The effect of hunting in Hokkaido on population dynamics of Steller sea lions in the Kuril Islands : a demographic modeling analysis

抄録

The Steller sea lions, Eumetopias jubatus, in the Kuril Islands, Russia, has declined in number since the early 1960s. One factor which may have caused their decline is hunting in the waters of Hokkaido, Japan. Hunting of sea lions has been conducted during their annual dispersal to Hokkaido in an attempt to reduce damage to local fisheries. In this study, we analyzed the effect of damage control hunting in Hokkaido on the sea lion population of the Kuril Islands from 1958 to 1994 using a demographic model which was developed as an extension of a Leslie matrix. Because a life table had not been calculated for the Kuril Islands population, we assumed that the population followed the vital parameter values estimated by Calkins & Pitcher (1982) and York (1994) for Steller sea lion populations in Alaska, USA. Data on hunted individuals were obtained from regional government records. In our simulation analysis we establish several different scenarios, and also investigate the sensitivity of our results due to uncertain parameters. Where we assumed an absence of hunting, the population was kept constant at 20, 000 sea lions, even when including the effects of environmental stochasticity on the probability of survival and fecundity. In the presence of hunting, the population decreased between 34% and 59% from the 1960s to the 1990s. When we included an effect for incidental catch by trawl vessels in the western Bering Sea and Okhotsk Sea, our analysis predicted a tendency for population declines similar to that which was observed. We conclude that the hunting of sea lions in Hokkaido most likely depleted the population in the Kuril Islands. When other factors such as incidental catch are combined with the pressures of hunting, the decline in sea lion population is even larger.