Variations in minke whale Balaenoptera acutorostrata diets in response to environmental changes in the Barents Sea

Summary

Substantial changes have occurred in the Barents Sea ecosystem over the past 30 years, the most conspicuous being related to the rise and fall of stocks of the two dominant pelagic shoaling fish species: capelin (Mallotus villosus) and herring (Clupea harengus). Based on data from annual studies, the effects of these ecological changes on the diet and food consumption of the minke whale (Balaenoptera acutorostrata), an important top predator in the system, were assessed for the period 1992-1999. Following a collapse in the capelin stock in 1992/1993, minke whales foraging in the northern Barents Sea apparently switched from a capelin-dominated diet to a diet almost completely comprised of krill (Thysanoessa sp. and Meganyctiphanes norvegica). The southern region of the Barents Sea represents important nursery areas for the Norwegian spring-spawning herring. Good recruitment to this stock gives strong cohorts and large numbers of young, immature herring (0-3 years old) which serve as the main food for minke whales feeding in the area. Recruitment failure with subsequent weak cohorts seems, however, to reduce the availability of immature herring to such an extent that minke whales switch to other prey items such as krill, capelin and, to some extent, gadoid fish. The annual changes in prey abundance and body condition, measured as a girth and blubber index, were weakly correlated. However, the comparison of minke whales' body condition in years of high and low abundance of immature herring showed that minke whales in the southern Barents Sea, particularly immatures and adult females, were in significantly better condition in years when the abundance of immature herring was high.