared with 80 mm mesh (Gray, 2002;Broadhurst et al., 2003;Gray et al., 2005). Generally, the L F compositions of catches reported here were consistent with direct observations of catches of G. tricuspidata in these fisheries (Gray, 2002;Gray & Kennelly, 2003;Gray et al., 2005). ...
... This was confirmed by the fact that the majority of the estimates of fishing mortality were greater than the estimated natural mortality. The general absence of larger and older fish in catches is not a gear selectivity issue; the selectivity of the nets used in the fishery does not exclude the capture of larger (>350 mm L F ) and presumably older G. tricuspidata (Broadhurst et al., 2003). ...
Commercial gillnet and beach-seine catches of Girella tricuspidata from seven estuaries in eastern Australia were examined for differences in fork length (LF), sex and age composition, and populations were assessed for growth and mortality. Fish 220–350 mm LF dominated landings across all estuaries sampled, regardless of gear type. Few fish >10 years of age were observed in the catches, with fish aged 3–5 years, and 4–7 years, being most abundant in the catches in the four most northern estuaries and three southern estuaries, respectively. There was considerable variation in the LF of G. tricuspidata at any given age and the oldest male and female were 21 and 24 years, respectively. There were no consistent differences between sexes or latitudinal regions in the growth and mean LF at age of fish in each individual age class between 3 and 8 years. Growth of females was greater than males in the northern region, but not elsewhere. Estimates of the instantaneous rate of total mortality (Z) were dependent on estuary and year, ranging from 0·30 to 1·01, whereas the corresponding estimates of fishing mortality (F) ranged from 0·12 to 0·90. Populations of G. tricuspidata appear to have been heavily exploited, primarily relying on young fish recruiting to the fishery. The ecosystem-wide effects of harvesting this dominant mobile teleost herbivore need to be assessed further.
... The potential for wide-scale mortalities to valuable species discarded from gillnets has raised concerns over the sustainability of populations (Gray, 2002). Attempts have been made to address this issue by improving the selectivity of the various configurations for the targeted teleosts and their sizes; mostly through changes to rigging, materials, and mesh sizes (Broadhurst et al., 2003; Gray et al., 2005 ). Although these changes have reduced significantly the discarding of some juvenile teleosts, the morphology of blue swimmer crabs precludes their defined selection in gillnets so, irrespective of the configurations, large numbers are still caught and discarded (Broadhurst et al., 2003). ...
... Attempts have been made to address this issue by improving the selectivity of the various configurations for the targeted teleosts and their sizes; mostly through changes to rigging, materials, and mesh sizes (Broadhurst et al., 2003; Gray et al., 2005 ). Although these changes have reduced significantly the discarding of some juvenile teleosts, the morphology of blue swimmer crabs precludes their defined selection in gillnets so, irrespective of the configurations, large numbers are still caught and discarded (Broadhurst et al., 2003). A more appropriate strategy for reducing the potential for negative impacts associated with these discards may be to promote operational and/or handling methods that maximize post-capture survival (Broadhurst et al., 2006). ...
Uhlmann, S. S., Broadhurst, M. K., Paterson, B. D., Mayer, D. G., Butcher, P., and Brand, C. P. 2009. Mortality and blood loss by blue swimmer crabs (Portunus pelagicus) after simulated capture and discarding from gillnets. – ICES Journal of Marine Science, 66: 455–461.
Two laboratory experiments were carried out to quantify the mortality and physiological responses of juvenile blue swimmer crabs (Portunus pelagicus) after simulated gillnet entanglement, air exposure, disentanglement, and discarding. In both experiments, all but control blue swimmer crabs were entangled in 1-m² gillnet panels for 1 h, exposed to air for 2 min, subjected to various treatments of disentanglement ranging between the forceful removal of none, one, two, and four appendages, then “discarded” into individual experimental tanks and monitored for 10 d. In Experiment 1, mortalities were associated with the number of appendages removed and the occurrence of unsealed wounds. In Experiment 2, live blue swimmer crabs were sampled for blood at 2 min and 6, 24, and 72 h post-discarding to test for the effects of disentanglement and appendage removal on total haemocyte counts, clotting times, protein levels (by refractive index), and blood ion concentrations. Compared with blue swimmer crabs that had sealed or no wounds, those with unsealed wounds had lower total haemocyte counts, protein, and calcium concentrations and increased clotting times and magnesium and sodium levels. Induced autotomy, as opposed to the arbitrary, forceful removal of appendages has the potential to minimize the mortality and stress of discarded, juvenile blue swimmer crabs.
... Unlike mullet, flathead are viewed as a primary target for recreational fishers and harvest rates in this sector are roughly equal to that reported from the commercial net fishery (Webley et al., 2015;McGilvray et al., 2018a;Leigh et al., 2019;Department of Agriculture and Fisheries, 2019f). Flathead stocks are highly accessible (Broadhurst et al., 2003;Gray & Barnes, 2015) and these species are at higher risk of experiencing a regional overfishing event due to cumulative fishing pressures. This is more likely to occur in regions with higher populations, including in south-east Queensland where areas like the Moreton Bay and Great Sandy Marine Parks remain popular recreational fishing locations. ...
... In C&R-type studies, it was found that hook localization and bleeding status, relative to specific fish species, are the most important factors influencing mortality rates [13,40]. Therefore, it is important to develop fishing tools and methods which cause the least damage to the fish, when they are returned to the water after capture, by taking into consideration the undesirable effects of hook size, bleeding, and so forth [35]. ...
