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Rapport2017Öppen tillgång

Behaviour of grey seals (Halichoerus grypus) and their prey in and near set traps

Calamnius, Linda


The seal populations of the Baltic Sea Area were at historically low levels in the 1970’s, due to two factors. The first was an extensive hunt and the second was emissions of organochlorines, which affected the reproductive abilities of the females. Laws and regulations were set in force to improve the marine environment and from the early 1990’s the populations of the grey seal (Halichoerus grypus), the harbour seal (Phoca vitulina) and the ringed seal (Pusa hispida) have recovered. It is a success for the management of the Baltic Sea Area environment that the seal populations have increased. Coupled with the increase of the populations, is also an increase of conflicts with the inshore fisheries. The seals frequenting the fishing gear takes fish and damage gear. It is mainly the grey seal which is the culprit and it is predominantly males. These males have been proven to be specialists. Two studies were conducted to learn more about the behaviour of seals and of their prey. The first study investigated the pattern of seal visits in the middle chamber of a herring pontoon trap. A camera filmed the seals which entered and the seals were identified. There were almost 1400 visits by 12 individuals. Of all visits, 84 % took place within 5 minutes of each other. Of all visits, 3.7 % were concurrent visits, i.e. two males inside the middle chamber at the same time. By studying these visits in detail, it could be concluded that there was a dominance hierarchy among the seals in the trap. A simulation of the visits was executed to examine whether the proportion of concurrent visits was random or non-random. The simulations used the same distribution pattern of the realized visits. If the visits had been random, then c 9.5 % of them would have been concurrent. This suggests that there is a pattern to their visits. The second study investigated the effect of a Seal Exclusion Device (SED) on seal visits and on catch. The experiment was conducted during two years. In 2012, using a SED with a diamond mesh and in 2016 using two SEDs; the diamond mesh and a square mesh - with the entire frame rotated 45°. The expectation was that the SEDs would reduce the number of seal visits, increase the catch and possibly deter larger fish from entering. In 2012, the diamond mesh had an effect on the size of trout, with larger trout entering the control trap, whereas large salmon were not affected by the presence of the SED. In 2016, larger salmon were caught in the traps with a SED. There was no significant result regarding the catch of trout, possibly due to small samples. The number of seal visits in both sets of experiments were too low to be able to draw any conclusions regarding presence of seals. The SEDs did not have any effect on the quantity of caught fish.


grey seal, inshore fisheries, mitigation means, Seal Exclusion Device, Baltic Sea Area

Publicerad i

Aqua introductory research essay
2017, nummer: 2017:1
ISBN: 978-91-576-9460-7
Utgivare: Department of Aquatic Resources, Swedish University of Agricultural Sciences