Spatial and temporal patterns in the Gulf of Alaska groundfish community in relation to the environment

Abstract

The GoA supports a rich demersal fish community dominated by gadids, pleuronectids, sablefish (Anoplopoma fimbria) and rockfishes (Sebastes spp.). This study describes the structure of the juvenile and adult groundfish communities of the Gulf of Alaska (GoA) in relation to their environment and along spatial and temporal gradients. Abundance data were obtained from trawl surveys of juvenile groundfishes in the nearshore areas of Kodiak Island (1991--1996), shrimp-trawl surveys in the same areas (1976--1995), and triennial bottom trawl surveys of adult groundfishes on the GoA shelf and upper slope (1984--1996). Species richness, species diversity, total abundance, and multivariate indices of species composition for each station sampled were statistically related to depth, temperature, salinity, sediment composition, geographic location, and time of sampling to identify spatial and temporal patterns in community structure. The observed patterns were then related to local and large-scale atmospheric and oceanographic trends. Both juvenile and adult groundfish communities were primarily structured along the depth gradient. The abundance of juvenile groundfishes decreased with depth from 0 to 100m, whereas the abundance of adults increased with depth to a peak at 150--200m. Species richness and diversity of the adult community had a significant peak at 200--300m. Spatial patterns suggested higher abundances, lower species richness and diversity, and a different species composition of demersal fishes in the western GoA compared to the eastern GoA. These large-scale spatial patterns appear to be related to differences in upwelling between the eastern and western GoA. A 40% increase in total groundfish biomass on the GoA shelf and upper slope was estimated between 1984 and 1996. Significant changes in species composition occurred in the nearshore areas of Kodiak Island in the early 1980s, from a community dominated by shrimp and small forage fishes to one dominated by large piscivorous gadids and flatfishes. The change in species composition in the nearshore community appeared to be linked to an increase in advection in the Alaska Current. Increased flow around the GoA may enhance the supply of nutrients and plankton on the shelf and upper slope, resulting in an increase in overall productivity of the pelagic and demersal biota.