What attracts Baltic sea grey seals to seal-safe cod pots and when do they attempt to attack fish in the pots?

Seals and fishermen share the role of top consumers in the Baltic Sea, leading to inevitable competition. One aspect of this is that fishermen use fishing gear to catch fish and seals raid these fishing gear. The fisheries lose out in terms of fish catches and also bear the significant costs of damage to the gear.

Researchers have been active for some years in developing ‘sealsafe’ fishing gear, which will be unattractive to seals and resistant to attacks. This study investigated the presence of grey seals (Halichoerus grypus) around cod pots and their attempts to take fish from them. Baited and camera-equipped cod pots of three designs including three netting types were set out close to a seal haul-out site east of the island of Gotland in the Baltic Sea.

The behaviour of visiting seals filmed with underwater cameras was observed and analysed using a generalized linear model (GLM). As well as the cod pot characteristics, the variables used for modelling included the time of day, whether bait fish were alive or dead, and the quantity of fish in each pot.

It was found that the most important cod pot-characteristic for both seal presence and ‘attack behaviour’ was the design of the cod pot. The design which attracted the most seal presence and the most fish-attacking behaviour had loose netting around the upper chamber, in contrast to the other two designs which had tightly stretched mesh.

Neither mesh size nor material showed any correlation with seal presence or attack behaviour. It was also found that the most important overall factor for predicting attack behaviour was the time of day. There was individual variation in seal behaviour. The behaviour was categorized into eleven groups, of which ‘investigation’ was the most commonly observed.

Most attack behaviours were targeted towards moving fish and no attacks occurred on dead fish. These results could suggest that seals are visiting cod pots because of curiosity and not primarily because of hunger.