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The Distribution and Trophodynamics of Demersal Fish from Spencer Gulf

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This paper describes the composition, distribution and diets of demersal fishes collected from Spencer Gulf during a quantitative Gulf-wide trawl survey in February 2007. A total of 132 fish species from 65 families were collected from the 120 trawl shots. Degens leather jacket, Thamnaconus degeni, was the most abundant species collected (627 @ 239ha-1 ). This fish was also dominant in terms of biomass, and accounted for more than 40% of the total catch weight. Most other species (95%)had mean biomasses ranging from less than 1 to 515 gha-1, andindividually contributed less than 3% to the overall catch. Correlation analyses revealed a broad latitudinal gradient in fish abundance, biomass and richness. All three parameters generally increase towards the south of the Gulf, in association with increasing water depths and decreasing water temperatures. Multivariate cluster analyses confirmed the presence of a strong environmental gradient between the north and south of the Gulf, and highlight the presence of three distinct fish assemblages(north, central and south) that are closely allied with changes in depth. Small subsets of species with restricted distributions characterised each regional assemblage. Dietary examinations of the stomach contents of 871 fish representing 107 species were also undertaken during this study. Five feeding guilds were identified from cluster analysis of the stomach contents. These included groups of fish feeding primarily on echinoderms (2spp), other fish (13spp), molluscs (11spp), crustaceans(52 spp)and worms(23spp). A trophic model was constructed to evaluate levels of food consumption for each fish, and to assess regional differences in the composition and total volume of prey eaten. Total food consumption was estimated to decrease progressively towards the top of the Gulf, with daily food consumption rates in the northern region (219.55 g ha-1 day-1 ) more than seven times lower than those in the south of the Gulf(1580.99gha-1 day-1 ). Crustaceans and annelids were the two most important prey items consumed by demersal fish in all three biomes of the Spencer Gulf (north, central, south), and comprised more than 30% and 18% of the total diet, respectively. Our model suggests that almost half a kilogram of crustaceans and worms are consumed daily per hectare in the south of the Gulf. All other prey groups (e.g.molluscs, bryozoans, echinoderms, sponges)are eaten in much smaller volumes, and are individually consumed at daily rates of less than 127 g ha-1 in each biome. Estimates of consumption were broadly consistent with other temperate Australia nestuaries. However, as no seasonal replication of sampling was undertaken it remains unclear if observed spatial patterns and trends are temporally robust.


Document Type: Research Article

Publication date: November 1, 2010

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