The eastern North Pacific right whale (NPRW) is the most endangered population of whale and has been observed north of its core feeding ground in recent years with low sea ice extent. Sea ice and water temperature are important drivers for zooplankton dynamics within the whale’s core feeding ground in the southeastern Bering Sea, seasonally forming stable fronts along the shelf that give rise to distinct zooplankton communities. A northward shift in NPRW distribution driven by changing distribution of prey resources could put this species at increased risk of entanglement and vessel strikes. We modeled the abundance of NPRW prey, Calanus glacialis, Neocalanus, and Thysanoessa species, using a dynamic biophysical food web model of nine zooplankton guilds in the Bering shelf zooplankton community during a period of warming (2006–2016). This model is unique from prior zooplankton studies from the region in that it includes density dependence, thereby allowing us to ask whether species interactions influence zooplankton dynamics. Modeling confirmed the importance of sea ice and ocean temperature to zooplankton dynamics in the region. Density-independent growth drove community dynamics while dependent factors were comparatively minimal. Overall, Calanus responded to environmental terms, with the strength and direction of response driven by copepodite stage. Neocalanus and Thysanoessa responses were weaker, likely due to their primary occurrence on the outer shelf. We also modeled the steady-state (equilibrium) abundance of Calanus in conditions with and without wind gusts to test whether advection of outer shelf species might disrupt steady-state dynamics of Calanus abundance; results did not support disruption. Given the annual fall sampling design, we interpret our results as follows: low ice-extent winters induced stronger spring winds and weakened fronts on the shelf, thereby advecting some outer shelf species into the study region; increased development rates in these warm conditions influenced the proportion of C. glacialis copepodite stages over the season. Residual correlation suggests missing drivers, possibly predators and phytoplankton bloom composition. Given the continued loss of sea ice in the region and projected continued warming, our findings suggest that C. glacialis will move northward, and thus, whales may move northward to continue targeting them.

Zooplankton species abundance (ind. m^-3) were obtained from fall NOAA cruises (mid-August – early October) on the Bering shelf from 2006 to 2016. Data collection is led by the NOAA EcoFOCI program. Zooplankton were collected using oblique tows of paired bongo nets (20 cm diameter frame with 153 μm mesh, and a 60 cm diameter frame with 333 or 505 μm mesh). These oblique tows spanned from ~10 m of the bottom to the surface from a boat moving at ~1.5 knots. Net depth was determined using a SeaCat or FastCat conductivity, temperature, and depth (CTD) sensor (Sea-Bird Electronics), and the volume filtered was estimated using a General Oceanics flowmeter mounted inside the mouth of each net. Onboard, zooplankton samples were preserved in formalin for later laboratory sorting and identification. Plankton were processed using an established splitting and subsampling protocol and were identified to the lowest taxonomic level and stage possible at the Plankton Sorting and Identification Center (Szczecin, Poland), and verified at the Alaska Fisheries Science Center, Seattle, Washington, USA. It is important to note that euphausiid abundances reported here are semi-quantitative as larger euphausiids are able to avoid capture. For each species, zooplankton tow data were averaged to a study grid comprised of fourteen contiguous 80 km² cells for each time-step. The cell size was chosen to maximize cell number while maintaining contiguity along the 70 m isobath.