Size and energy content of <i>Calanus</i> spp. copepods: linking copepod phenology to variability in their quality as prey

Abstract Relating changes in lipid ecology of Calanus spp. copepods to temporal variation in their energy content is crucial for assessing energy that is available to zooplanktivores. In this study, statistical models were constructed to predict individual energy content from copepod prosome length and oil sac morphometrics, and ultimately characterize temporal changes in the energy content of Calanus finmarchicus stage C5 (Cfin5)and Calanus hyperboreus stage C4 and C5 copepodites. Direct measurements of individual energy content and lipid morphometrics were obtained for copepodites collected in the southern Gulf of St. Lawrence (sGSL), a foraging habitat of an endangered zooplanktivore, the North Atlantic right whale (Eubalaena glacialis, NARW), in summer 2019, early spring 2022 and late spring 2023. Estimates of individual energy content inferred from oil sac volume were comparable with direct energy measurements from the same collections. Inferred individual energy content increased as a function of prosome length but the relationship varied temporally, with 1.5–2-fold differences in energy content of average-sized copepodites, with ranges of ca. 6–9 J ind−1 in Cfin5 and ca. 9–15 J ind−1 and 33–65 J ind−1 in C. hyperboreus C4 and C5 respectively. Smaller lipid-poor Cfin5 were found in the upper 50 m of the water column in the sGSL. Regional comparisons were also made from Cfin5 collected from a NARW foraging area in the Nantucket Shoals (NShoals) in winter 2020, and with previously published observations in traditional foraging areas in the Gulf of Maine and western Scotian Shelf, to begin establishing potential upper and lower bounds of energy availability in known NARW foraging areas. In comparison to the sGSL, individuals in NShoals were relatively small and lipid-poor, indicating lower prey quality. Our results assess the extent to which Calanus species- and stage-specific lipid dynamics contribute to prey energy availability in important contemporary foraging areas for NARW.