The production and trophic ecology of shallow-water fish assemblages in southern Australia II. Diets of fishes and trophic relationships between fishes and benthos at Western Port, Victoria

Abstract

A total of 5113 fishes belonging to 91 species was collected from seagrass and unvegetated habitats at Western Port for dietary analysis between August 1989 and November 1990, with 720 animals having empty guts. None of the common species was found to have a highly specialised diet. Crustaceans were the dominant component in the diets of the majority (69%) of the 88 species with non-empty guts, with five species (a girellid, a mugilid, a monacanthid and two gobiids) consuming large amounts of algal material, and only one species [the garfish Hyporhamphus melanochir (Valenciennes)] ingesting seagrass in any quantity. In general, dietary differences between juveniles and adults of a species were as great as dietary differences between fish belonging to different species when at the same body size. Amongst the crustacean feeders, diets generally changed from predominantly copepods to predominantly peracarids (amphipods, isopods and mysids) at ≈0.1 g wet body weight, and from predominantly peracarids to predominantly crabs and shrimps at ≈100 g. Across the range of species examined, there was a close relationship between the size of ingested prey and fish biomass. Prey length averaged 7.5% of predator length. The dominant species in gillnet catches, the mullet Aldrichetta forsteri (Cuvier & Valenciennes), possessed an unusual diet because prey were smaller than for other fishes of the same body size and large quantities of algal material were also consumed. Similar trophic pathways leading to fishes were found in different habitats, with the major linkages being from benthic microalgae and detritus through epifaunal crustaceans to the smaller fishes. The major predators of small fishes were locally-resident species, the rock flathead Platycephalus laevigatus Cuvier & Valenciennes in seagrass beds and the sand flathead Platycephalus bassensis Cuvier & Valenciennes at unvegetated sites. Crustaceans supplied most of the dietary intake for the small fish communities at all sites, with molluscs and polychaetes also important but supplying >25% of food consumed at only two sites. When daily rates of consumption were compared with daily production of food, the production of crustaceans >1 mm sieve size was calculated to be all consumed by fish. The production of the non-crustacean benthos was an order of magnitude higher than that consumed by fish predators. These results indicate that the availability of high quality crustacean prey may limit the production of fishes, and that fishes are likely to compete diffusively for crustacean prey. Additional support for this hypothesis is provided by the observations that the condition of seagrass-associated fish declined, and mortality rates were high, during the autumn season when total fish consumption could not be supported by crustacean production.