Fish assemblage structure in relation to macrophytes and filamentous epiphytes in shallow non-tidal rocky- and soft-bottom habitats

Synopsis The fish assemblage in nineteen shallow water (0–3 m) areas on the Swedish west coast, including an estuarine zone, was assessed during spring and autumn 1989 and autumn 1990, using semi-quantitative survey nets. Samples of macrovegetation were collected concurrently for estimates of species composition and biomass. Nine stations had rocky-bottom substrata and ten had soft-bottom substrata all characterized by high coverage of macrovegetation and variously overgrown with epiphytic filamentous algae. Fish assemblage structures were compared and related to vegetation biomass, substrata and estuarine influences. At rocky-bottom stations total fish biomass was positively correlated with total vegetation biomass and negatively correlated with the proportion of filamentous algae during autumn samplings. In soft-bottom habitats variation in vegetation was small between stations, and no correlation existed between vegetation biomass and fish biomass. However, the number of fish species in soft-bottom habitats decreased significantly with increasing dominance of filamentous algae. The component species of the fish assemblage varied in their relation to the vegetation biomass and structure suggesting differences in degree of association with vegetation at the species level. Multivariate analysis based on fish species composition and on vegetation assemblages at the individual stations, yielded two major groups in accordance with division of the substrate into rocky- and soft-bottom habitats. Vegetation biomass superimposed on the fish assemblage ordination indicated a relationship between vegetation biomass and fish assemblage structure. Location of stations, in relation to the estuary was reflected in subgroups formed in the fish assemblage based cluster and ordination, suggesting a substantial estuarine influence on the fish assemblages. Thus, substrate type, vegetation biomass and structure, and estuarine influence are all potential structuring factors for the fish assemblages. In our study, vegetation structure seems to be of major importance and changes such as increased dominance of filamentous algae, like that observed in coastal areas in Sweden, might cause significant changes in fish assemblage structure.