Tidal and subtidal currents affect deep aggregations of right whale prey, Calanus spp., along a shelf-basin margin

North Atlantic right whales Eubalaena glacialis foraging on deep (>100 m) popula- tions of diapausing Calanus spp. in the Roseway Basin right whale Critical Habitat (south of Nova Scotia, Canada) are most often located along the southern margin of the basin. We investigated the physical and biological oceanographic characteristics that make this margin a lucrative feed- ing ground. Bottom-moored acoustic Doppler current profilers equipped with conductivity, tem- perature, and depth sensors were deployed cross-isobath on the southeastern slope of the basin to simultaneously measure variation in Calanus spp. concentration, current velocity, and water mass characteristics in time and space. Variation in upslope tidal advection of deep-basin water with densities >1026 kg m �3 and containing highly concentrated copepod aggregations was the most important process influencing cross-isobath variation in copepod concentrations on the slope. The aggregations were maintained in the slope region through time despite extensive along-isobath advection, implying the existence of copepod re-supply mechanisms to the southern margin. Pro- posed mechanisms include immigration from surface populations, horizontal advection, and gyre- like re-circulation within the basin. A simple empirically-driven particle tracking model illustrated tidally-forced particle convergence between the 100 and 140 m isobaths on the southeastern slope, followed by upward and along-isobath advection. Cross-isobath 'compression' of copepods coupled with the maintenance of their vertical position through neutral buoyancy is proposed as a mechanism resulting in accumulation on the slope, making the area uniquely beneficial to right whales feeding at depth along the slope margin.