Habitat Use and Community Structure in an Assemblage of Cottid Fishes

I applied an observational approach, using strip transects, to document the abundance and the spatial distributions of members of an assemblage of subtidal fishes (Scorpaeniformes: Cottidae) in the San Juan Islands, Washington, USA. I then assessed the extent to which species—specific distributions were correlated with habitat availability and the potential for interspecific competition to modify patterns of abundance and resource use. The six common cottid species were Artedius fenestralis, A. harringtoni, Asemichthys taylori, Chitonotus pugetensis, Icelinus borealis, and Jordania zonope. A comparison of their observed distributions with the distributions of microhabitats along the transects revealed that all species demonstrated significant affinities for or avoidance of many of the predominant substrates: clean rock, silt rock, shell sand, gravel, mud, and algae. Using a stepwise regression technique, I found that the densities of individual cottid species were significantly correlated with the percentage cover of these substrate types along the transects and with transect depth. Several lines of evidence suggest that interspecific competition does not play a strong role in this assemblage. Species pairs that co—occurred on at least some transects did not show disjunct distributions. Microhabitat overlap of these species pairs was higher in sympatry (comparison of transects with both species present) than in allopatry (comparisons of transects with only one species present), counter to the expectation of niche divergence. Correlations of species densities after correcting for habitat heterogeneity with the Schoener—Crowell—Pimm technique revealed only one significant negative density correlation out of thirty potential correlations. Cottid diversity was not correlated with structural diversity along transects in spite of strong associations of each species with habitat structure. This suggests that the transect areas were undersaturated in the number of individuals and/or species of cottid fishes. The apparent lack of competition among subtidal cottids contrasts with studies on intertidal fish communities dominated by cottids for which there is observational evidence of interspecific competition.