Home About Browse Search

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

Calamnius, Linda (2017). Behaviour of grey seals (Halichoerus grypus) and their prey in and near set traps. Drottningholm Lysekil Öregrund: (NL, NJ) > Department of Aquatic Resources, Sveriges lantbruksuniversitet. Aqua introductory research essay ; 2017:1

[img] PDF


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.

Authors/Creators:Calamnius, Linda
Title:Behaviour of grey seals (Halichoerus grypus) and their prey in and near set traps
Series Name/Journal:Aqua introductory research essay
Year of publishing :2017
Number of Pages:73
Place of Publication:Drottningholm Lysekil Öregrund
Publisher:Department of Aquatic Resources, Swedish University of Agricultural Sciences
ISBN for printed version:978-91-576-9460-7
Publication Type:Report
Article category:Other scientific
Full Text Status:Public
Agris subject categories.:M Aquatic sciences and fisheries > M40 Aquatic ecology
Subjects:(A) Swedish standard research categories 2011 > 4 Agricultural Sciences > 401 Agricultural, Forestry and Fisheries > Fish and Aquacultural Science
Agrovoc terms:halichoerus grypus, seals, behaviour, predation, animal population, baltic sea
Keywords:grey seal, inshore fisheries, mitigation means, Seal Exclusion Device, Baltic Sea Area
Permanent URL:
ID Code:14203
Faculty:NJ - Fakulteten för naturresurser och jordbruksvetenskap
Department:(NL, NJ) > Department of Aquatic Resources
Deposited By: SLUpub Connector
Deposited On:28 Mar 2017 06:31
Metadata Last Modified:09 Sep 2020 14:17

Repository Staff Only: item control page


Downloads per year (since September 2012)

View more statistics