Comparisons of the diets of four species of sparid on the central and lower west coast of Australia

Dietary data was collected, collated and synthesised for four species of commercially and/or recreationally important sparid (Acanthopagrus latus, Rhabdosargus sarba, Acanthopagrus butcheri and Pagrus auratus), that are abundant in estuaries and/or marine waters on the lower and central west coast of Australia. Thus, gut samples that had previously been collected for A. latus and R. sarba were subjected to dietary examination, while published (Sarre et al., 2000), and unpublished (Gary Jackson, Fisheries of Western Australia) data on the diets of A. butcheri and P. auratus, respectively, was re-analysed for comparison with the other two species of sparid. Acanthopagrus latus is abundant in mangrove and unvegetated rocky habitats in nearshore marine waters of Shark Bay on the central west coast of Australia. Rhabdosargus sarba co-occurred with A. latus in the latter habitat type in Shark Bay, and was also common in both nearshore marine and estuarine waters on the lower west coast of Australia. In terms of estuaries, R. sarba was particularly abundant in the entrance channel of the permanently open Swan River Estuary, while large populations of A. butcheri, which complete their life cycle in that estuary, are essentially restricted to the upper and middle part of the estuary. Pagrus auratus is distributed in marine waters on both the lower and central (Shark Bay) west coasts of Australia. In the mangrove habitats of Shark Bay, in which A. latus ranged from 77 to 223 mm in size, their diet consisted mainly of plant material, crabs and gastropods. Furthermore, marked size-related changes in dietary composition occurred, with amphipods and teleosts being far more important in larger than smaller fish. Sizerelated movement to slightly more offshore waters, which contained rocky, rather than sandy, substrates and no mangroves, coincided with the ingestion of large amounts of mytilid bivalves. The subsequent increase in size of those fish, which occurred in those living over rocky substrates, was not reflected in changes in the dietary composition. The strong seasonal changes for the mangrove habitats, but not rocky unvegetated nearshore waters, suggest that A. latus is feeding more opportunistically on the prey present in this comparatively sheltered mangrove environment. Moreover, the lack of any size-related or seasonal changes in the diet of this sparid in rocky nearshore waters strongly implies that mytilid molluscs are the preferred food source of this species once they have reached size of ca 150 mm, and that this prey remains sufficiently abundant throughout the year. In the lower Swan River Estuary where R. sarba is abundant, the dietary composition differed between this environment and other marine environments in south-western Australia, presumably reflecting differences in the types of food sources in those different habitats and that R. sarba tends to feed opportunistically in each environment. Moreover, within the lower estuary in autumn and winter, when the salinity were relatively low, differences between the two season, with more algae being consumed in autumn and amphipods in winter, probably reflects seasonal changes in the abundance of amphipods. In the upper Swan River Estuary where A. butcheri is common, the diet of this sparid differed markedly with that of R. sarba with the latter species consuming greater amounts of amphipods that the former species, which ingest more mytilid molluscs. Marked size-related changes in dietary composition were evident for both species with R. sarba tending towards herbivory with increasing body size while A. butcheri essentially remained an omnivore throughout life. Thus, if these species do co-occur, such as in certain estuaries on the south coast of Western Australia, this would mean that food resources would be partitioned amongst those species. This is in direct contrast with those R. sarba that cooccurred in Shark Bay with A. latus, which were shown to have similar dietary compositions. It is therefore likely that mytilid bivalves, upon which both species fed extensively, are sufficiently abundant throughout the year in Shark Bay to reduce the potential for competition among those two sparid species. The fourth sparid Pagrus auratus, which is common in the inner gulf of Shark Bay frequently ingests hard-shelled crustaceans and teleosts. This is presumably facilitated by its robust dentition and the large maximum size it attains in comparison to the other three sparid species. The data collected and compared for the four species of sparid that are abundant in estuaries and marine waters of the central and lower west coast of Australia demonstrates that the diet of those species can be strongly influenced by the environment in which they are found. Furthermore, when those sparids are present in environments where mytilid molluscs occur on the substrate in abundance, they apparently prefer to consume these prey in preference to other food sources. However, the data presented in this thesis have also highlighted the gaps that existed in the dietary information for those sparids. Thus, such gaps include that for the smaller individuals of each species, and particularly P. auratus, while seasonal data is still lacking for R. sarba and P. auratus. Furthermore, there is no information on the diets of larger R. sarba, when they occur on offshore reefs, and it is likely that the dietary composition of these fish would differ from those in nearshore waters