Attack behavior and presence of grey seals around pots : a search for seal-safe cod-pots in the Baltic sea

University essay from SLU/Dept. Of Aquatic Resources

Abstract: The seals and the fishery are forced to share the role as top-predators in the Baltic waters since they both depend on the same fish stocks, which leads to an unavoidable competition. Seals are raiding traps, leading not only to costly damaged fishing gears and loss in fish catch but also to the seals themselves getting caught and drowned in the gears as by-catch. In order to solve the conflict research is working on developing ’seal-safe’ fishing gears, which are well-working fishing gears that also are unattractive to seals and that are resistant to attacks. This study investigated seal presence and attack behavior of grey seals in relation to codpots in the Baltic sea. Baited camera-equipped cod-pots of three shapes and mesh types were set out in the vicinity of a seal colony outside the east coast of the Swedish island Gotland, and the behavior of the visiting seals caught on tape was observed and analysed. It was found that the most important cod-pot component for both seal presence and attack behavior was the shape. The shape with most attack behavior and seal presence had an upper part of loose netting, in contrast to the other two shapes. Neither mesh size nor material showed any significance for seal presence or attack behavior. It was also found, through a Generalized Linear Model, that the most important factor for proportion seal presence was time of the day, where the morning had greatest proportion. Furthermore it was found that the proportion seal presence was not affected by if the bait fish were dead or alive, at the same time as no attack behavior on dead bait fish could be found. This could suggest that seals are present because of curiosity and not because of hunger. This study provides insight in how grey seals behave around different cod-pots and what factors that might affect their behavior. It shows that cod-pot-components but also factors such as the time of the day can effect the behavior of the seal.

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