DOI: 10.5061/dryad.hqbzkh1nn
title: Joint species distribution modeling reveals a changing prey landscape for North Pacific right whales on the Bering shelf

description: 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.
description: Zooplankton species abundance (ind. m<sup>-3</sup>)
 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 km<sup>2</sup> cells for each time-step. The
 cell size was chosen to maximize cell number while maintaining contiguity
 along the 70 m isobath.