Effect of codend mesh sizes on the size selectivity and exploitation pattern of cocktail shrimp (Trachypenaeus curvirostris) in shrimp trawl fishery of the South China Sea

This study aims at improving fishery management by testing and comparing the size selectivity and exploitation pattern of diamond-mesh codends, with mesh sizes ranging from 25 to 54 mm, for cocktail shrimp (Trachypenaeus curvirostris) in the South China Sea (SCS). Beginning from 25 mm, the minimum mesh size regulated by the shrimp trawl fishery industry, we investigated how the mesh sizes of the codends would affect the size selectivity and exploitation pattern in reference to the minimum conservation reference size (MCRS, 7.0 cm total length) of cocktail shrimp in the SCS. According to our results, the selective properties of the codend mesh sized at 25 mm in line with the regulation failed to protect the undersized individuals of cocktail shrimp because of its 50% retention length of 5.85 cm (confidence interval, CI: 5.16–6.18 cm), far less than the MCRS of cocktail shrimp. As the retention probability of a shrimp with a MCRS length was as high as 97.45 % (CI: 86.86–100.00%), more than 40% of the undersized shrimp was retained. It was proved that increasing the codend mesh size did not significantly improve the size selectivity and exploitation pattern which might be improved by other gear modifications.publishedVersio

Escape, discard and landing probability of Nephrops norvegicus in the Mediterranean Sea creel fishery

Size selection in creel fishery consists of two processes: the first taking place in the creel on the seabed and the second made by the fisher on the vessel. However, no study has ever considered both processes when assessing the size selection in creel fisheries. This study presents a framework for including both and demonstrates it to predict the effect of mesh size and shape on the creel fishery targeting the Norway lobster (Nephrops norvegicus) in the Mediterranean Sea. For this specific fishery, we demonstrate that both processes play a role in the overall size selection. Furthermore, we predict an optimal creel mesh size, which potentially eliminates the second process taking place on the vessel, while maintaining high efficiency for the first process on the seabed for the targeted sizes of Nephrops. The approach here presented can be also applied to other creel fisheries

Size-dependent escape risk of lumpfish (Cyclopterus lumpus) from salmonid farm nets

In the last decade, the salmon aquaculture industry has considerably increased the use of lumpfish juveniles as cleaner fish. Potential escape of reared lumpfish into the wild may spread diseases or genetically contaminate wild stocks. The guidelines for minimum sizes of cleaner fish to use in aquaculture cages are currently based on simple mesh penetration tests. However, these guidelines do not consider the potential compressibility of fish or changes in mesh state due to factors such as sea conditions and maintenance operations. This study shows that the industry-recommended minimum stocking sizes for a given mesh size may result in escape risk and that ignoring fish compressibility and mesh state can lead to underestimation of the lumpfish sizes that are able to escape. Our results can be used to develop new guidelines that will contribute to reduced escape of lumpfish from salmonid farms and lessen the potential environmental consequences.acceptedVersio

New codend designs to improve the size selectivity of fyke net for narrow-clawed crayfish (Pontastacus leptodactylus)

