The fish community in the shallows of a temperate australian estuary: Relationships with the aquatic macrophyte Ruppia megacarpaand environmental variables

Abstract

The fish fauna in the shallows of the lower region of Wilson Inlet, south-western Australia, was sampled at night in each month between February 1988 and March 1989 to determine whether the number of species, species diversity, density and biomass of fish in beds of the aquatic macrophyte Ruppia megacarpa were greater than over adjacent bare sand. The 41 377 teleosts caught during this period, which represented 23 species and 16 families, were dominated by three species of atherinid and three species of goby. The number of species and species diversity were lower in dense Ruppia than over bare sand, whereas the opposite was true for the density and biomass of fish. Density and biomass of fish were positively correlated with the leaf height and weight of Ruppia and negatively correlated with temperature. Classification and ordination of the species density data indicated that the composition of the fauna differed among habitats and seasons, mainly as a result of the immigration of the juveniles of certain marine species into the shallow sandy areas during the summer. More restricted sampling indicated that the above relationships between the four community variables (number of species, species diversity, density and biomass) and the dense Ruppia and bare sand areas occurred during the day as well as night and in two regions further from the estuary mouth. In addition, each of the community variables decreased with distance from the estuary mouth. The number of species and species diversity were greater in each of the habitats at night than during the day. Since the fine blades of Ruppia are not extensively colonized by epifaunal invertebrates, they apparently provide a less productive habitat for foraging fish than other aquatic macrophytes. However, the fact that densities of the more abundant species of fish were greater in Ruppia than over nearby bare sand, indicates that this macrophyte was important in providing cover for fish.