Assessments of the seal population in the Baltic Sea are based on annual aerial counts. Performed when seals are moulting, aerial counts provide an efficient and quick way of assessing the size of the population.
The count results are lower than the true size of the population, since some seals are in the water, beyond the reach of the count, regardless of the weather conditions. Despite this, aerial counts spot most individuals and are therefore repeated in the same manner every year to indicate the direction in which the population is developing.
Gray seals are counted from photos
Grey seals are counted in May and June, at a time when they can be found on islets on the outer fringe of the archipelago. Counts are performed simultaneously within a period of two weeks in almost every Baltic Rim countries. The census period is short, in order to minimise multiple counts of the same individuals which may move from one place to another. The population’s core areas are counted two or three times, and the largest daily count is taken into consideration when determining the size of the population. The number of grey seals is counted from photos taken during flights, and the figure thus achieved is considered the minimum size of the population.
In Finland’s territory, the Natural Resources Institute Finland is responsible for counting the size of the grey seal population.
Ringed seals are counted while on the ice
The size of the ringed seal population in the Baltic Sea is established by transect surveys after mid-April and counting the number of seals basking on the remaining ice cover. Located at even distances from each other, transect lines make up a random sample of the ice cover. Counts of the ringed seal population therefore generate an assessment based on random sampling, since the counting routes cover only part of the ice cover.
Counts of the ringed seal population are carried out in the Bay of Bothnia, the seals’ principal distribution area. While the Swedish Museum of Natural History is responsible for performing these counts, the Natural Resources Institute Finland also participates in them.
Metsähallitus is responsible for monitoring the Saimaa ringed seal population.
Photo on top of the page: Mervi Kunnasranta