A biophysical model of Calanus hyperboreus in the Gulf of St. Lawrence: Interannual variability in phenology and circulation drive the timing and location of right whale foraging habitat in spring and early summer

Since 2015, North Atlantic right whale (NARW) occurrences have considerably increased in the Gulf of St. Lawrence (GSL), likely driven by a decrease in the availability of their historically preferred prey, the lipid-rich Calanus spp. (Copepoda), in their traditional foraging habitats. In the southern GSL (sGSL) where most NARW sightings occurred in recent years, the large bodied and lipid-rich Calanus hyperboreus dominates the Calanus spp. biomass in the spring and early summer, with important interannual variability hypothesized to be driven by variations in phenology and transport. This study examined the seasonal and interannual variability of phenology and relative distribution of C. hyperboreus biomass in the sGSL during the spring and early summer from 2016 to 2019, using a 3-D coupled biophysical model. We simulated the development and advection of the new cohort of C. hyperboreus from first copepodite (CI) stages in early spring to mid-July, a period during which they enter diapause as lipid-rich fourth copepodite (CIV) stages. Our study confirmed that C. hyperboreus distribution in the sGSL is mainly influenced by transport from adjacent deeper areas through interactions between ocean circulation and early copepodite stages during the spring. Model results indicated variable numbers of simulated individuals from the Laurentian Channel were transported annually to the sGSL during their active stages, entered diapause, and were retained near the seafloor in the summer. Biophysical simulation outputs could therefore contribute to seasonal-scale advice on locations of NARW potential foraging habitat formation and improve understanding NARW seasonal dynamics in the sGSL.