The aim of this study is to determine the bleeding status, handling time, and initial mortality caused by the anatomical status of fish hooks in a fish's oral mouth. Observations were conducted during the course of a total of 30 amateur recreational fishing expeditions to catch Squalius cephalus between July, 2013, and December, 2015. Anatomical conditions of the hooks as lodged in a fish's jaws (lower, upper, external, upper and lower jaws), hook removal times, and overall length of the captured fish were compared on the basis of the type of lure and hook used (i.e., spinner, spoon). Only four (of the 111 fish caught in total) were observed to have died within one hour of capture. All of these deaths occurred in fish caught by spinner. It was observed that the bleeding rates proportionally increased with the increase in the amount of time required to remove the hook. The difference between the anatomical status and the time of removal from the hook was statistically found to be significant (ANOVA, F = 8.842 p <0.01). In the fish that were hooked on both jaws, the time to remove the hook was longer. Therefore, bleeding (and mortality) was more prevalent in these fish. To create more sustainable recreational fisheries , it can be concluded that the awareness of fisher-men (and state authorities) should be focused on greater selectivity in the types of hooks that are used; fish hooks that cause less damage to the fish lead to an increase in overall fish welfare, and ultimately facilitate a more sustainable fishing industry.
... This is only allowed during the winter months when it is believed that the cold water reduces the mortality of the bycatch and improves the quality of retained species. This method targets mainly Platycephalus fuscus (flathead) as well as the three species listed above (Broadhurst et al., 2003;Gray et al., 2005). 1 9 5 5 1 9 5 7 1 9 5 9 1 9 6 1 1 9 6 3 1 9 6 5 1 9 6 7 1 9 6 9 1 9 7 1 1 9 7 3 1 9 7 5 1 9 7 7 1 9 7 9 1 9 8 1 1 9 8 3 1 9 8 5 1 9 8 7 1 9 8 9 1 9 9 1 1 9 9 3 1 9 9 5 1 9 9 7 1 9 9 9 2 0 0 1 2 0 0 3 2 0 0 5 2 0 0 7 Year Removals (Wild Harvesting) of the Biological Resources from Systems 261 10 vessels takes place in the middle and outer estuary; long lining from boats for skate and cod is carried out mainly in winter in the middle and lower reaches; fishing for crabs using pots occurs in the outer estuary; eel trapping using fyke nets or traps takes place in the middle and upper estuary and tribu taries; netting for salmon and sea trout, which had almost died out, is increasing once more as the fish return to the now less polluted waters of the estuary; and, finally, commercial cockle gathering is permitted on one bed on the south bank of the estuary (HERAG, 2004). Recreational angling, both from the shore and from boats, is very popular and it is estimated that there are more than 20 000 anglers in the Humber catchment (see also Section 8.09.6.2). ...
This chapter outlines the current and historical importance of the many types of fishing carried out in estuaries around the world. The types of gear used by small- and large-scale fishers and examples of different fisheries are described. The fisheries productivities of temperate, subtropical, and tropical systems are compared. The effects of fishing on the target organisms, nontarget species, nursery functions, trophic structure, habitats, water quality, the human environment, and extinctions are reviewed using case studies. Finally, the importance of fishing in estuaries is discussed in relation to both conservation and economic issues.
... 200 tonnes per annum (Henry and Lyle, 2003;Rowling et al., 2010;Roelofs et al., 2012). Concerns over high rates of mortality and exploitation and the long-term sustainability of dusky flathead populations have seen several investigations into the development of more selective fishing gear and practice (Broadhurst et al., 2003(Broadhurst et al., , 2009Gray et al., 2005;Butcher et al., 2008) and changes in management strategies (Gray et al., 2004;Roelofs et al., 2012) for the species over the past decade. However, some fundamental, yet vital, demographic and biological parameters for the species have yet to be resolved. ...
The dusky flathead (Platycephalus fuscus) is an important teleost harvested by recreational and commercial fishers throughout its endemic distribution along eastern Australia. This study indicates that the species has an extended spawning period throughout the austral summer, with females in spawning condition occurring in lower estuarine and coastal waters. Total length (L50) and age (A50) at which 50% (±1 SE) of the population was mature was 31.72 (±1.08) cm TL and 1.22 (±0.44) years for males and 56.75 (±0.60) cm TL and 4.55 (±0.13) years for females. The von Bertalanffy growth parameters differed significantly between sexes; females grew faster and attained a greater maximum TL and age than males. The largest female was 98.5 cm TL (7.5 kg), and the oldest 16 years, whereas the largest male was 61.5 cm TL (1.58 kg) and 11 years of age. A tag-and-release study identified the exchange of sub-adult and mature-sized individuals among estuaries. Determinations of length-based management regulations for the species are compounded by the large gender-based differences in growth and length-at-maturity. Current minimum legal lengths of 30–40 cm TL protect approximately 3–9% of the female spawning population. Alternative management options, including harvest slot sizes, need to be investigated and tested.
... Mugilidae species represents more than 56% of the total fishery landings in Greece (Koutrakis et al., 2007; Katselis et al., 2010). M. cephalus is one of the main species fished in the Mediterranean lagoons, is an important component of the bycatch in Australian lagoons (Broadhurst et al., 2003) and constitutes 72.4% of the regional fishery of fish species in the Gulf of Mexico (Guzman and Huidobro, 2002). The taxonomic similarity between lagoons, including in the same geographical area, tends to be very low for all taxonomic groups. ...
Coastal lagoons are considered to be among the most productive ecosystems in the world. Many of them support important fisheries, and some of them maintain intensive and extensive aquacultural exploitations. Their particular features, such as shallowness, relative isolation and protection from the sea, and the presence of boundaries with strong physical and ecological gradients help explain this high productivity. Despite the fact that coastal lagoons are among the most studied ecosystems in the world, our knowledge on fisheries in coastal lagoons or why some lagoons capable of maintaining profitable fisheries seem inadequate for intensive aquaculture remains limited. Scarce, too, is our knowledge of the characteristics of target species, their exploitation status, and the ecological processes that are affected by fisheries or that influence them, including the impact of human activities or climatic change. Here, we review present day knowledge on lagoon fisheries and analyse gaps in the science, stressing the need for adequate management of these important resources.
