University of Eastern Finland, press release
The doctoral dissertation of Master of Science Sari Oksanen provides novel and detailed information of the movements of the ringed and grey seals in the Baltic Sea. The results reveal that Baltic ringed seals range over large areas during the open water season, but most grey seals remain on smaller areas near their terrestrial resting sites.
The stocks of the Baltic grey and ringed seals collapsed during the 20th century and in 1970s both stocks consisted of only circa 5000 seals. The populations have been recovering 1990s and at the same time the losses that seals can cause to the coastal fisheries and fish farming have increased.
The differences in the behavior of the seal species influence the effectiveness of the measures to mitigate the seal-fishery conflict and sustainable management of seal stocks. Only male grey seals were observed to visit the pontoon traps. Grey seals made trips between their terrestrial resting sites and foraging areas. Their foraging areas were situated to river estuaries and other shallow coastal areas, which are also important for coastal fishery. The observed fidelity to given foraging areas suggest that removal of individuals near the fishing gear could be one method to mitigate the damage caused by grey seals. Such removal would focus on those individuals that repeatedly visit the vicinity of fishing gear. The ringed seals did not exhibit similar fidelity, but they ranged over larger areas and had several spatially distinct foraging areas. This suggests that removal of ringed seals near the fishing gear may not be effective in reducing losses caused by ringed seals. Therefore, development of fishing gear and practices could provide more effective mitigation measures.
Although ringed seals move over wide areas during the open water season, especially adults are quite sedentary in the breeding season in winter. In contrast, the grey seals left their open-water home ranges during the ice formation and occupied areas with little ice-cover. Therefore, although the extent of overall movement was similar in both species, their seasonal patterns of movement differed.
Ways to safeguard the future of the seal stocks
The other side of the seal-fishery conflict is the by-catch mortality of seals in the fishing gear. A device to reduce seal by-catch in trap nets, so-called seal sock, was tested. The sock enables a seal to have access to breathe on the surface while it is trapped inside a trap net. The sock proved to be an effective way of reducing the ringed seal mortality in trap nets, but it did not work as well with the grey seals. Reducing incidental by-catch is one method to develop sustainable fisheries. For example the MSC-certificate that is granted for fisheries or fish products take reduction of by-catch into account. Although both of the seal stocks have been increasing in recent decades, climate change, for example, can give rise to new threats and especially ringed seal is very dependent on the snow and ice in its breeding. Information on the movements can also been used in protecting parts of the important habitats of the seals.
The movements of Baltic seals were investigated with GPS satellite telemetry in co-operation with Natural Resources Institute Finland.
The doctoral dissertation of Master of Science FM Sari Oksanen entitled Spatial ecology of the grey seal and ringed seal in the Baltic Sea – seeking solutions to the coexistence of seals and fisheries will be examined at the Faculty of Science and Forestry. The opponent in the public examination will be Doctor Jonas Teilmann, Aarhus University and the custos will be Professor Raine Kortet, University of Eastern Finland.
Date and venue: 20.11.2015 at 12 noon, N100, Natura, Joensuu campus