Trophic interactions of seagrass-associated fish in Cockburn Sound, Western Australia

Dylan J Bennett

Seagrasses are a vital component of coastal ecosystems, providing habitat, shelter and direct and indirect food sources for prey and predator species. A variety of different fishes utilise seagrass habitats, with some families displaying specialised morphological traits to partition resources, particularly food, to reduce competition. Measuring morphological traits can help determine how species partition resources by identifying methods of prey capture. While most dietary studies focus on extremes or partitioning among closely-related species, this study determined the dietary composition and morphometrics of a diverse range of 38 seagrass-associated species and evaluated the extent to which interspecific competition and partitioning of resources occurred among 20 species with adequate replicates. Sampling was conducted in Cockburn Sound, a temperate marine embayment in Western Australia using a benthic sled. Examination of 666 guts revealed that arthropods were the most ingested prey taxa split into small (e.g. amphipods and copepods) and large crustaceans (e.g. brachyurans and penaeids), whilst macrophytes, molluscs, insects, teleosts, echinoderms and annelids made smaller contributions to the diet of the species examined. While most species utilised a generalist feeding strategy feeding on a wide range of prey, some were small crustacean (e.g. pipefishes and clingfishes) or macrophyte specialists (e.g. leatherjackets). Despite evidence of partitioning, most species consumed small crustaceans, indicating there is competition for this important food source, and, to a lesser extent, filamentous algae. Statistical analysis of morpho-anatomical traits assigned 78 species (including soft sediment-associated species) to 26 groups, with seagrass-associated species possessing a particular suite of traits. There was a significant relationship between morphology and diet, with five traits being particularly important in seagrass-associated fish i.e. oral gape size and position, body transversal surface, pectoral fin position and caudal fin aspect ratio. The full suite of 36 species was classified into 17 functional feeding guilds on the basis of their diet, feeding strategy, and feeding behaviour (derived from morphology). Such data can be used in determining food web position and trophic flow and subsequently used as inputs for future models in heavily utilized areas.

Trophic interactions of seagrass-associated fish in Cockburn Sound, Western Australia

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