... Alternatively, more selective and appropriately configured gillnets (e.g. height, material and hanging ratio of nets; see Hamley, 1975;Millar and Fryer, 1999;Broadhurst et al., 2003;Godoy et al., 2003) that minimise the capture and mortality of non-target species and undersized individuals could be developed. This, however, would be more expensive than the simpler options of altering existing mesh sizes and soak times. ...
A scientific observer program was used to quantify relationships among the compositions and rates of retained and discarded catches taken in a temperate estuarine commercial gillnet fishery in southeastern Australia. Sampling was stratified across six estuaries and three fishing seasons that corresponded to different permitted setting practices (set and immediate retrieve versus overnight set). A total of 265 fishing trips was sampled throughout 2001, yielding 57 species (52 finfish, 3 invertebrate, 1 bird and 1 tortoise), of which 42 and 45 species were retained and discarded, respectively. More species and a greater mean number and weight of total individuals were retained than discarded during each fishing season. Throughout the entire survey, 6.2% by number and 3.3% by weight of catches were discarded. Average retained catches ranged from 37 to 609 kg fisher-day−1 and discarded catches from 1 to 10 kg fisher-day−1. Mugil cephalus and Girella tricuspidata accounted for 85% by number of total observed catches, with a further 10% being contributed by Acanthopagrus australis, Platycephalus fuscus and Portunus pelagicus. G. tricuspidata, A. australis and P. pelagicus below the minimum legal length collectively accounted for 69% by number and 49% by weight of all discards observed. Discarding was greatest in the smallest mesh sizes and during winter when nets can be set overnight. Increasing the minimum permitted mesh size as well as decreasing the maximum soak time of nets set in winter will further reduce the low levels of discarding in this fishery.
... Hamley and Regier (1973) described how, for walleye Sander vitreum (a congeneric species of pikeperch), a very spiny species, tangling can be as important a means of capture as wedging. A bimodal length frequency distribution derived from gillnet catches owing to tangling, has been described for Atlantic cod Gadus morhua (Hovgärd, 1996a,b; Hovgärd et al., 1999; Holst et al., 2002), for sole Solea solea (Madsen et al., 1999), for flathead mullet Mugil cephalus (Gray et al., 2005), a toothy species, Glossobogius giuris (Pet et al., 1995) and for dusky flathead Platycephalus fuscus, a species with a body characterised by prominent morphological discontinuities (large teeth, opercular spines, maxillaries; Broadhurst et al., 2003). These circumstances may result in a higher probability of percid species being caught and retained in gillnets compared with smooth-bodied fish such as cyprinids (Cyprinidae) or salmonids (Salmonidae) and further bias the species compo- sition. ...
Overestimation of the number of percid fishes taken by gillnets was studied in eight reservoirs in the Netherlands and the Czech Republic during 1998–2006. Overestimation was defined as a higher proportion of percids (percids/(percids + cyprinids)) in gillnets than in the reference community (catches by seines on the same beach and night as the gillnet catches). In total, 97 pairs of catches were compared and overestimation was found in more than 80% of cases. The overestimation ranged from a few percent to more than 1,000%, being dependent on the proportion of percids in the fish community. Overestimation was highest in reservoirs with the lowest proportions of percids. Overestimation was proved for perch Perca fluviatilis, but not for pikeperch Sander lucioperca and ruffe Gymnocephalus cernuus. A correction factor was developed, for the proportion of perch in the gillnet catches, using an empirical cubic function. Analysis of the direct mechanisms by which fish were enmeshed in the gillnets showed that most fish were wedged, one quarter were gilled and only 1.5% were tangled. Percid species were relatively more frequently tangled and gilled than cyprinids but not to an extent that can completely explain the total overestimation. Furthermore, the overestimation was not caused by a higher probability of perch being retained in the gillnet, as was evident from an experiment with retaining perch and roach Rutilus rutilus in the gillnet. Overestimation of perch is most likely caused by a higher probability of them encountering the gillnet, in comparison with cyprinids, which is related to their greater activity during dusk and dawn.
... All control fish were removed from the codend by researchers using scoop nets or gloved hands, tagged, measured, transported in 200-l holding tanks and then released into the remaining two control sea cages. Few dusky flathead were caught in the seine, and so additional individuals were captured using bottom-set gillnets (Broadhurst et al., 2003) deployed for less than 25 min in Botany Bay. All fish placed in the sea cages were fed chopped pilchards at a rate of 1% biomass per day and monitored for mortalities twice daily over 10 days. ...
