Loads applied to aquaculture nets by the biting behaviour of Atlantic cod (Gadus morhua)

Atlantic cod (Gadus morhua) bite at netting materials in sea-cages, which is known to weaken and destroy net materials, contribute to the formation of net holes and lead to escapes. We determined the lateral pulling forces applied to nets through biting by cod in a laboratory set-up. A sensor for automatic logging of the lateral pulling force was used to obtain data for over 1000 undisturbed, objectively collected biting events. Lateral pull force varied between cod of different size (180 and 610 g) and different net types, ranging from 0.9 to 9.4 N. Further, pull forces were on average 1.5–2.5 times stronger during lengthy biting events compared to single, brief bites. Maximum lateral pulling forces exerted by 610 g cod were similar to their body weight in air, while 180 g fish were able to pull with a force equivalent to twice their own weight in air. All recorded lateral pull strengths significantly exceeded the known force needed to break single nylon fibres of the netting material. Our results advance understanding of mechanisms and forces involved when cod interact with sea-cage nets and provide important baseline data for the design and production of more bite-resistant netting.