Research Document 2020/033
Significance of dominant zooplankton species to the North Atlantic Right Whale potential foraging habitats in the Gulf of St. Lawrence: a bio-energetic approach
By Lehoux, C., Plourde S., and Lesage, V.
Abstract
The North Atlantic Right Whale (NARW) is an endangered species that feeds primarily on Calanus finmarchicus in their traditional feeding areas in the western North Atlantic. In recent years, the Gulf of St. Lawrence (GSL) was identified as an important feeding ground for this population. In this region, the large and lipid-rich C. finmarchicus, C. hyperboreus and C. glacialis are abundant species, but other small calanoids such as Pseudocalanus spp. and Temora spp. and krill are also numerically abundant. Since the diet of NARW is unknown in the GSL, the potential relative contribution of these prey to NARW in early and late summer between 2006 and 2017 was assessed using a bioenergetics model. The model used a 3D preyscape, which was predicted using the depth-integrated biomass of zooplankton and species-specific vertical distributions. The model assessed the energy gained according to prey density in each 10 m-depth bins, and the energy requirement of mature females that were resting, pregnant or lactating. A suitable habitat was defined as a positive difference between the energy gained and the energy required divided by the energy required. The difference in habitat suitability between the period before and after 2010 was assessed as well as the sensitivity to model parameters. The effect of zooplankton vertical distribution on their availability to NARW, and the relationship between habitat suitability and bathymetry were also examined. C. hyperboreus was identified as the prey contributing the most energy to NARW compared to the other prey species. This was true with the exception of the southwestern GSL in August-September when C. finmarchicus contribution increased relative to the June-July period. The contribution of other prey species was overall negligible. The southwestern GSL was identified as the most suitable habitat for NARW in June-July, whereas the northern GSL, and more specifically the area to the north of Anticosti Island was identified as the most suitable habitat in August-September. Using the model parameters minimizing net energy gain, there were almost no habitats that provided sufficient energy for NARW to meet their daily energy requirements. When using the model parameters maximizing the net energy gain, very large areas that encompassed more than 50% of the southwestern GSL in June-July and the northern GSL in August-September appeared suitable. While the energy gained did not markedly decrease in the northern GSL between 2006-2010 and 2011-1017, it did decrease in the southwestern GSL, resulting in a lesser persistence of NARW suitable habitat in the region during the latter period. The distribution of NARW suitable habitats changed between the two periods, with suitable areas in the Shediac Valley and on the Orphan Bank in June-July, and to the north of Anticosti Island in August-September being larger in 2011-2017 compared to the 2006-2010 period. In recent years, the abundance of large Calanus species decreased while the abundance of small calanoids tended to increase. If this trend continues, the energy available to NARW is likely to decrease and possibly not be sufficient to meet their energy requirements.
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