The effect of grid size on catch efficiency and by-catch in the demersal trawl fishery for Norway pout (Trisopterus esmarkii)

Norway pout (Trisopterus esmarkii) is caught with large trawls with small meshed codends, inevitably causing the fishery to have large by-catch issues. To reduce the amount of by-catch, a rigid sorting grid was made compulsory in 2010. However, there is still a severe by-catch issue, as well as the loss of target species because of the grid. A possible cause might be clogging of the grid, which could be solved by increasing the grid area. Therefore, this study compared the size selectivity of by-catch species and target species in a double-trawl configuration in which one trawl was equipped with a standard grid (6.30 m2), and the other trawl was equipped with a grid that had a 50% larger surface area (9.45 m2). The results demonstrated that the size selectivity and catch efficiency of the target species were unaffected; neither was there any significant difference between the two grids in terms of wanted by-catch species [blue whiting (Micromesistius poutassou), horse mackerel (Trachurus trachurus), and greater argentine (Argentina silus)]. However, the larger grid caught significantly more unwanted by-catch species [haddock (Melanogrammus aeglefinus), whiting (Merlangius merlangus), and hake (Merluccius merluccius)]. Approximately one-third of the accumulated catches comprised the target species, one-third of wanted by-catch species, and one-third of unwanted by-catch species, demonstrating the by-catch challenges in this fishery. Simultaneously, none of the by-catch limits were exceeded.publishedVersio

Managing size selectivity: the relevance of compulsory and alternative selection devices in the Northeast Atlantic bottom trawl fishery

Two decades of use of the compulsory selectivity gear configuration in the Northeast Atlantic bottom trawl fishery, which consists of a rigid sorting grid followed by a diamond mesh codend, have revealed problems with performance and efficiency. Size selectivity gear alternatives to this configuration are being pursued, and recent studies of codends with shortened lastridges have reported promising results. In this study, we compared the size selectivity and catch efficiency for cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) caught using a sorting grid with 55 mm bar spacing followed by a 130 mm knotted diamond mesh codend (i.e. the compulsory gear) that same configuration but considering only the catch in the 130 mm diamond mesh codend (regular codend), and an alternative knotless codend with shortened lastridge ropes and a thinner twine. We also addressed the question of whether size-selective gear is even needed to avoid exceeding the bycatch limits of the fishery. Our results demonstrated that the alternative codend improved size selectivity by reducing the loss of fish above minimum legal size (MLS), with a minor increase in the retention of fish below MLS, compared to the compulsory combined grid and codend configuration. The regular codend also reduced the loss of fish above MLS, but the retention of haddock below MLS approached the legal limit of 15% by number. Our results also showed that the abundance of cod below MLS in the fishing area during the trials was low. They also suggested that spatial and seasonal closures in combination with a flexible choice of selection device and modern monitoring tools could be an alternative to rigid enforcement of the use of selective fishing gear.publishedVersio

Accounting for Uncertainties in Biodiversity Estimations: A New Methodology and Its Application to the Mesopelagic Sound Scattering Layer of the High Arctic

Measures of biological diversity (biodiversity) are important for monitoring the state of ecosystems. Several indices and methods are used to describe biodiversity from field observations. Marine faunal biodiversity is often quantified based on analysis of samples collected using a trawl during research surveys. To monitor spatial and temporal changes in biodiversity between surveys, samples are generally collected from a series of stations. Inference regarding changes in biodiversity must account for uncertainties in the estimation of the values for the different biodiversity indices used. Estimation for a single station is affected by spatial-temporal variation in the species composition in the area and by uncertainty due to the finite sample size taken by the trawl. Therefore, variation between stations needs to be accounted for when estimating uncertainty for values of different indices during a survey as total or as mean for the survey. Herein, we present a method based on nested bootstrapping that accounts for uncertainties in the estimation of various indices and which can be used to infer changes in biodiversity. Application of this methodology is demonstrated using data collected in the mesopelagic sound scattering layer in the high Arctic.publishedVersio

Effect of the Nordmøre grid bar spacing on size selectivity, catch efficiency and bycatch of the Barents Sea Northern shrimp fishery

The introduction of the Nordmøre grid in shrimp trawls has reduced the bycatch of non-target species. In the Norwegian Northern shrimp (Pandalus borealis) fishery, the mandatory selective gear consists of a Nordmøre grid with 19 mm bar spacing combined with a 35 mm mesh size diamond mesh codend. However, fish bycatch in shrimp trawls remains a challenge and further modifications of the gear that can improve selectivity are still sought. Therefore, this study estimated and compared the size selectivity of Nordmøre grids with bar spacings of 17 and 21 mm. Further, the effect of applying these two grids on trawl size selectivity was predicted and compared to the legislated gear configuration. Experimental fishing trials were conducted in the Barents Sea where the bottom trawl fleet targets Northern shrimp. Results were obtained for the target species and two by-catch species: cod (Gadus morhua) and American plaice (Hippoglossoides platessoides). This study demonstrated that reducing bar spacing can significantly reduce fish bycatch while only marginally affecting catch efficiency of Northern shrimp. This is a potentially important finding from a management perspective that could be applicable to other shrimp fisheries where flexibility in the use of different grid bar spacings may be beneficial to maximize the reduction of unwanted bycatch while minimizing the loss of target species.publishedVersio

The effect of grid size on catch efficiency and by-catch in the demersal trawl fishery for Norway pout (Trisopterus esmarkii)

