End-of-Century projections of Calanus species biomass in the Gulf of St. Lawrence, southern Newfoundland, Scotian Shelf and northeast Gulf of Maine

The goal of this study was to describe the potential trajectory of Calanus species total biomass in response to future environmental conditions in eastern Canadian waters toward to the end of the 21st century. This was accomplished by using an approach combining Species Distribution Models (SDMs) and regional climate ocean simulations based on the Representative Concentration Pathway 8.5 scenario to project interdecadal abundance and individual body weight. We first built SDMs of C. finmarchicus, C. glacialis and C. hyperboreus with data from the Gulf of Maine to the Gulf of St. Lawrence using covariates extracted from three simulations of a regional climate ocean model driven with downscaled forcing from different Earth System Models. These SDMs were then used to perform past and future decadal projections of abundance and their associated uncertainty among the three regional climate ocean simulations. These abundance projections were then combined with those of individual body weight to describe the interdecadal changes in total Calanus biomass. Our SDMs performed similarly well among the three regional climate scenarios. While greater uncertainty was associated with the end of the projection period, total Calanus biomass was predicted to decrease toward the end of the century across Canadian waters. However, marked regional differences were noted in amplitude due to varying interplay between local environmental conditions, Calanus species composition and individual body weight. In the Fundy-northeast Gulf of Maine and western Scotian Shelf regions in the south, a 60% decrease in total Calanus biomass was predicted in 2080-2089 relative to 2000-2009 and was solely driven by a decrease in C. finmarchicus abundance and individual body weight. This sharp decrease thus indicated that environmental conditions in these regions would become unfavorable for this species in the future. In the Gulf of St. Lawrence, a 40% decrease in C. hyperboreus biomass was partly compensated by a small increase in C. finmarchicus biomass, resulting in a 25% decrease in total Calanus biomass over the same period, indicating that environmental conditions in this colder region would gradually become less and more favourable to C. hyperboreus and C. finmarchicus respectively. The decrease in total Calanus biomass in the southern Newfoundland region was similar (25%) to the Gulf of St. Lawrence, whereas the decrease forecasted in eastern Scotian Shelf was intermediate (35%) between the southern and northern regions. Assuming that no drastic changes in the ocean circulation occur during the 21st century, our long-term projections suggested that North Atlantic Right Whales (NARW) foraging conditions in the Fundy - northeast Gulf of Maine and western Scotian Shelf will continue to deteriorate at a rate greater than in more northern regions. These predicted patterns of change in future foraging conditions could therefore reinforce the pattern in NARW habitat use and distribution observed since 2010.

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