In situ zooplankton density estimates at a foraging site in the Canadian Arctic are below minimum prey thresholds for adult bowhead whales (Balaena mysticetus)

Bowhead whales (Balaena mysticetus) are an iconic Arctic species with a critical ecological role as a top predator. Bowheads can reach up to 80 tonnes and 20 m, yet feed on zooplankton four orders of magnitude smaller. Arctic zooplankton community composition and distribution are changing, which may have direct impacts on bowhead foraging. Data on the threshold prey density for successful bowhead feeding are needed to predict these impacts. However, zooplankton densities are patchy temporally and spatially, influenced by oceanographic conditions that alter the location of energetically profitable patches. We assessed spatio-temporal patterns in zooplankton abundance and distribution using a multi-frequency echosounder following a systematic and opportunistic survey near feeding whales in Iqalujjuaq Fjord, Cumberland Sound, Nunavut (65.66°N, 65.20°W) during August 2023. Zooplankton net samples were used to validate the acoustic data. There was a strong link between copepod distribution and environmental variables (e.g. water depth and tidal cycle) (generalized additive models, P < .001). Copepods were present in 49.8% of the fjord, with a median density of 3240 copepods m⁻³ and 0.26 g C m⁻³. Based on published prey density requirements, this site provides feeding opportunities for juveniles but is insufficient for the needs of adults (>0.44 g C m⁻³).