The Baltic ringed seal population has increased fivefold since the 1970s when long-term overhunting and environmental toxins endangered the future of the species. A new statistical model now revises the population estimate.
Since its lowest point in the 1970s, the Baltic ringed seal population has grown from around 5,000 to 25,000 individuals. Behind this increase are hunting bans and the phasing out of environmentally hazardous substances such as PCB and DDT. The decline in environmental toxins has allowed the reproductive ability of ringed seals to recover to normal levels, typical in a healthy environment.
"Our choices can also have a positive effect on nature. Without the corrective decisions made in the 1970s, the ringed seal population would not be where it is today," says Professor Jarno Vanhatalo of the University of Helsinki's Faculty of Biological and Environmental Sciences and Faculty of Science.
A recent study led by Prof. Vanhatalo explored how human activity and environmental changes - such as the reduction of pollutants in the Baltic Sea - affect the structure and development of the ringed seal population. The researchers developed a new statistical method that integrates several long-term datasets. This allowed the ringed seal population to be studied in greater detail than would be possible using any single dataset. The key datasets included aerial counts as well as samples collected from dead ringed seals, which offer information on the age and gender structure of the population and the reproductive history of the females.
"Modern techniques enable digital population modelling, which allows us to estimate how the population structure and the behaviour of the individuals change in response to variations in the weather, hunting pressure and other external factors," says Vanhatalo.
Hunting significantly affects ringed seal populations
The ringed seal numbers observed in aerial counts have varied considerably in recent years. Modelling indicates that this variation can be largely explained by changing ice conditions linked to climate change, which is thought to affect seal behaviour. Ice coverage influences how many ringed seals are on the ice and thus visible during the surveys.
Modelling suggests that the resumption of seal hunting in 2015 has also considerably affected the population size and growth rate.
"Hunting affects the ringed seal population. We estimate that, without hunting, the population would grow annually by about 7%. However, hunting has lowered the growth rate to less than 5%, and even small increases in hunting quotas are likely to cause a further decline," says the article's lead author Murat Ersalman.
The study demonstrates that modern modelling is an effective tool for monitoring animal populations and aids in decision-making, such as setting hunting quotas. The results also support conservation efforts in the Baltic Sea and the work of the Baltic Marine Environment Protection Commission (HELCOM).
The study was carried out in collaboration with Natural Resources Institute Finland, the University of Eastern Finland, the Turku University of Applied Sciences and the Swedish Museum of Natural History.
Original article
Ersalman M, Kunnasranta M, Ahola M, Carlsson AM, Persson S, Bäcklin B, Helle I, Cervin L, Vanhatalo J () Integrated population model reveals human and environment driven changes in Baltic ringed seal Pusa hispida botnica demography and behavior. Mar Ecol Prog Ser :0-0 https://doi.org/10.3354/meps14886