Norway pout (Trisopterus esmarkii) is caught with large trawls with small meshed codends, inevitably causing the fishery to have large by-catch issues. To reduce the amount of by-catch, a rigid sorting grid was made compulsory in 2010. However, there is still a severe by-catch issue, as well as the loss of target species because of the grid. A possible cause might be clogging of the grid, which could be solved by increasing the grid area. Therefore, this study compared the size selectivity of by-catch species and target species in a double-trawl configuration in which one trawl was equipped with a standard grid (6.30 m2), and the other trawl was equipped with a grid that had a 50% larger surface area (9.45 m2). The results demonstrated that the size selectivity and catch efficiency of the target species were unaffected; neither was there any significant difference between the two grids in terms of wanted by-catch species [blue whiting (Micromesistius poutassou), horse mackerel (Trachurus trachurus), and greater argentine (Argentina silus)]. However, the larger grid caught significantly more unwanted by-catch species [haddock (Melanogrammus aeglefinus), whiting (Merlangius merlangus), and hake (Merluccius merluccius)]. Approximately one-third of the accumulated catches comprised the target species, one-third of wanted by-catch species, and one-third of unwanted by-catch species, demonstrating the by-catch challenges in this fishery. Simultaneously, none of the by-catch limits were exceeded

Understanding and predicting the effect of entrance cone diameters on the catch efficiency of snow crabs (Chionoecetes opilio) in conical pots

Conical pots with a plastic entrance cone on the top are a type of fishing gear used to harvest snow crabs (Chionoecetes opilio) in Arctic regions. In this study, we assessed the effect of pot entrance diameter on the catch efficiency of snow crabs. We used catch data collected during sea trials of pots with different entrance cone diameters, and we conducted laboratory experiments where morphological characteristics of snow crabs were investigated to estimate the extent of the different body parts of the animal that are decisive for the catch efficiency of the pot entrance. The results show that body parts larger than the carapace affected pot entrance efficiency. We predicted that entrance diameter is a key factor determining catch efficiency and that the experimental pots used in this study would experience significant reductions in the catch of marketable size crab, compared to pots currently used in the fishery.publishedVersio

Understanding and predicting the effect of entrance cone diameters on the catch efficiency of snow crabs (Chionoecetes opilio) in conical pots

Conical pots with a plastic entrance cone on the top are a type of fishing gear used to harvest snow crabs (Chionoecetes opilio) in Arctic regions. In this study, we assessed the effect of pot entrance diameter on the catch efficiency of snow crabs. We used catch data collected during sea trials of pots with different entrance cone diameters, and we conducted laboratory experiments where morphological characteristics of snow crabs were investigated to estimate the extent of the different body parts of the animal that are decisive for the catch efficiency of the pot entrance. The results show that body parts larger than the carapace affected pot entrance efficiency. We predicted that entrance diameter is a key factor determining catch efficiency and that the experimental pots used in this study would experience significant reductions in the catch of marketable size crab, compared to pots currently used in the fishery

Understanding and predicting the effect of entrance cone diameters on the catch efficiency of snow crabs (Chionoecetes opilio) in conical pots

Conical pots with a plastic entrance cone on the top are a type of fishing gear used to harvest snow crabs (Chionoecetes opilio) in Arctic regions. In this study, we assessed the effect of pot entrance diameter on the catch efficiency of snow crabs. We used catch data collected during sea trials of pots with different entrance cone diameters, and we conducted laboratory experiments where morphological characteristics of snow crabs were investigated to estimate the extent of the different body parts of the animal that are decisive for the catch efficiency of the pot entrance. The results show that body parts larger than the carapace affected pot entrance efficiency. We predicted that entrance diameter is a key factor determining catch efficiency and that the experimental pots used in this study would experience significant reductions in the catch of marketable size crab, compared to pots currently used in the fishery

Understanding and predicting the effect of entrance cone diameters on the catch efficiency of snow crabs (Chionoecetes opilio) in conical pots

Conical pots with a plastic entrance cone on the top are a type of fishing gear used to harvest snow crabs (Chionoecetes opilio) in Arctic regions. In this study, we assessed the effect of pot entrance diameter on the catch efficiency of snow crabs. We used catch data collected during sea trials of pots with different entrance cone diameters, and we conducted laboratory experiments where morphological characteristics of snow crabs were investigated to estimate the extent of the different body parts of the animal that are decisive for the catch efficiency of the pot entrance. The results show that body parts larger than the carapace affected pot entrance efficiency. We predicted that entrance diameter is a key factor determining catch efficiency and that the experimental pots used in this study would experience significant reductions in the catch of marketable size crab, compared to pots currently used in the fishery

Effect of the Nordmøre grid bar spacing on size selectivity, catch efficiency and bycatch of the Barents Sea Northern shrimp fishery

The introduction of the Nordmøre grid in shrimp trawls has reduced the bycatch of non-target species. In the Norwegian Northern shrimp (Pandalus borealis) fishery, the mandatory selective gear consists of a Nordmøre grid with 19 mm bar spacing combined with a 35 mm mesh size diamond mesh codend. However, fish bycatch in shrimp trawls remains a challenge and further modifications of the gear that can improve selectivity are still sought. Therefore, this study estimated and compared the size selectivity of Nordmøre grids with bar spacings of 17 and 21 mm. Further, the effect of applying these two grids on trawl size selectivity was predicted and compared to the legislated gear configuration. Experimental fishing trials were conducted in the Barents Sea where the bottom trawl fleet targets Northern shrimp. Results were obtained for the target species and two by-catch species: cod (Gadus morhua) and American plaice (Hippoglossoides platessoides). This study demonstrated that reducing bar spacing can significantly reduce fish bycatch while only marginally affecting catch efficiency of Northern shrimp. This is a potentially important finding from a management perspective that could be applicable to other shrimp fisheries where flexibility in the use of different grid bar spacings may be beneficial to maximize the reduction of unwanted bycatch while minimizing the loss of target species.publishedVersio