A field experiment was done to quantify the mortality of fish released during a recreational angling tournament in Botany Bay, Australia. Participating boat-based anglers were divided into two groups, each representing different typical catch-and-release events. The first group (termed the ‘live weigh-in group’) retained the largest two individuals of 4 species (dusky flathead, Platycephalus fuscus, yellowfin bream, Acanthopagrus australis, sand whiting, Sillago ciliata, and trevally, Pseudocaranx dentex) in onboard holding tanks and then presented these to researchers at designated weigh-in times and stations. Gear, operational and handling data were collected before 125 fish were tagged using plastic t-bar tags, returned to the anglers and then released into two sea cages. The second group (termed the ‘immediate-release group’) immediately released 224 fish into two sea cages, after they were tagged and relevant data recorded by onboard observers. This group represented those fish routinely discarded (i) as part of catch-and-immediate-release tournaments and/or (ii) due to minimum legal sizes and/or personal quotas. Appropriate species and numbers of ‘control’ fish were seined and placed into two sea cages. All fish were monitored for mortalities over 10 days. Dusky flathead, yellowfin bream, trevally and snapper, Pagrus auratus accounted for more than 85% of the total catch. Their adjusted mortalities ranged between 0% and 36.6%. Irrespective of the treatment, most yellowfin bream and snapper deaths occurred within 3 h of being hooked and released into the cages, while trevally and dusky flathead showed a delayed mortality over 4 days. Owing to confounding effects due to their confinement, dusky flathead were excluded from further analyses. Anatomical hook location and the time between capture and release were significant predictors of mortality for yellowfin bream and trevally, respectively (p < 0.01), but none of the various gear, operational or handling factors examined were significant for snapper (p > 0.05). The results are discussed in terms of species-specific variabilities in mortalities, their causal effects and better management of catch-and-release events.
In this review we highlight the relevance
of biodiversity that inhabit coastal lagoons, emphasizing
how species functions foster processes and
services associated with this ecosystem. We identified
26 ecosystem services underpinned by ecological
functions performed by bacteria and other
microbial organisms, zooplankton, polychaetae
worms, mollusks, macro-crustaceans, fishes, birds,
and aquatic mammals. These groups present high
functional redundancy but perform complementary
functions that result in distinct ecosystem processes. Because coastal lagoons are located in the interface
between freshwater, marine and terrestrial ecosystems,
the ecosystem services provided by the biodiversity
surpass the lagoon itself and benefit society
in a wider spatial and historical context. The species
loss in coastal lagoons due to multiple human-driven
impacts affects the ecosystem functioning, influencing
negatively the provision of all categories of services
(i.e., supporting, regulating, provisioning and
cultural). Because animals’ assemblages have unequal
spatial and temporal distribution in coastal lagoons,
it is necessary to adopt ecosystem-level management
plans to protect habitat heterogeneity and its biodiversity,
ensuring the provision of services for human
well-being to multi-actors in the coastal zone.
Responsible stock enhancement initiatives require baseline data of wild population demographic conditions that can be used in testing management outcomes. This study provides the first fishery-independent assessment of length- and age-based biological characteristics of exploited populations of Platycephalus fuscus in eastern Australia prior to stock enhancement. Sampling was conducted over seven estuaries spanning seven degrees of latitude and the geographical range of proposed stock enhancements. Populations in all estuaries showed evidence of length and age truncation, especially those subject to commercial fisheries where young individuals of both sexes dominated populations. Maximum longevities were 12 and 11 years for females and males respectively, but few females >5 years and males >3 years were generally sampled. Females dominated populations, and on average, the mean lengths and ages of females were greater than males within each estuary and across all age classes. Sexually dimorphic variation in growth was evident across all estuaries, with females attaining greater maximum lengths than males. Estuary-specific differences in individual growth were not identified. On average, over 50% of females sampled in each estuary were > the minimum legal length (MLL), but the opposite was evident for males. In contrast, over 25% of males in each estuary were > the mean length at maturity (L50), whereas in all but one estuary <13% of the females were > the L50. Stocked male and female P. fuscus should recruit to the fishery in 2 and 3 years, and contribute to the spawning stock in 1.5 and 4.5 years, respectively. This study provides important historical baseline data that can contribute to testing stock enhancement outcomes on populations.
Very little work has determined the relative importance of uncontrolled environmental factors for affecting fish biology, and how these might influence gillnet catches. This study addresses this deficit for an important Southeast Asian cyprinid (Barbonymus schwanenfeldii). Fish were caught monthly for 12 months using gillnets of three different mesh sizes, each of which was deployed in duplicate at the surface of one of three randomly selected sites in Lake Kenyir, Malaysia, concurrent with determining various environmental parameters and the abundance of phytoplankton (chlorophyll–a). Results indicated that growth co-efficient of B. schwanenfeldii was positively influenced by dissolved oxygen and negatively influenced by total inorganic nitrogen, whereas an opposite result was observed in case of the hepatosomatic index of fish. Water turbidity was a limiting factor only for small fish (mean total length: 15.74±1.10 cm). B. schwanenfeldii could best be caught during the period of high phytoplankton abundance or at the location of high phytoplankton density in the water. Water temperature negatively influenced the gillnet catches of the fish. The remaining environmental factors such as water depth, pH, and phosphate had a weak and insignificant influence (P >0.05) on the biology and gillnet catches of fish. The observed results can be very useful for the ecological monitoring and conservation plans for this species in relation to climate change. Furthermore, the utility of the similar data for other species would be useful not only for regional but also for international fishery by optimizing catches considering environmental conditions.
This article reviews the factors that affect the catching efficiency and selectivity of gillnets, a method widely used throughout the world for harvesting many fish species. These factors include 1) biotic features related to fish or aquatic organisms such as species, morphologies, sizes, behaviors, distributions, and fish abundance, and 2) operational features related to gear characteristics and fishing methods such as mesh sizes, net materials, twine diameters, hanging ratios, soaking times, and fishing periods. These factors are important in the consideration of appropriate and effective gillnet fishery management.
The national reporting framework used in the Status of key Australian fish stocks reports 2014 was developed collaboratively by fisheries scientists from around Australia. This framework uses standardised terminology and reference points for stock status classifications. Following release of the Status of key Australian fish stocks reports 2012, a review was undertaken to document the lessons learned. Feedback from stakeholders suggested that national reporting on the status of key Australian fish stocks should continue, and that effort should be made to incorporate reporting on fishery-level environmental, economic, governance and social aspects.
