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Utilisation of the shallows of a south-western Australian estuary by fish, with special reference to the influence of the aquatic macrophyte Ruppia megacarpa

Humphries, Paul (1991) Utilisation of the shallows of a south-western Australian estuary by fish, with special reference to the influence of the aquatic macrophyte Ruppia megacarpa. PhD thesis, Murdoch University.

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The fish faunas associated with bare sand and patchy and dense beds of the aquatic macrophyte Ruppia megacaipa were sampled in the shallows of the lower region of Wilson Inlet, south-western Australia, between March 1988 and January 1990. The 41,377 fish collected in monthly night-time samples between March 1988 and February 1989 comprised 26% that were taken over bare sand, 34% in patchy Ruppia and 40% in dense Ruppia. A total of 23 species, representing 16 families, were recorded. Sixteen species were caught in bare sand and 17 in both patchy and dense Ruppia. The mean density and biomass of fish were greater in dense Ruppia than over bare sand, whereas the mean number of species and species diversity were lower in dense Ruppia than over bare sand. This difference is due to the fact that only a few species in Ruppia occur in very large numbers and the small juveniles of certain marine species are generally restricted to bare sand. Density and biomass of fish were positively correlated with Ruppia biomass, but negatively correlated with temperature. Classification and ordination demonstrated that the composition of the fish fauna was related to the type of habitat and season. Results from more limited sampling indicated that trends shown by the number of species, species diversity, density and biomass of fish at night in the lower estuary were maintained during the day in this region and during the night in both the middle and upper regions in Wilson Inlet. The fish fauna of the shallows of the lower region of Wilson Inlet was dominated by three species of atherinid and three species of goby, each of which was found in abundance within the system at all times of the year. Density and biomass of the atherinids Leptatherina wallacei and Atherinosoma elongate and the goby Favonigobius suppositus were greater in dense Ruppia than over bare sand, whereas the opposite was the case with Leptatherina presbyteroides and Favonigobius lateralis. The density and biomass of Pseudogobius olorum were greatest in patchy Ruppia. The diets of each of the six species differed with habitat, reflecting the available food in the habitat in which the fish were caught. The three species of atherinids each fed at different levels in the water column, whereas the gobies foraged predominantly on the benthos. Correlations and/or overlaps between the diets of the six abundant species were generally few but occurred mostly between congenerics. An analysis of the stomachs of a large teleost species (flathead) and four species of cormorants collected over two successive summers showed that fish were an important component of their diets, but that the flathead and the two small cormorant species also consumed large numbers of shrimp. Although dietary correlation and overlap were high between the last three predator species, flathead consumed substantial quantities of both atherinids and gobies, little pied cormorants ingested mostly gobies and little black cormorants ate mostly atherinids. Laboratory experiments showed that the choice of bare sand or artificial seagrass by the three species of goby was influenced by time of the day, density of fish and the particular combination of fish species in the tank. When alone, each species chose seagrass but when F. suppositus was present, F. lateralis chose sand. When disturbed all three species took cover in seagrass. Sampling of the atherinids at different levels in the water column confirmed that L. presbyteroides positioned itself higher in the column than the other species. The diets of the three species of atherinid reflected differences in feeding morphology and the position in which they fed in the water column. Irrespective of differences in the feeding morphology of the species, the ability of the resident fish to alter their foraging locations facilitates their coexistence in the shallows of Wilson Inlet.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Biological and Environmental Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Potter, Ian
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