Associations between North Atlantic right whales and their prey, Calanus finmarchicus, over diel and tidal time scales

Abstract

Temporal variability in the distribution and abundance of North Atlantic right whales Eubalaena glacialis and their copepod prey, late-stage Calanus finmarchicus, was monitored at an oceanographic station in Grand Manan Basin of the lower Bay of Fundy for 29 h on 2 separate occa- sions. The vertical distribution of C. finmarchicus was measured at 1 /2 h intervals with an optical plankton counter (OPC) and at 6 or 12 h intervals with a MOCNESS. Right whale abundance was estimated from periodic point scans. Late-stage C. finmarchicus exhibited diel vertical migration in the upper 100 m of the water column, but the bulk of the population remained at depths below 100 m throughout both the day and night and was likely in diapause. Diel vertical migration is unlikely to be influenced by right whales, but may instead be motivated by abundant, near-surface food resources and avoidance of visual predators. Right whale sighting rate was correlated with OPC- detected C. finmarchicus fifth copepodite (C5) abundance at mid-depths (90-140 m); variability in both right whale sighting rate and C. finmarchicus C5 abundance in this depth stratum appeared to have similar periodicity to that of the tide. Energetic considerations suggest that right whales continue to feed on deep, diapausing layers of C. finmarchicus during the night, but the occasional presence of exploitable near-surface concentrations of C. finmarchicus suggests that nighttime near-surface feeding might sometimes occur.