Bayesian estimation of the abundance of northern fur seals Callorhinus ursinus in the wintering and feeding ground off Sanriku on the Pacific coast of northern Japan

The abundance of northern fur seals in their wintering and feeding ground off the Pacific coast of northern Japan was estimated using standard line-transect modeling. Several nested modeling components were simultaneously estimated in a single Bayesian model, which enabled us to explicitly estimate the uncertainty of total abundance. Median abundance estimates (with 95% credible interval) were 8191 (5441:13873) in 2004, 1288 (725:2756) in 2005, and 5672 (3111:13543) in 2007. Sanriku is well known as one of the most productive fishing grounds in the world. This is the first attempt to estimate absolute abundance in this region, despite this species being a key predator in the ecosystem, which were commercially hunted before 1950s when regulation was tightened. Although these values represent only a part of the population migrating from breeding rookeries, the waters off Sanriku are their southernmost habitat, which provides important feeding grounds during winter and spring. Our estimates indicate that the northern fur seal was one of the most abundant marine mammals in the region.