Backcasting population sizes of ringed and grey seals in the Baltic and Lake Saimaa during the 20th century

摘要

Baltic Sea populations of the ringed seal (Phoca hispida) and grey seal (Halichoerus grypus) have been dramatically declining throughout this century. They are currently recovering from a population low in the 1970s, but sizes before the decline remain unknown. The land-locked Saimaa ringed seal (Phoca hispida saimensis) has similarly been declining over much of the century. We use past bounty statistics to estimate sizes of these three populations at the beginning of this century, and investigate the sensitivity of our results to assumptions on population growth, density dependence, the true extent of kills, and the age of killed individuals. The possible range extends from 50 000 to 450 000 individuals for Baltic ringed seals, from 30 000 to 200 000 for grey seals, and from only 100 to 1 1300 individuals for Saimaa seals. The larger estimates apply if hunting tended to remove adult individuals with high reproductive value, and if the initial population was regulated by density dependence rather than kept below its true carrying capacity by nineteenth-century hunting. The low estimate for the Saimaa seal may either indicate that it was initially less abundant than thought, or that known hunting statistics simply do not capture the true magnitude of the true magnitude of the decline.