The narrow-clawed crayfish (Pontastacus leptodactylus) is one of the most ecologically and economically important freshwater species in Turkey. However, the harvest of this species has declined from 7937 t in 1984 to 696 t in 2019. One of primary reasons for this decrease in stock is the use of fishing gears with poor size selectivity. In this study, new codend designs were investigated to improve the size selectivity of fyke nets for P. leptodactylus. Seven codends of three different designs were tested: (1) a commercially used standard codend with 34 mm mesh size (Com34); (2) sorting grids with 20 (SG20), 25 (SG25) and 30 mm (SG30) bar spacing attached to the last circle of the net; and (3) a stiff rigged net, the last part of the fyke nets (codend) with 34 (SRN34), 42 (SRN42) and 50 mm (SRN50) mesh size. The average length at 50% retention probability (L50) and selection range (SR) (L75–L25) values of Com34 were 9.4 cm and 3.1 cm, respectively. For the SG20, SG25 and SG30 grids, the L50 values were 11.3 cm, 11.6 cm and 12.0 cm, while the SR values were 1.4 cm, 1.3 cm and 0.6 cm, respectively. For the SRN34, SRN42 and SRN50 codends, the L50 values were 10.9 cm, 11.6 cm and 11.6 cm, while the SR values were 2.1 cm, 2.1cm and 1.1 cm, respectively. Overall, the commercial codend resulted in lower L50 values when the minimum conservation reference length of 10 cm was considered. However, all tested new codend designs showed improved selectivity compared with the standard, with optimum results obtained with SG20. SG20 grid decreased discarding by 15.7% compared to the classic commercial fyke net; thus, this is a very important result for the sustainability of natural P. leptodactylus stocks.publishedVersio

Prediction of goldsinny wrasse (Ctenolabrus rupestris) minimum size required to avoid escape through salmon (Salmo salar) farm nets

The salmon (Salmo salar) aquaculture industry has used various practices to treat salmon louse (Lepeophtheirus salmonis) infections, including mechanical, thermal, and chemical treatments. However, due to the environmental and fish welfare issues, and limitations associated with these practices, the use of cleaner fish has become a popular complementary delousing method. The goldsinny wrasse (Ctenolabrus rupestris) is the smallest among the delousing species used, which makes it most susceptible to escape through the salmon cage netting. Escape of goldsinny wrasse into the wild would lead to additional costs for farmers and could spread diseases and genetically contaminate wild stocks, as these cleaner fish are often transported over large distances from the capture site to the aquaculture site. The Norwegian aquaculture industry sets the goldsinny wrasse minimum size for use with different salmon cage mesh sizes using guidelines based on simple penetration tests that do not consider the potential compressibility of fish or changes in mesh state due to factors such as sea conditions and maintenance operations. Using morphological measurements of 100 fish, results of 47,800 penetration tests, and computer simulation, we predicted the minimum sizes required to avoid escape of goldsinny wrasse through and array of salmon farm nets. Our results show that the minimum stocking sizes for goldsinny wrasse in the current guidelines used by the industry may pose escape risk. Further, our findings illustrate the importance of considering fish compressibility and mesh state when estimating the fish minimum sizes required to avoid escape via cage netting mesh penetration. Finally, the results presented here provide the salmon farming industry with new guidelines that will contribute to reducing goldsinny wrasse escape from the cages and ultimately lessen the potential environmental consequences of such escape. For the two most typical mesh-size used by the aquaculture industry in Norway i.e. 30 and 50 mm and under the worst-case scenario i.e. slack state, the minimum recommended lengths for goldsinny wrasse would be ~128 (~28) and ~ 206 mm (~123 g). These sizes represent an underestimation of 16 and 24% in the sizes of goldsinny wrasse necessary to avoid escape compared to the current guidelines based on stiff meshes.publishedVersio

Gillnet colour affects catch efficiency in pearlspot (Etroplus suratensis) tropical estuary fishery

Gillnets are one of the most commonly used fishing gears in both marine and inland waters. Different colour filament nettings are often used in gillnets. However, the effect of gillnet colour on catch efficiency is unclear for most fisheries. Therefore, in this study we investigated the effect of gillnet colour on the catch efficiency in pearlspot (Etroplus suratensis) fishery in Vembanad lake, India. Gillnet colours tested were transparent, green and blue. Results showed that in this fishery, the catch efficiency of gillnets of the three colours tested differed significantly. The highest catch efficiency of pearlspot was shown by green gillnets compared to transparent or blue netting. Specifically, green gillnets on average were estimated to be 74% more efficient compared to transparent gillnets. For the green compared to the blue gillnets, the catch efficiency was estimated to be higher by 81%. These results demonstrate that gillnet colour can be an important factor significantly affecting the catch efficiency of this fishing gear