Preliminary Analysis of Pacific Coast Demersal Fish Assemblages

partment of Fish and Wildlife (ODFW) otter trawl survey, designed to sample flatfishes; and the portion of the West Coast Joint Agency Rockfish Survey covering a similar area of continental shelf. Whether assemblages persist through a time scale of years is an im­ portant fisheries resource question. If assemblages are relatively stable, mul­ tispecies management strategies could possibly be devised taking asemblage composition into account. Cluster Analysis Techniques Two techniques were used to reduce variability and skewness in the biomass frequency distributions arising from the contagious distribution of many rockfish species. First, data from three adjacent sites, each within a 36.6-m (20-fathom) depth interval parallel to the coast, were pooled or clumped (Tyler and Stephenson!) into 55 clumps from 115 sites. Second, data were transformed to loglo values. Clump similarities were described using a Bray-Curtis dissimilarity index (Clif­ ford and Stephenson, 1975). The index for each pair of clumps was calculated from species biomass data expressed as a proportion ofthe total biomass at each site. A group-average fusion strategy (Clifford and Stephenson, 1975) was used to link similar clumps into clus­ ters. The linked clusters were then dis­ played in a dendrogram (Fig. I).