Population structure of bowhead whales (balaena mysticetus) in Disko Bay, west Greenland

• Main Author: Rekdal, Silje Larsen
• Other Authors: Lutz Bachmann og Øystein Wiig
• Format: Master Thesis
• Language: English
• Published: 2012
• Subjects: VDP::470; Canada; Greenland; Balaena mysticetus; bowhead whale; Disko Bay
• Online Access: http://hdl.handle.net/10852/34530; http://urn.nb.no/URN:NBN:no-33238

Skin biopsies of bowhead whales (Balaena mysticetus) were sampled during the spring aggregation in Disko Bay, West Greenland, over a period of 13 years, and analyzed regarding gender and genetic diversity at the mitochondrial D-loop region and at 11 microsatellite loci. By identifying recaptures through matching sex, mitochondrial haplotype and microsatellite genotype, individual interannual revisits to the bay were confirmed. These were further utilized to provide a mark-recapture population size estimate that applies to the source of the local aggregation in Disko Bay, which yielded 1219 bowhead whales (SE=278, 95%CI: 673-1765) and a corresponding estimate of 1087 female bowhead whales (SE=290, 95%CI: 518-1656) for 2012. Given that each adult female bowhead whale in the stock(s) between eastern Canada and West Greenland visits the sampling area during the reproductive cycle, the latter estimate is assumingly valid for the adult female proportion of the bowhead whales in these waters. A skewed sex ratio in the Disko Bay aggregation was observed, where females constitute an estimated proportion of 79% of the bowhead whales in the sampling area. As recent observations and early whaling records state that few calves are found in the area, Disko Bay is believed to serve as a feeding and mating ground, where adult females regain fat depots for their next calving period. The cyclicity in the female returns to the bay was thus assessed, which may arise from a multi-year migration pattern in relation to the female reproductive cycle. Although no conclusive results were obtained, a calving interval of four years would be most consistent with the data. Further, to test whether there was any substructuring of the stock in which different demes visit the bay in different years, the sampling years were analyzed with respect to both mitochondrial haplotypes and microsatellite genotypes. Global FST-value and an exact test of population differentiation were significant when based on the mitochondrial haplotypes across sampling years. When the microsatellites were investigated however, no global differentiation was detected. Slight differentiation was yet found among a few pairs of sampling years in both instances, although not coinciding among the markers. Hence, no obvious substructuring could be inferred from the data. The computer program STRUCTURE was additionally applied to the female bowhead whales sampled each year, finding that six clusters in the aggregation was the most likely number of clusters given the data. This was likely spurious and resulting from between-year recaptures, linked loci and close relatedness between the whales. In line with a star-shaped haplotype network and a sudden increase in the abundance in Disko Bay around the turn of this millennium, a population expansion was consequently implied. Although a recovery of the bowhead whale stocks after the extensive whaling during the 18th and 19th century is evident, close monitoring of the species is recommended in order to understand and manage it properly. This thesis is contributing to an extended dataset of bowhead whales in Disko Bay, of which most of its biology remains to be unveiled.