The summer distribution of coastal minke whales ( Balaenoptera acutorostrata) in the southern outer Moray Firth, NE Scotland, in relation to co-occurring mesoscale oceanographic features

Abstract

Data on the distribution of northern minke whales ( Balaenoptera acutorostrata) in the coastal waters of the southern outer Moray Firth in northeast Scotland were collected from May to October, 2000 to 2004 inclusive. During this period, 127 encounters with B. acutorostrata were recorded during systematic boat-based surveys. The encounters were subsequently converted to number of animals sighted per unit of survey effort (SPUE) for comparison with mesoscale oceanographic features. Using Advanced Very High Resolution Radiometer (AVHRR) and Sea-viewing Wide Field-of-view Sensor (SeaWiFS) imagery, monthly composite images of sea surface temperature (SST) and chlorophyll- a concentration (Chl-a) were obtained. In the SST imagery two oceanographic features appeared to dominate the Moray Firth region: a cold water current and a warm water plume. The results show that the SPUE of B. acutorostrata was significantly higher during warm plume events than when the cold current was dominant (Mann–Whitney U, P = 0.0167). Levels of phytoplankton biomass also appeared to be substantially greater during warm water plume events. In conclusion, it is hypothesised that those areas of highest minke SPUE are directly related to the presence of targeted prey attracted by high densities of phytoplankton. Although these whales were found to occur across the survey area, throughout the study period, their distribution is concluded to be highly dependent upon the presence of co-occurring mesoscale features. These, as well as other physiographic environmental determinants, are all thought to be important in forming this heterogeneous coastal ecosystem.