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

Cheng, Zhaohai (2020) Improving the size selectivity of trawl codends for northern shrimp (Pandalus borealis) and redfish (Sebastes spp.) fisheries in the North Atlantic. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

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, size selectivity was undetermined due to small catches at those sizes. In my third experiment, I compared the performance of four different codends on the size selectivity of redfish in eastern Canada (Unit 1, Gulf of St. Lawrence). I evaluated a traditional diamond mesh codend with a nominal mesh size of 90 mm and three experimental T90 codends of different nominal mesh sizes (90, 100, 110 mm). My results demonstrated that the traditional codend was not size selective, catching greater than 97% of redfish over all of the length classes observed. Overall, my results reveal that T90 codends improve size selectivity in which large proportions of undersized fish are successfully released. In my final experiment, I examined the hydrodynamic performance of full-scale T0 and T90 codends with and without a cover net using the flume tank located at the Fisheries and Marine Institute. I measured flow velocity, mesh shape, mesh opening, and drag at various towing velocities. The results showed that the flow velocity inside each codend was lower than the towing velocity. T90 codends had higher flow velocity and better mesh opening than the T0 near the terminal end of the codend. The total drag of each T90 codend was significantly higher than the T0. With the cover net, the flow velocity in the area between codend and the cover did not change significantly for the T0 codend, but was significantly different for the T90 (90 mm) codend. In summary, the findings from this thesis confirm the importance of codend design on the size selectivity of bottom trawls. Changes in mesh size and mesh orientation in particular, were shown to significantly affect the size selectivity of northern shrimp (Iceland) and redfish (Iceland and Canada). These results could prove helpful in the pursuit of sustainable fisheries, whereby smaller undesirable or non-targeted animals can be released from codends during towing operations, preventing their unnecessary capture and mortality.

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