Structure and diversity of fish assemblages on rocky and soft bottom shores on the Swedish west coast

Abstract

Density and biomass of fishes, from shallow rocky and soft bottom habitats on the Swedish west coast, showed large seasonal variation with low values in winter and spring and with peaks in June. Season was also the most important factor determining the fish assemblage structure. Within season, however, there was a clear separation in assemblage structure between rocky‐and soft‐bottom habitats. There were significantly higher total fish abundances and biomasses during night compared with day catches. On soft bottoms density and biomass of fishes decreased with increasing depth, but no such pattern was seen in rocky habitats indicating that the distribution of fishes was related to vegetation cover. Altogether, 53 fish species were recorded of which 30 were common to both habitats. Species richness was similar on rocky and soft bottoms. Of the 10 most abundant species found in rocky habitats four belonged to the Labridae and three to the Gadidae. The fish assemblage on soft bottoms were of a more mixed nature with representatives among the 10 dominants from six families (Clupeidae, Cottidae, Gadidae, Gobiidae, Labridae and Pleuronectidae). When ranking the 10 dominant fish species on rocky bottoms according to biomass c. 50% of the mass was Labridae, 19% Gadidae and 13% Cottidae. In soft bottom habitats, fish biomass was mainly distributed between six families. Pleuronectidae and Gadidae were dominant and each made up 25% of the biomass, whereas Labridae only contributed 4% of the fish mass. It is concluded that the fish assemblage in rocky habitats is dominated by permanent non‐commercial species (63% of biomass), whereas soft bottoms mainly function as nurseries for juvenile fishes and as feeding grounds for seasonal migrants of commercial species (80% of biomass).