MEPS 497:69-85 (2014)  -  DOI: https://doi.org/10.3354/meps10584

ABSTRACT: North Atlantic right whales Eubalaena glacialis aggregate in Roseway Basin on the western Scotian Shelf where their energy-rich food sources, diapausing copepods Calanus spp., are concentrated at depth. The Canadian Species at Risk Recovery Strategy for right whales provides provisional Critical Habitat (CH) boundaries for Roseway Basin based on right whale occupancy, with the stipulation that the boundaries be refined using spatially explicit indicators of CH, specifically the environmental, oceanographic, and bathymetric conditions responsible for the copepod aggregations and distribution. We measured the concentration, energy density, spatial distribution, and extent of the right whale food base and the related oceanography at depth in the Basin during late-summer 2008 with the goal of refining the spatial extent of right whale CH. We show that the diapausing copepods were distributed throughout the Basin, with elevated concentrations located at depth toward the northeast and along the southern Basin margin slope. The aggregations were associated with warm, salty, high-density (26.0 to 26.2 σ_t, kg m^-3), continental slope-influenced water masses and not with cold, fresh, low-density (<26.0 σ_t) water masses originating on the Scotian Shelf. Tidally driven variation in the copepod aggregations across the southern slope was coincident with the movement of the 26.0 σ_t isopycnal. We propose a mechanism, based on water mass density and advection, that explains the spatial and temporal (e.g. inter-annual) variation in diapausing Calanus energy density (joules per unit ocean volume) distribution, and by proxy, variation in right whale occupancy of the Basin. We further propose that the provisional CH boundaries, partly conditioned on vessel-strike mitigation measures, be extended to better encompass the critical feeding habitat. This could be achieved without compromise regarding vessel traffic and existing vessel-strike mitigation.

KEY WORDS: North Atlantic right whale · Critical Habitat · Roseway Basin · Water mass density · Slope water · Calanus finmarchicus · Calanus hyperboreus · Conservation