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The fish and invertebrate fauna of adjacent weed and sand areas in the swan Estuary

Shaw, Jenny (1986) The fish and invertebrate fauna of adjacent weed and sand areas in the swan Estuary. Honours thesis, Murdoch University.

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The invertebrate and fish fauna was sampled in adjacent weed and sand areas in the middle Swan Estuary. Invertebrates were sampled with a corer, pump net and plankton tow, while fish were captured with the pump net, beach seine and gill net. Sampling was carried out in the day and night at three monthly intervals between February 1982 and February 1983. A total of 96 species comprising 34,000 counted individuals, with an estimated biomass of 238,050g wet weight were collected. The weed area, which consisted predominantly of the seagrass Halophila ovalis, contained a greater density and biomass of individuals than the sand habitat. The major taxa caught during the study were molluscs, crustaceans, polychaetes and fish.

The most abundant molluscs in the weed were species such as those belonging to the Mytilidae Ce. g. Musculista senhausia), which attached to the substrate or surfaces. By contrast, burrowing species such as Spisula triqonella were more abundant in the sand. The burrowing gastropod Nassarius birchardi was the only mollusc which was clearly shown to exhibit diel movements, moving up into the surface sediment at night. The amphipod fauna in the study site were dominated by tubiculous burrowing species Ce.g. Neomicrodeutopus sp. and Corophium minor) which were more abundant in the sand than weed. Free-swimming and nestling species <Melita spp. and Caprella scaura) predominated in the weed habitat. Data from plankton tows shows that many amphipod species exhibited diel swimming activity, moving from the sediment to the water column at night. The most abundant polychaete, Ceratonereis aeguisetes, was in greater density in the weed than the sand. The most abundant invertebrates in the sand tended to be those which "buried" into the sediment. The substrate in sandy areas would be easier to penetrate than in the weed where seagrass roots, rhyzomes and mollusc byssus mats are present. Thus, protection from predation in the weed habitat is provided by the cover of the seagrass, whereas in the sand it is provided by the substrate in which the animal burrows.

The total density of teleosts was greater in the weed than sand and during the night than day. The two major species of teleost Apoqon rueppellii and Torguiqener pleuroqramma, were more abundant in the night and day respectively. Gut analyses of these species showed that the most abundant prey ingested were those found in the weed area. Thus, A. rueppellii fed mainly on amphipods found predominantly in the weed, while T. pleuroqramma ingested large numbers of mytilid molluscs and polychaetes. Some of the more abundant fish species appeared to exhibit diel movements on and off the shallow banks.

Item Type: Thesis (Honours)
Murdoch Affiliation(s): School of Environmental and Life 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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