Species composition and biomasses of fishes in different habitats of a tropical Northern Australian estuary: Their occurrence in the adjoining sea and estuarine dependence

Abstract

The fish communities of the five habitats comprising the Embley estuary in tropical north-east Australia were studied for two and a half years. The fish faunas of each habitat were significantly different in both biomass and species composition. Mean biomasses were estimated as 7·1 g m −2 to 16·1 g m −2 for open water channels, 5·0 g m −2 for sandy mud beaches, 0·5 to 1·8 g m −2 for seagrass areas, 8·2 g m −2 for small mangrove creeks and inlets, and 70·6 g m −2 for intertidal mudflats adjacent to mangroves. The species composition and biomass of the fish population in the estuary were compared with those of offshore waters in adjoining Albatross Bay. Of the 197 species recorded in the estuary, 91 were also recorded in the bay. They fell into six species categories: (1) juveniles found only in the estuary, (2) juveniles found only offshore, (3) juveniles that live both in the estuary and offshore, (4) adults found only in the estuary, (5) adults that live only offshore, and (6) adults that occur in both areas. Of the 106 species caught only inside the estuary, 59 also occur in shallow marine areas, which could not be sampled by trawling. This estuarine/shallow marine component formed at least one-third of the biomass in all estuarine habitats. The juveniles of 17 species of this group were found only in the estuary. Thirty species from the Embley (17 of which were Gobiidae) were considered truly estuarine. The number of species recorded, the biomasses in the various habitats and the differences between the fish faunas of the habitats are compared with published data from other tropical estuaries. The relatively high number of species from the Embley and the variations in biomasses and communities emphasize the importance of adequate sampling of all estuarine habitats. The results are also discussed in relation to the concepts of ‘estuarine dependence’ and ‘estuarine opportunism’. We concluded that ‘estuarine dependence’ is a valid concept and that at least one-third of the species from the Embley estuary are ‘estuarine-dependent’: they make up at least half the fish biomass in all estuarine habitats.