The national framework for these reports considers both the abundance (number or biomass [weight]) of fish in a stock and the level of fishing pressure (rate of fishing) applied to a stock. The status classifications assess whether the current abundance of fish in a stock is adequate—that is, whether there is a large enough proportion of the original adult stock remaining that the production of juveniles is not significantly reduced. They also assess whether the amount of fish currently being removed through fishing is adequately controlled to ensure that stock abundance is not reduced to a point at which production of juveniles is significantly reduced. The framework makes these assessments against the biomass reference point of 'recruitment overfished', which is the point at which the spawning stock biomass has been reduced by fishing so that average recruitment (that is, the production of juveniles) is significantly reduced. There are five classification categories (refer to Introduction for full descriptions):
Sustainable stock —indicates that biomass (or biomass proxy) is at a level sufficient to ensure that, on average, future levels of recruitment are adequate (that is, the stock is not recruitment overfished) and that fishing pressure is adequately controlled to avoid the stock becoming recruitment overfished.
Transitional –recovering stock—indicates that biomass is recruitment overfished, but management measures are in place to promote stock recovery, and recovery is occurring.
Transitional–depleting stock —indicates that biomass is not yet recruitment overfished, but fishing pressure is too high and moving the stock in the direction of becoming recruitment overfished.
Overfished stock —indicates that spawning stock biomass has been reduced through catch, so that average recruitment levels are significantly reduced (that is, the stock is recruitment overfished). Current management is not adequate to recover the stock; or adequate management measures have been put in place but have not yet resulted in measurable improvements.
Environmentally limited —indicates that spawning stock biomass has been reduced to the point where average recruitment levels are significantly reduced, primarily as a result of substantial environmental changes or disease outbreaks (that is, the stock is not recruitment overfished). Fisheries management has responded appropriately to the environmental change in productivity.
Undefined stock —indicates that insufficient information exists to determine stock status.
A substantial decline in the commercial catches and catch rates of black bream Acanthopagrus butcheri in the Gippsland Lakes, Australia, signified a depletion of the stock and prompted management to increase the legal minimum length (LML) from 26 cm to 28 cm (total length) in December 2003. The effectiveness of this increase depends on the postrelease survival rate of undersized fish. The present study estimates the rates of release and initial survival (1 h after capture) of undersized A. butcheri during commercial fishing by gill nets in the Gippsland Lakes. Field capture and holding trials were also used to estimate initial and delayed survival (72 h after capture). Almost 6,000 A. butcheri from 347 gill net shots were caught during 2005–2006. The average total length increased by 1 cm from 2005 to 2006, increasing the proportion of the catch that was retained in 2006 (75%) compared with 2005 (50%). The best estimates of initial and delayed survival were 97.2% (SE, 0.3%) and 94.4% (SE, 3.8%), respectively. The total survival rate, which combines the initial survival rate from observer work with the delayed survival rates from the trials, was estimated as 90.8% (SE, 3.8%). The increase in the LML from 26 to 28 cm afforded significant protection to fish between these size limits, but the level of protection will be short term (1 or at most 2 years) for each year-class because of the growth rate of the species. Released, retained, and postrelease survival rates are discussed for various LMLs and with regard to their implications for fishery management.
Samples of 16 key fish species targeted by oceanic prawn trawlers and oceanic and estuarine beach seiners in southeastern Australia were measured for up to 10 morphological dimensions that included: total (Wt) and eviscerated weights (EWt); total (TL), fork (FL), natural (NL), standard (SL), dorsal standard (DSL), and anal standard lengths (ASL); and maximum body height (MH), width (MW), and girth (MG). All partial lengths were indexed against TL to provide linear relationships and, where there were sufficient data, tested for intra‐specific sexual dimorphism. Nearly all species that were sexed demonstrated at least some morphological divergence, and especially for transverse measures; probably owing to different reproductive conditions. Sex‐specific or combined regressions of DSL and ASL and transverse measures versus TL were used to identify the most appropriate: (1) partial length for compliance purposes; and (2) mesh sizes and configurations for the different fishing gears used. Using relevant data describing the smallest commercial‐sized fish species (stout, Sillago robusta and red spot whiting, S. flindersii— 150mm TL) retained in prawn trawls, a new square‐mesh codend (made from 35‐mm mesh hung on the bar) was designed and constructed. The selectivities of this square‐mesh codend and a conventional diamond‐mesh design (40‐mm mesh) were then modelled and compared in a case study. The conventional codend was demonstrated to be non‐selective for the sizes of eastern king prawns and red spot whiting targeted. In contrast, the square‐mesh codend selected red spot whiting across narrow selection ranges (18.88–15.67 mm) and at lengths of 50% retention (143.39–146.32mm TL) that closely corresponded to the estimated maximum mesh opening. The square‐mesh codend also improved selection for eastern king prawns, without impacting on commercial catches. Based on these results, similar predictions were made for appropriate mesh sizes and configurations in oceanic and estuarine beach seines; both of which were demonstrated to comprise minimum mesh sizes that would be non‐selective for the sizes of all targeted fish.
Two experiments were done in an Australian estuary to quantify the mortalities and contributing factors for key species discarded during 8 and 9 deployments of commercial beach (or shore) seines and gillnets, respectively. In both experiments, bycatches (2347 individuals comprising 16 species) were handled according to conventional practices and assessed for immediate mortalities before live samples of selected species were discarded into replicate cages along with appropriate controls, and monitored for short-term mortalities (< or =10 d). All of the seined or gilled fish were alive prior to discarding. During the beach seine experiment, 20% of caged seined-and-discarded surf bream Acanthopagrus australis (n = 290) were dead after 5 d, with most mortalities occurring between the second and fifth day. In the gillnet experiment, 42 and 11% of gilled-and-discarded A. australis (n = 161) and lesser salmon catfish Neoarius graeffei (n = 67), respectively, died during a 10 d monitoring period, mostly within the first 5 d. There were no deaths in any controls for these fish. Mixed-effects logistic models revealed that the mortality of A. australis discarded from both gears was significantly (p < 0.01) and negatively correlated with their total length, while N. graeffei had a significantly (p < 0.05) greater (5-fold) probability of dying when jellyfish Catostylus sp. were present in the gillnet. Simple modifications to the operations of beach seines and gillnets and/or post-capture handling procedures, such as close regulation of size selectivity for the target species, careful removal of fish from meshes, and abstention from setting during high abundances of jellyfish will maximise the survival of discarded bycatch.
