Improving the size selectivity of trawl codends for northern shrimp (Pandalus borealis) and redfish (Sebastes spp.) fisheries in the North Atlantic

• Main Author: Cheng, Zhaohai
• Format: Thesis
• Language: English
• Published: Memorial University of Newfoundland 2020
• Subjects: Canada; Iceland; North Atlantic; northern shrimp; Northwest Atlantic; Pandalus borealis
• Online Access: https://research.library.mun.ca/14463/; https://research.library.mun.ca/14463/1/thesis.pdf

A bottom trawl is a towed fishing gear that is designed to catch commercially important species that live in close proximity to the seafloor. In the Northwest Atlantic, bottom trawls are widely used to harvest shrimp, redfish, and various groundfish species. Coastal fishing fleets in both Canada and Iceland have been using bottom trawls to harvest northern shrimp (Pandalus borealis) and redfish (Sebastes spp.) for several decades. The codend of these fishing gears plays an important role in reducing unintended bycatch of non-targeted species and sizes of animals. Careful design and engineering of these codends is a necessary step in the fishing gear development cycle. In this thesis, I conducted different experiments, including laboratory and field work, to improve the size selectivity of codends for northern shrimp and redfish in the North Atlantic. In my first experiment, I compared the performance of different codends on the size selectivity of shrimp in the coastal fishery of Iceland. I compared codends of same nominal mesh size (42 mm) constructed using netting in the traditional orientation (T0, two-panel) against experimental codends constructed using netting rotated 45ᴼ (T45, two-panel) and 90ᴼ (T90, four-panel). My results revealed that the T90 codend retained significantly less shrimp between 9 and 19 mm carapace length than the T0 codend, and between 15 and 19 mm than the T45 codend. Since discarding of undersized shrimp is prohibited in Iceland, using the T90 codend would enable fishers to use their quotas more efficiently. In my second experiment, I compared the performance of two different codends on the size selectivity of redfish in a commercial fishery off the south coast of Iceland. The codends varied in their design, mesh size (inside-knots measurement), and construction (i.e., knotted vs. knotless). My results showed that there was no significant difference in size selectivity between the codends at lengths greater than 29 cm for S. norvegicus and 19 cm for S. viviparous. At smaller lengths, ...