Comparisons between the influence of habitat type, season and body size on the dietary compositions of fish species in nearshore marine waters

Fish were collected from over bare sand in nearshore shallow waters at three sites that varied in the extent to which they were exposed to wave activity and which were located within a 45-km stretch on the lower west coast of Australia. Sampling was undertaken within a 4-6-week period in each season. The volumetric contributions of different prey to the stomach contents and the mouth characteristics of four species, i.e. Sillago bassensis and Sillago vittata (Sillaginidae), Spratelloides robustus (Clupeidae) and Pseudorhombus jenynsii (Bothidae), were determined. Overall, the dietary compositions of the four species differed significantly from each other and those of fish at both the three different sites and in four consecutive seasons were also significantly different. In comparison with S. bassensis, the morphologically similar S. vittata fed to a relatively greater extent on polychaetes than zooplankters, presumably reflecting in part its greater ability to extend its upper jaw downwards towards the benthos. S. robustus typically targeted calanoid copepods in the plankton, whereas P. jenynsii fed on larger benthic prey taxa, reflecting the large differences in mouth morphology and feeding behaviour of these species. Although the diets of S. bassensis and S. vittata were strongly influenced by habitat type and season, the former variable was slightly more important for both of these species. However, the reverse applied with S. robustus. Season strongly influenced the dietary composition of P. jenynsii at the one site at which it was regularly caught. The diets of the two Sillago species and P. jenynsii underwent pronounced size-related changes, which would help distribute the food resources among the individuals of the different size classes of each of these species. In contrast, all size classes of S. robustus fed predominantly or exclusively on calanoid copepods at all sites and in all seasons, except at the most sheltered site in winter when these zooplankters were not found in samples taken from the water column. The seasonal and habitat variations recorded in the diets of the fish species in this study imply that these species are able to feed opportunistically, a characteristic that would be of particular value to fish that live in nearshore waters where the relative abundance of the different prey types varies with habitat type and season.