Size selection investigations were performed in the Sound with experimental, multipanel gill nets equipped with a broad range of mesh sizes targeting Atlantic herring (Clupea harengus). Each of the 20 experimental fishery surveys covered the central Sound from Helsingor-Helsingborg (north) to Drogden-Klagshamn (south) during the autumn, winter, and spring periods from 1993 to 1998. There was high variability in catch sizes both between and within surveys. The overall best fits, in terms of minimum total deviance across all sets, were achieved using a unimodal normal scale model. Almost all surveys showed high between-set variation, which was accounted for when estimating the mean selectivity curves using a random effects model. The selectivity curves fitted to individual sets were compared with those fitted to the combined data sets. It was demonstrated that analysing pooled data sets resulted in overdispersion and bias in the selection parameters. Irregular time series of the estimates of the selectivity parameters in the investigation period indicated negative autocorrelation with a seasonal pattern.
In response to claims that fish trawls off New South Wales, Australia, caught excessive quantities of under-size fish, the catches of finfish by a conventional fish trawl (constructed of 90-mm mesh in the body) were compared with those by a fish trawl constructed of 100-mm mesh in the body. Catches by the 100-mm trawl showed a 27% reduction in all by-catch and a 28% reduction in the numbers of retained tiger flathead, compared with catches by the conventional trawl. The 100-mm trawl also showed a 48% and 47% reduction in the numbers and weights respectively of discarded tiger flathead and a 57% and 63% reduction in the numbers and weights respectively of discarded rubberlip morwong. For john dory, however, at a particular locality where large numbers occurred, the 100-mm trawl caught significantly more fish than did the conventional trawl (a mean increase in weight of 66%). There is a need to determine species-specific mesh selectivities and to study the behaviour of fish in trawls. The importance of the results for the future management and operational efficiency of trawl fisheries is discussed.
The probability of catching a fish in a gill net may be separated into two components: (1) the probability of the fish encountering the net and (2) the probability of the fish being caught and retained in the net. We consider the probability of encounter to be directly proportional to the distance travelled by the fish during the sampling period. This distance will increase with fish size if different-sized fish swim for the same amount of time because swimming speed increases with fish size. Routine swimming speed measured in the laboratory for three size-classes of bloater (Coregonus hoyi) increased with length to the 0.8 power. Corrections for encounter probability were incorporated in gill net selectivity calculations for samples of cisco (Coregonus artedii), a species closely related to bloater. These corrections can significantly increase the proportional estimates of smaller relative to larger animals in the estimated population structure. The approach should also be applicable to other passive fishing gear, such as longlines and set nets.
The effects of varying net length and set time on fish catches in gill nets were studied at two sites in the Bay of Quinte, Lake Ontario. A latin square design was used (four locations per site over four nights); either net length was varied, from 25 to 200 yd over 12 h, or set time was varied, from 1.5 to 12 h with 50 yd of net. Treatment, location, and date effects on catches from each site were analyzed by a multiple regression model. Net length effects on catch per unit of effort (CPUE) were negligible for total and some fish species catches; CPUE for walleyes Stizostedion vitreum decreased with increasing net length. Set-time effects on total CPUE were significant; the CPUE of white perch Morone americana increased with set time, but CPUEs for yellow perch Perca flavescens, alewives Alosa pseudoharengus, and walleyes decreased. The results were attributable to specific differences in activity patterns and net-saturation effects. We concluded that use of gill-net catches as indices offish abundance must be validated species by species.
A comparative fishing experiment was conducted to investigate the capture efficiency and selectivity of bottom-set gillnets and trammel nets. Twelve gillnets and trammel nets of different mesh sizes (7.6, 8.8, 10.1, and 12.6 cm stretched mesh) and hanging ratios (1:1, 1:2, and 1:3) were fished from May 1990 to September 1991 in coral reef (inner and outer reef) and mangrove areas off La Parguera, Puerto Rico. Significant differences in capture due to mesh size and hanging ratio for the inner and outer reef were observed. However, catches in mangroves were only influenced by hanging ratio. Significant interaction between mesh size and hanging ratio influencing the catches of both gears was found. The interaction behaved in an oppositive manner between the two gears. Largest catches in gillnets were obtained by combination of high hanging ratio (1:3) with large meshes (12.7 cm) or low hanging ratio (1:1) with small meshes (7.6). For trammel nets, the largest catches were obtained with low hanging ratio (1:1) with large meshes (12.7 cm) or high hanging ratio (1:3) with small meshes. This oppositive interaction may arise from the different mode of capture of these gears.Selectivity curves were generated for four species: white grunt (Haemulon plumieri), bluestriped grunt (Haemulon sciurus), stoplight parrotfish (Sparisoma viride) and sea bream (Archosargus rhomboidales). The selectivity curves of gillnets and trammel nets were unimodal. Optimum length increases as the mesh size increases, and the selection range becomes wider as hanging ratio increases.
