Population genetics and behavioural ecology of North Atlantic minke whales (Balaenoptera acutorostrata)

Abstract

Regional habitat use by a species, dictated by the spatial and temporal availability of resources, influences its distribution patterns and ultimately population genetic structure. Seasonal migrations between geographically separated breeding and feeding areas, as occur in many baleen whales, can complicate these relationships. Here I try to integrate the population structure of minke whales over the whole North Atlantic with regional habitat use and behavioural adaptations to a particular summer feeding ground, the Hebrides off West Scotland. Whereas no genetic differentiation could be found between separate feeding areas as far apart as Canada, the UK and Svalbard, using microsatellites and mtDNA, the presence of two cryptic breeding populations was detected, which form mixed assemblages on feeding grounds across the North Atlantic. This implies fidelity to at least two breeding grounds irrespective of proximity to feeding areas, i.e. extensive seasonal migrations (over half the North Atlantic or more), which may require a re-assessment of current management stocks. These findings were consistent with the mobility and flexibility in habitat use and behaviour observed within the Hebrides. Results from Generalized Additive Models indicated that minke whale distribution was dependent largely on temporally variable parameters (temperature in spring, chlorophyll concentration in autumn), besides depth and, to a lesser extent, topography. However, fine-scale foraging behaviour was dictated primarily by the strength and direction of tidal currents. Distribution patterns according to environmental parameters changed through the season, but were largely consistent between the entire Hebrides (cell resolution of 4min) and a smaller core study area (2min), and over a time period of 15 years. Significantly higher sighting rates in areas of likely sandeel presence in spring, but not during the rest of the season, combined with prey samples from the core study area consisting almost entirely of sprat in August/September, indicate a switch in diet between early and late season and are consistent with the changes in habitat use. Site fidelity within the core study area was high only during periods of high feeding activity, but low at other times and between years, so that individual specializations to fine-scale feeding areas, as observed off Washington State, seem unlikely. Significant interannual changes in minke sighting rates between 2003-07, both within the core study area and over the entire Hebrides, were paralleled by changes in phytoplankton concentration, local sprat landings by the fishing fleet, and seabird breeding success and numbers counted at sea, particularly common guillemots. Auks were also the seabird guild that minke whales were most likely to associate with during foraging, taking advantage of tight bait-balls concentrated by them. The significant relationships with primary productivity make bottom-up control the most likely scenario for dictating concentrations of whale and seabird prey species in West Scotland. The ability to switch between different prey according to their availability through the season, and a distribution influenced by temporally variable parameters (temperature and chlorophyll concentration), combined with adjustments in foraging activity dependent on variable conditions at fine spatial scales (tides), enable minke whales to optimise exploitation of patchy prey concentrations.