Influence of environmental factors on capelin distributions in the Gulf of Alaska

Capelin (Mallotus villosus) are an important mid-trophic link within marine food webs, yet there is limited information describing fluctuations in distributions and abundances of capelin in the Alaskan North Pacific. The influence of physical and biological environmental factors on capelin distributions was investigated over continental shelf waters in the central Gulf of Alaska (CGOA). Acoustic, oceanographic, and trawl sampling were conducted in summer and fall of 2011 and 2013 as part of the Gulf of Alaska Integrated Ecosystem Research Program. Continuous acoustic sampling across bathymetric gradients showed capelin distributions on the CGOA shelf were primarily concentrated over submarine banks less than 100 m in depth. Acoustic densities of capelin were higher over banks in summer of both years compared to in deeper troughs (>= 100 m depth). Length-based distributions indicated that capelin occurring over banks were mostly age-1 fish, while larger age-2 + capelin were more prevalent in troughs. Environmental factors that influenced occurrence and density of capelin in summer 2013 were identified at two spatial resolutions associated with systematic sampling at discrete stations and continuous sampling along transects. At the station resolution (37 km), the probability of capelin occurrence was associated with relatively warmer bottom temperatures and increased chlorophyll concentrations, while capelin density was associated with close proximity to the edges of banks and reduced thermal stratification. At the transect resolution (0.5 km), increases in capelin density were associated with increases in density of a potential competitor (age-0 pollock, Gadus chalcogrammus) over banks and with increases in prey (macrozooplankton) and predator (semi-demersal groundfish) densities in troughs. We infer that in summer, age-1 capelin concentrate over shallow banks on the CGOA shelf in waters that are well-mixed and more productive to feed on abundant copepods, whereas age-2 + capelin primarily occupy deeper waters in troughs where they consume larger euphausiid and amphipod prey. This suggests that distributions of both life stages over the shelf were likely driven by changes in the availability and composition of prey.