Le sud du golfe du Saint-Laurent comme habitat d'alimentation de la baleine noire de l'Atlantique Nord

Abstract

The spatial distribution of North Atlantic Right Whales (NARW, Eubalaena glacialis) on northwest Atlantic foraging grounds shifted in the early 2010s, with the southern Gulf of St. Lawrence (sGSL) becoming especially important for NARW since 2015. Recent DFO field- and model-based research is synthesized within this document to assess the sGSL as important habitat for the NARW, focusing on the foraging function. Three species of Calanus spp. copepods, the preferred prey for NARW, are seasonally abundant in the sGSL. During late fall and winter, when Calanus spp. are in diapause in deep water, Calanus spp. abundances are low in the sGSL. Transport from the adjacent Laurentian Channel, mainly via the Gaspé Current, resupplies the sGSL when Calanus spp. emerge from diapause and return to the surface layer in an active state. Calanus hyperboreus and C. glacialis are transported into the sGSL in circa March-July, and both transport and local production supply C. finmarchicus to the sGSL from circa March through summer and early fall. The magnitude of resupply depends on upstream concentrations and circulation patterns during the Calanus spp. active periods. Aggregation of copepods to high concentrations required for NARW feeding (i.e., on the order of 1000 individuals m-3 or greater) can occur through interactions of vertical movements and local circulation features or barriers (e.g., the seafloor). Dense near-bottom layers composed mainly of C. hyperboreus, with energy levels around the threshold for profitable NARW foraging, have been observed in the sGSL as early as mid-May, and dense mixed-species layers have been observed locally in the sGSL in summer. These observations and simulations indicate that the environmental, oceanographic, and bathymetric conditions of the sGSL can support and aggregate high concentrations of prey for NARW. Characterizing the processes driving distribution and dynamics of Calanus spp. in the sGSL provides a basis for assessing foraging habitat and its vulnerability to future environmental and circulation changes.