How does twine thickness and mesh size affect catch efficiency and ways of capture in the Northeast Arctic cod (Gadus morhua) gillnet fishery?

Abstract

The aim of this study is to establish a general pattern for how round-fish get caught in gillnets. Understanding these patterns might further help establishing which gillnet parameters are important to consider when developing new biodegradable materials for gillnets. Developing biodegradable materials is important to reduce impacts from lost, abandoned, and discarded gillnets made of non-biodegradable materials. The study was conducted by comparing catch efficiency and way of capture of Northeast Arctic cod (Gadus morhua) in different gillnet types with two different twine thicknesses (0.7 mm and 0.8 mm) and two different mesh sizes (210 mm and 230 mm). The fishing trials were conducted onboard commercial fishing vessel ‘Karoline’ during the main season for Northeast Arctic cod in Northern Norway, lasting from late January to mid-March 2022. Furthermore, circumference measurements were collected for Northeast Arctic cod to establish a relationship between fish morphology, gillnet mesh size and fish length dependent capture in gillnets during a cruise with research vessel ‘Helmer Hanssen’. During the same cruise, to further support establishing gillnet capture patterns for Northeast Arctic cod, a laboratory experiment was conducted with a gillnet and dead cod by investigating the point of capture when letting fish through a gillnet mesh. The results from the gillnet fishing experiments did not detect any significant differences in catch efficiency between the two twine thicknesses for either mesh size, implying that capture efficiencyhas some tolerance regarding this design parameter and therefore also regarding the associated twine elasticity/stiffness. Analysing the ways of capture showed that this was dependent on the length of the fish. Overall, 76 % of the fish were caught by either gilling or wedging, the latter being the overall most dominant of all ways of capture. The remaining fish were mainly caught by snagging, while entangling constituted a minor proportion. Furthermore, the way of capture probability was not affected by an increase in twine thickness, but a significant change was observed when increasing the mesh size, more specifically snagging and wedging. Fall-through results showed similar patterns regarding length dependent ways of capture as the gillnet fishing trials and can therefore be used to supplement investigations based on the latter. For the dominant ways of capture, gilling and wedging, the cod circumference was approximately 20 % larger than mesh circumference. This knowledge can be applied to select the mesh size for the gillnets, dependent on what size of cod that are targeted at the specific area. The results from this study have increased the understanding of the effect of twine thickness and mesh size for capture patterns. This is important knowledge for the further development of biodegradable material used in gillnets as an alternative to nylon. We now know that we can operate within a range of twine thicknesses for the same mesh size without significantly influencing the capture pattern.