Canadian journal of fisheries and aquatic sciences, ISSN 0706-652X, vol. 52, nr. 1, 75-83 We compared the catches in a series of six vertical gill nets (6.25- to 18.75-mm bar mesh) with estimates of fish abundance derived from hydroacoustics. A total of 47 combined acoustic and gillnet surveys were collected between July and November 1987-1992 in archipelago areas of the northern Baltic Sea. Fish abundance determined by hydroacoustics was significantly correlated with gillnet catches but explained only 28% of the variance in catch per unit effort corrected for gillnet selectivity and fish swimming speed. Visibility (measured as Secchi disk depth) significantly decreased the likelihood of fish being caught in gill nets, while the effect of temperature was positive but nonsignificant. These data imply that gillnet catches will be a biased indicator of changes in fish abundance over time or space, if visibility and possibly also temperature change as well.
Catches of Atlantic cod (Gadus morhua) from Greenland gill-net surveys were analyzed by a two-step approach. in the initial step the form of the selection curve was identified as binormal, which was caused by fish being gilled or caught by the maxillae. Both capture processes could be described by normal distributions and could be related to mesh size in accordance with the principle of geometrical similarity. In the second step the selection parameters were estimated by a nonlinear least squares fit. The model also estimated the relative efficiency of the two capture processes and the fishing powers of different netting sections. Gilling was about three times as efficient as maxillae catching and fishing power could be related to the ratio of twine diameter to mesh size. It is proposed that information on how fish are caught should be included when modelling gill-net selectivity, as lack of such data may lead to the formulation of unrealistic selection models.
The relative fishing powers of experimental gill nets were estimated for shorthorn sculpin (Myoxocephalus scorpius), Greenland cod (Gadus ogac), and Atlantic cod (Gadus morhua). The results suggested that fishing power was negatively correlated to the ratio between twine diameter and mesh size. Similar differences in fishing power could be inferred for American plaice (Hippoglossoides platessoides) where no mesh-size selection was evident.
Commercial landings of dusky flathead (Platycephalus fuscus) from four estuaries in New South Wales (NSW), Australia, were sampled for data on sex, length and age composition between February and July each year for 2–3 years between 1995 and 1997. Landings primarily contained female fish, ranging from 55% to 93% by number for different estuaries. Flathead sampled in commercial catches ranged from 30 cm to 96 cm total length (TL), but the majority were 33–50 cm TL. Fish >40 cm TL were primarily female and male fish >45 cm TL were uncommon. The length composition of catches differed between gillnets of different mesh sizes, with the average length of fish being least in the smallest allowed mesh size of 70 mm. Fish were aged by otolith interpretation and the analysis of marginal increments indicated that one opaque and one translucent growth zone was formed each year; the opaque zone being deposited in June–August (winter) and first observed in September–October (spring). Commercial landings included fish aged 2–11+ years, but fish aged 2–4+ years dominated landings in all estuaries. The total mortality of dusky flathead in each estuary was estimated by catch curve analysis and was relatively high, ranging from 0.45 to 1.64. The data indicate that dusky flathead may be heavily exploited in NSW.
Gillnet selectivity in the Pacific herring roe fishery was examined using experimental apparatus. Estimates of size-specific selectivity of monofilament gillnets were calculated by comparing numbers and lengths of fish retained in nets with those that swam through the net into a trap. Most fish less than 19 cm were not captured; most fish greater than 21 cm were captured. Factors affecting selectivity included size, sex and maturation (or gonad size). There was also a difference in selectivity between locations, perhaps associated with biological differences between populations. Fish that swam through the nets suffered little injury and scale-loss. Laboratory and field tests indicated high survival of fish that encountered, but were not captured by, the nets. Estimates of long-term survival were unavailable because disease became widespread throughout the holding facilities.
Commercial fishers are permitted to set gill nets overnight in New South Wales (NSW) estuaries during the winter months of June, July and August. Scientific observers sampled the retained and discarded catches from 55 fishing trips sampled across six estuaries throughout the 1999 winter fishing season. A total of 69 fish and 3 invertebrate species was identified in catches, with 38 species being retained by fishers and 64 species being discarded. Discards included the juveniles of fish species of commercial and recreational importance, as well as some species of little commercial value. Discards accounted for 19% of total catches by weight and 33% by number, and were mostly alive (>82%) when nets were retrieved from the water. Subsequent mortality after release was not assessed. The mesh size of the nets observed ranged between 80 and 250mm and the size compositions of catches generally portrayed the relative selectivity of the different mesh sizes. The data are discussed in terms of their consequences for managing the estuarine fisheries resources of NSW.
Le pouvoir pechant relatif de filets maillants experimentaux a ete evaluee dans le cas du chaboisseau a epines courtes (Myoxocephalus scorpius), de l'ogac (Gadus ogac) et de la morue franche (Gadus morhua). D'apres les resultats, il existerait une correlation negative entre le pouvoir pechant et le rapport entre le diametre du fil et la taille de la maille. Des differences similaires au niveau du pouvoir pechant pourraient etre deduites dans le cas de la plie canadienne (Hippoglossoides platessoides) lorsqu'aucune selection de la taille de la maille n'est evidente.
This paper reviews studies on gillnet selectivity: the relevant characteristics of fish and nets, graphical and mathematical representation of selectivity curves, and methods of estimation. F. I. Baranov started these studies 60 yr ago, and most subsequent work has been consistent with his axioms that a) gillnets catch fish whose head girths are smaller but maximum girths larger than the mesh perimeter, and b) selectivity curves for all mesh sizes have the same shapes and heights. Unfortunately these axioms are inaccurate. The heights of selectivity curves increase with mesh sizeSelectivity depends mainly on fish size and shape and mesh size, but is also affected by the thickness, material, and color of net twine, hanging of net, and method of fishing. The left slopes of selectivity curves represent smaller fish wedged in the meshes; the right slopes, larger fish mainly tangled by head parts. The curves may be very skewed or multimodal for fish that are easily tangled.The most reliable, though expensive, estimates of gillnet selectivity are by "direct" methods of fishing a population of known size-frequency distribution. The more popular but biased "indirect" estimates compare catches by two or more mesh sizes. Other methods used are prediction from girth measurements and the DeLury method.
