Linking fish assemblages and attributes of mangrove estuaries in tropical Australia: Criteria for regional marine reserves

Abstract

Quantifying natural features important in structuring fish assemblages is an essential pre-requisite to implementation of measures to conserve regionally representative whole ecosystems. Tropical estuaries are high in priority for protection, but fish-ecosystem relationships have rarely been analysed at multiple scales. Fishery-independent surveys (gill nets 19 to 152 mm mesh) were replicated seasonally (wet, dry) in 11 large estuaries along 1400 km of coast adjacent to the Great Barrier Reef. Fundamental differences in physical forces (river, wave, tidal) apparently generated variation in abiotic attributes, which distinguished Tide- from Wave-dominated systems. These trends correlated with patterns observed among estuarine fish assemblages, mediated by ecological and biological processes. Tide-dominated systems (n = 7), located in drier catchments, had greater mangrove area, wide deltaic mouths, and muddy substrate; in these systems, diversity was high with Ariidae, Belonidae and Haemulidae characterising the fish assemblages. Wave-dominated systems (n = 4), located in higher rainfall catchments, had constricted mouths, less mangrove area, and sandy substrate. For Wave-dominated systems, deltas (n = 2), had high diversity with Megalopidae characterising the fish assemblages, while true estuaries (n = 2) were relatively depauperate. Notably, none of the estuarine attributes explained the greater catch rates of Carangidae and Carcharhinidae observed in east Cape York. This trend may be attributed to negligible fishing pressure in the remote Cape York region. Overall, a substantial amount of the variation in fish assemblages (42.9%) was related to catchment hydrology, configuration of the estuary mouth, substrate and mangrove area. The results here support management programs incorporating a surrogate approach aimed at conserving the biodiversity of estuarine fish assemblages by combining regional and estuary scales in defining networks of reserve systems. © Inter-Research 2005.