Habitat Selection and Niche Characteristics of Rorqual Whales in the Northern Gulf of St. Lawrence (Canada)

Abstract:

Habitat selection influences the distribution patterns of animals and how they partition their ecological niches. However, studies of habitat selection seldom model temporal variability and focus primarily on terrestrial ecosystems where habitat patches change over comparatively long time scales. In marine ecosystems, ignoring the time-varying characteristics of habitats might lead to a poor understanding of ecological relationships. Blue (Balaenoptera musculus), finback (B. physalus), humpback (Megaptera novaeangliae) and minke (B. acutorostrata) whales occur in sympatry in the Gulf of St. Lawrence (Canada) during the feeding season. I combined boat surveys with remote-sensing technology and computer ocean models to study habitat selection and niche characteristics of these rorqual whales for the years 1997-2002. All four species responded to the temporal variability of their environment by selecting a combination of time-varying and static factors that differed from the available habitat and were likely linked with prey availability, namely shallow banks, steep seabed slope, cold and saline surface waters, deep and cold intermediate layer. Moreover, distribution of whale sightings was highly correlated with thermal fronts, a dynamic meso-scale process that increases biological productivity and aggregates prey. Spatial distribution differed among species, likely reflecting differences in feeding strategies that could have evolved to alleviate inter-specific competition. There was clear spatial partitioning of the habitat use between species, and some separation in time. Fine-scale selection of dynamic variables appears to be the main mechanism facilitating cooccurrence. Minke whales had a specialised niche centred on shallow coastal waters. Deeper offshore waters and a colder intermediate layer characterised the niches of finback and humpback whales. The endangered blue whale had the narrowest niche, characterised by high salinity cold surface temperature and slow currents. This likely results from its specialised diet and may render it vulnerable to environmental change and competitive pressure. In humpback whales, inter-individual variation was an important part of intra-population variation and contributed to the large niche width of that species. Individual specialisation, probably at the level of foraging techniques and diet preferences, could be an important mechanism for finer subdivision of the species’ niche. These findings greatly increase our understanding of habitat selection in rorqual whales, and encourage a wider use of dynamic environmental variables in future studies and management plans. 

Doniol-Valcroze, T. 2008. Habitat Selection and Niche Characteristics of Rorqual Whales in the Northern Gulf of St. Lawrence (Canada). PhD Thesis, McGill University, Montreal, Canada. 146 p.
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