The selectivity of gillnets is usually estimated "indirectly" from comparisons of catches by two or more mesh sizes, assuming that the selectivity curves for all meshes have the same shape and amplitude. We estimated selectivity "directly" by fishing marked walleye (Stizostedion vitreum vitreum) with gillnets of (3.81–11.43-cm) mesh. The selectivity curves were bimodal but each could be resolved into two unimodal components representing fish caught by wedging and tangling, respectively. We fitted the selectivity of each mesh to a pair of Pearson’s Type I curves. As the mesh size increased, amplitude of the selectivity curves increased rapidly and tangling occurred over a progressively narrower range of (fish girth)/(mesh perimeter) ratio. Our estimates agreed with data from four out of seven other gillnetting studies, indicating a wider applicability but also a need to consider additional variables or to standardize fishing methods. Because the assumption of equal amplitudes is not realistic, the indirect methods overestimate selectivity curves on the left and underestimate them on the right.
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 ...
An outdoor tank experiment was conducted to study the influence of soaking time on the catch efficiency of gillnets and entangling nets (trammel and semi-trammel nets). The catch efficiency of the nets was also compared with detecting the difference between entangling nets and gillnets in relation to soaking time. Nets of the same mesh sizes were set simultaneously in separate outdoor tanks containing 200 rainbow trout (Oncorhynchus mykiss) whose composition size was known. The experiment was carried out at night using four soaking times; 0.5, 1, 3 and 5 h. The variations of the catch efficiency of all nets had a U-shaped trend, showing that the efficiency increased at the shortest time (0.5 h), decreased at the intermediate time (1 h), and increased again at the longest soaking time. The average catch efficiency of the semi-trammel net appeared to be slightly higher than that of the gillnet.
Five hundred pre-measured brown trout (Salmo trutta), 20 of each cm-class from 5 to 29 inclusive, were released. They were recaptured with gillnets of mesh size 8.0, 10.5, 16.0, 19.5 and 24.0 mm, measured between adjacent knots. The nets were made of clear, nylon monofilament. The modal lengths and the general shape of the selectivity curves were in agreement with earlier findings. The height of the selectivity curve (S) increased exponentially with mesh size in mm (M): S= 0.047e0.075M.
Gill-net size selectivity for roach (Rutilus rutilus (L.)) was estimated directly in a small eutrophic lake by fishing with littoral and pelagic gill nets. The number of roach in the population was estimated by a mark-recapture method. The estimated height of the selectivity curves increased with increasing mesh size, i.e. a pronounced size selectivity. In explaining the observed differences in catch efficiencies, the encounter probability may be less important than the probability of capture and retention. In assessment of length-frequency distribution of roach populations by gill-net sampling, the numbers of large roach will be considerably overestimated relative to small roach if the catch data are not corrected for size selectivity.
The effects of net length and soak time on fish catches of gillnets and trammel nets were studied in a coral reef off La Parguera on the southwestern corner of Puerto Rico. Gillnets and trammel nets were varied in length from 100 to 300 fathoms in intervals of 100 fathoms. Net length significantly affects the number of fish caught by these nets, but no significant differences in catch per unit effort were observed on a per length basis. For the soak time experiment, gillnets and trammel nets of 100 fathoms were set for 24 h. A visual censu of the nets was carried out during the afternoon, at sunset and at sunrise. The saturation effect of decreased catch with increasing soak time did not occur. The efficiency of gillnets was variable over time but differences were not significant. The efficiency increased at the shortest time (6–10 h), decreased at the intermediate time (10–15 h), and increased again at the longest time (15–20 h). Changes in trammel net catch rates were negligible. A significant difference was found between visual census and gillnet catch in the number of individuals and species observed. These results appear to be influenced by some unmeasurable and unrecoeded factors such as method of deployment of the nets, fish behavior around the nets and type of substrate.
The size selectivity of sole nets was measured indirectly on a Danish commercial gill net vessel in the North Sea by using nets made in seven different mesh sizes fished simultaneously. Maximum likelihood procedure was used to fit different forms for the selection curve to the catch data for sole, plaice and cod for each setting of the gear. It was found that a bi-normal form for the selection curve gave the best fits. Mean selection curves were then estimated by combining sets using a model of between-set variation. The ratio between length of maximum retention and mesh size was estimated to be 3.28 for sole, 2.60 for plaice and 4.56 for cod. Selection curves were also fitted to the catch data pooled over all sets. The model deviance for the sole and plaice data indicated lack of fit when pooling the catch data.
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Management implications of discarding in an estuarine multi-species gill net fishery Variations in sex, length and age composi-tions of commercial catches of Platycephalus fuscus
- C A M K Gray
- Broadhurst
Gray, C. A. 2002. Management implications of discarding in an estuarine multi-species gill net fishery. Fish. Res. 56: 177–192 Arch. Fish. Mar. Res. 50(3), 2003 M. K. Broadhurst et al. Gray, C. A.; Gale, V. J.; Stringfellow, S. L.; Raines, L.P. 2002. Variations in sex, length and age composi-tions of commercial catches of Platycephalus fuscus (Pisces: Platycephalidae) in New South Wales, Australia. Mar. Freshwater Res. 53: 1091–1100.
- Arch
- Fish
Arch. Fish. Mar. Res. 50(3), 2003