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Dietary comparison of the tropical herbivore Siganus fuscescens and a range of temperate seagrass-associated omnivorous fishes

Climate change has been linked to marine and terrestrial pole-ward shifts in the distribution of tropical biota, causing an increase in the proportion of tropical to temperate taxa in a specified region, known as tropicalisation. Due to increasing ocean temperatures, tropicalisation has been predicted to increase plant and animal biodiversity in temperate seagrass meadows until a new equilibrium state is reached.

Temperate seagrass meadows of south-western Australia are typified by extensive Amphibolis and Posidonia meadows. Studies on fish assemblages within these meadows have shown that different seagrass species provide habitat and food for a range of fish species. Many fish species are known to commonly graze on seagrass, however, temperate herbivores predominantly consume algae on reefs, or epiphytes on seagrasses. These temperate fishes primarily feed on algae, with small amounts of seagrass being consumed to obtain the epiphytes and epifauna. In temperate Western Australian waters these fish species exert very little, if any, direct control on seagrass production.

In recent years the tropical herbivore Siganus fuscescens has been observed in increasing numbers in temperate Australian waters. This grazer inhabits rocky reefs, algal flats and seagrass meadows and is known to aggregate in relatively large numbers to feed on algae and seagrass leaves, with potential consequences for competition within fish assemblages in temperate seagrass meadows. There is limited information on the diet of S. fuscescens in both temperate and tropical regions in Australia, and how it could overlap with temperate omnivorous species.

The aim of this study is to determine if the pole-ward range shift of the tropical herbivore S. fuscescens could result in competition for food sources with temperate seagrass-associated omnivorous fish. This will be done by determining the similarity in diets of a range of temperate and tropical herbivorous/omnivorous fishes inhabiting southwest Australian temperate seagrass meadows, and by determining the tropical to temperate shift in diet of S. fuscescens.

Project duration

August 2016 to August 2018

Researchers

Mr Casper Avenant (Masters student)

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