Occurrence, behavior and genetic diversity of bowhead whales in the Western Sea of Okhotsk, Russia

Abstract

The Okhotsk Sea (OS) stock of bowhead whales was severely reduced by commercial whaling in the late 19th century and again by illegal Soviet pelagic whaling in the 1960s. Estimates of the population prior to exploitation range from 3,000 to 6,000. A crude estimate of the population in the 1980s was 100-300. Two seasons were spent in field camp near Cape Ukurunru in the Shantar Archipelago in the northwestern Sea of Okhotsk to begin to define summer habitat use, behavior, and genetic diversity of the OS bowheads. Bowheads were seen in both Ulbanskiy and Konstantina Bays, with more whale groups seen in Ulbanskiy Bay. Bowhead groups appeared to be concentrated around Cape Ukurunru. Whale groups were seen at an equal rate in August and September. Underwater feeding behavior was observed most frequently. Genetic diversity of the OS stock was compared to the Bering-Chukchi-Beaufort Seas (BCBS) stock with mitochondrial DNA sequences and nine nuclear DNA microsatellite loci. Individuals identified by mitochondrial haplotype, sex, and nuclear DNA microsatellite genotypes were used in a mark-recapture estimate of population size. Sixty seven individuals were identified from the OS stock, resulting in an estimate of 566 with variance from 67 to 189,000. A minimum population estimate with 95% confidence is 247. Seven mitochondrial DNA haplotypes were identified from the OS stock. The OS stock was significantly different from the BCBS population with both an exact test of population differentiation and an AMOVA. A nested clade analysis of haplotype frequency and geographic distribution of haplotypes with the OS and BCBS populations suggested reduced gene flow with segregation by geographic distance and contiguous range expansion between the OS and BCBS at low-level clades, but was unable to differentiate the populations at high-level clades. These results suggest that the OS and BCBS stocks share common ancestry, and have mixed repeatedly throughout the evolution of bowheads in the north Pacific.