Linking community structure of small demersal fishes around Kodiak Island,Alaska, to environmental variables

Juveniles and small adults of at least 75 species of demersal fishes were i dentified in trawl catches from the nearshore waters of Kodiak Island, Alas ka, in August 1991 and 1992. We derived several indices to characterize com munity structure at each site, identified key environmental gradients along which community structure was organized, and identified those species whos e abundances varied most strongly along these gradients. We related species richness, species diversity, and total catch per unit effort to environmen tal variables observed at each site. Species richness and diversity were hi ghly variable among sites, but decreased significantly with salinity and we re significantly higher on heterogeneous sediments. Standardized catch per unit effort for all species combined differed significantly among 5 geograp hic areas and was significantly higher on sediments with a high sand and/or mud content. Indices of species composition for each sampling site were ob tained as the scores of ordination axes based on non-metric multidimensiona l scaling of Bray-Curtis dissimilarities between sites. The indices summari zed different aspects of community composition and were associated with dif ferent species groups. The first and major index was primarily related to t he depth-temperature gradient and contrasted a shallow, warm water species group with a deep, cold water group. Non-linear depth effects on most indic es suggest relatively rapid changes in species composition in shallow water (0 to 50 m), and more gradual changes in the lower part of the depth range . While the depth-temperature gradient was the most important gradient alon g which species composition was structured, sediment composition and geogra phic area accounted for a significant proportion of the variance of each of the indices. While species composition changed most strongly along the dep th-temperature gradient, species richness, diversity, and total abundance w ere not related to depth or temperature, suggesting that species compositio n changed independently of the overall abundance and of species richness an d diverstity.