Variability in species composition and distribution of forage fish in the Gulf of Alaska

Abstract

In the Gulf of Alaska (GOA), forage fish species, such as age-0 walleye pollock (Gadus chalcogrammus), capelin (Mallotus villosus), Pacific herring (Clupea pallasii), and mesopelagic fishes (e.g. Myctophidae), are ecologically important as both consumers of zooplankton, and as prey for fish, seabirds, and marine mammals. As part of the Gulf of Alaska Integrated Ecosystem Research Program, an acoustic-trawl survey was conducted in the summer and fall of 2011 and 2013 to quantify variability in species composition, density, and distributions of forage fish over the continental shelf and slope in the central and eastern regions of the GOA. The forage fish community in 2011 was characterized by the absence of age-0 pollock and lower densities of capelin, herring, and mesopelagics compared to observations in 2013. Age-0 pollock were abundant across both regions in summer 2013, but were rarely observed in fall. In contrast, summer observations of herring were rare, while aggregations of herring were observed over the eastern GOA shelf in fall of both years. Seasonal changes in community composition are attributed to the transport of age-0 pollock from offshore waters in summer to nearshore waters in fall, and to immigration of herring to the eastern GOA shelf in fall. Forage fish spatial patterns varied within and between regions due to intra- and interspecific differences in horizontal and vertical distributions that were correlated with bottom depth. Observed spatial and temporal variability in community composition and distributions of forage fish species may potentially impact predator foraging in the GOA, as well as the effectiveness of monitoring to detect changes in forage fish biomass.