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The distribution, abundance and trophic ecology of the fishes of Darwin Harbour mangrove habitats

Martin, Julie M. (2005). The distribution, abundance and trophic ecology of the fishes of Darwin Harbour mangrove habitats. PhD Thesis, Charles Darwin University.

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Author Martin, Julie M.
Title The distribution, abundance and trophic ecology of the fishes of Darwin Harbour mangrove habitats
Institution Charles Darwin University
Publication Date 2005
Thesis Type PhD
Subjects 0704 - Fisheries Sciences
Abstract Darwin Harbour features one of the largest and most diverse mangrove communities in northern Australia. The harbour is also the centre of a thriving recreational fishery. This study examined the role of mangroves in the distribution, abundance and trophic relationships of the fish of Darwin Harbour. Fish were sampled in three different mangrove habitats in three locations in Darwin Harbour for two years, using three methods: trammel nets, light traps and pit traps. Forest structural complexity and aquatic environmental variables were also measured. Diet composition was analysed and trophic groups identified using multivariate techniques. A preliminary trophic model was also developed. At high spring tides, the mangrove forest was used extensively by a wide range of fish. At low tide, only resident species appeared to remain in pools, as pit trap samples comprised the Gobiidae and Pseudomugilidae. Trammel net captures were dominated by the Ariidae, Mugilidae, Clupeidae and Engralulidae families. Fish captured in the light traps were dominated by Engraulidae, Clupeidae, Atherinidae and Gobiidae. Small fish, including juveniles and larvae, were more abundant in the seaward habitats, whereas larger fish, including predatory species, were captured throughout the mangrove forest. This suggests that smaller fish were not seeking refuge from predation in the shallower, more complex habitats. Fish abundance and distribution did not appear to be influenced by small scale variations in structural complexity (stem density and sizes) or mangrove productivity. Instead, large scale differences in forest characteristics, such as mangrove species composition and general topographical features, along with aquatic environmental variables appeared to be more important. All habitats appeared to be important feeding areas for fish and seven trophic groups were identified. Three major groups were separated according to feeding mode (nekton, benthic and surface feeders) and within these, seven groups were defined by the main prey items in their diets. The Ecopath with Ecosim trophic model, although preliminary and based on incomplete information, synthesised information about Darwin Harbour from a wide variety of sources, indicated information gaps and identified areas in which future research should be focussed. Some species, such as Toxotes chatareus and two Ariid catfish, were more strongly associated with the higher intertidal habitats. This association may be important ecologically, due to their high levels of predation on the sesarmid crab, Perisesarma darwinensis, which is also found more often in the higher intertidal habitat and is known to consume mangrove leaf litter. Overall, the results show that a diverse array of fish use the mangroves of Darwin Harbour. Some are economically valuable species themselves and many are prey species for fish of recreational importance. There appear to be close trophic links between fish and all habitats and these links must be considered in future management of Darwin Harbour.

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