Behavioural observations of foraging minke whales (Balaenoptera acutorostrata) in the outer Moray Firth, north-east Scotland

Abstract

The minke whale (Balaenoptera acutorostrata) is a widespread, opportunistic species showing spatial and seasonal variations in diet according to local availability of prey. Although previous research has been conducted on the foraging strategies of this small rorqual whale, in terms of prey aggregation and assimilation, relatively little has been published on the foraging association of this species with coastal seabirds. Over the past five years, minke whales occurring along the outer coastline of the southern Moray Firth in north-east Scotland during the summer and autumn months have been recorded foraging in the presence of seabirds, such as kittiwakes (Rissa tridactyla), herring gulls (Larus argentatus), guillemots (Uria aalge) and shearwaters, which form dense feeding rafts at the water's surface. The formation of bird rafts notably occurs independently of the presence of B. acutorostrata, believed instead to be the successive result of prey concentrated at the surface by predatory schooling fish from below rather than by activities of the whales themselves. In this area of the North Sea, schooling mackerel (Scomber scombrus) constitute the most significant component of the summer fish biomass and are believed to perform the role of compacting targeted sandeel (Ammodytes spp.) prey into concentrated bait balls almost exclusively. The resulting ball of prey is consequently available to the foraging whale, which can be seen opportunistically utilizing successive bird rafts rather than expending unnecessary energy corralling the Ammodytes prey by traditional, active entrapment methods. The role of the mackerel in increasing both the rate and density of sandeel ball formation (as indicated by the presence and activity of associated bird rafts) is therefore thought to be very significant in this inshore Scottish location. Changes in oceanographic variables such as water temperature have been directly correlated with the migration of S. scombrus. The observed inter-annual variability in B. acutorostrata distribution in the outer Moray Firth may subsequently be related to the respective distribution and abundance of these migratory, pelagic fish species.