Fish communities and the nursery role of the shallow inshore waters of a tropical bay in the gulf of Carpentaria, Australia

Abstract

The species composition of the fishes of the inshore zone between a tropical estuary and offshore trawling grounds were studied. The objectives were to investigate the affinities of the fishes with those of adjacent areas, to examine the influence of abiotic factors on species composition, and to assess the role and importance of the zone as a nursery area. The shallow (<5 m) inshore waters south of the Embley estuary were sampled in two wet, two dry and two pre-wet seasons between 1989 and 1993. A total of 7934 fish of 118 species were caught with gill nets (5741 individuals of 95 species) and beach seines (2193 individuals of 45 species). Forty-two of the species caught have not been reported in the adjacent offshore prawn or fish trawling grounds, 32 species have not been found in the nearby Embley estuary, and 15 species were caught only in the inshore zone. The dominant species caught by gill netting in the inshore zone were Scomberoides commersonianus (5.47 g m-1 h-1), Chanos chanos (3.67 g m-1 h-1) and Eleutheronema tetradactylum (3.57 g m-1 h-1). Overall, fish catch rates and biomasses were less than in the adjacent estuary and offshore waters, but greater than in tropical sea-grass localities. The number of species in the inshore zone (118) is less than in the Embley estuary (197), the clear inshore waters of Groote Eylandt in the western Gulf of Carpentaria (156) and in the offshore waters (237), but greater than in the Norman estuary (100). These differences are probably due to habitat diversity. Water turbidity, tidal range, wind and day versus night were the only abiotic factors that correlated with the relative abundances of some species. The inshore zone acts as a nursery for many species and is the first nursery ground for at least 11 species whose adults live mainly offshore. The overall juvenile component of the inshore fauna consists of fishes apparently restricted to the inshore habitat, larger juveniles that have emigrated from the estuary, and fish that occur in all three habitats. The inshore zone is not primarily or solely a transition zone between estuary and offshore. It is a habitat with characteristic juvenile and adult fish communities, but it supports some fish that move between offshore and estuarine habitats. It is also an important spawning area for some species as well as a foraging zone for a number of piscivores. © 1995 Academic Press Limited.