Abundance Estimates of Dalli-Type and Truei-Type of Dall’s Porpoise Phocoenoides Dalli in the Western Central Part of the Sea of Okhotsk, July–September Between 1990 and 2010

The abundance of dalli - and truei -types of Dall’s porpoise Phocoenoides dalli was assessed based on the multicovariate distance sampling method, using line-transect survey data collected in the western central part of the Sea of Okhotsk between 1990 and 2010. The estimated abundance are 77,856 individuals in 1990, 72,303 in 2009 and 111,402 in 2010 for dalli -type, and 120,198 individuals in 1990 and 101,173 in 2009 for truei -type. Abundance was not estimated for truei -type in 2010 because of small sample size. During this period, abundance seemed to fluctuate, but statistically significant trends could not be detected in abundance. Considerable small numbers of sightings of truei -type in 2010 could be attributed to insufficient survey effort in the southeastern region and warmer temperature in the southwestern region. We surmise that the abundance of Dall’s porpoise migrating into the Sea of Okhotsk during summer depends on interannual shifts in environmental conditions and resulting prey availability.