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The fish fauna of two mangrove systems in the Eastern Gulf of Shark BY, Western Australia

King, Justin (2003) The fish fauna of two mangrove systems in the Eastern Gulf of Shark BY, Western Australia. Honours thesis, Murdoch University.

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Mangrove habitats are widely regarded as important fish nurseries and feeding grounds for a wide variety of species, particularly in subtropical conditions. While the fish communities in subtropical mangrove systems along the eastern coast of Australia have been extensively studied, those of subtropical mangrove habitats in Western Australia have received little attention.

The fish assemblages in subtropical mangrove habitats in Shark Bay, located at ca 26°S on the coast of Western Australia, were sampled bimonthly between May 2001 and August 2002. The aims of this study were to describe the fish fauna of these mangrove habitats, and examine variations in the fish assemblages between areas, sites, tidal phases, and months. Furthermore, the nursery functions of these mangrove areas were investigated in relation to commercially and recreationally important species.

Sampling was undertaken in two relatively small mangrove areas, namely Herald Bight and Dubaut Creek, located in the eastern gulf of Shark Bay. A salinity gradient along the eastern gulf resulted in consistently greater salinities at Dubaut Creek than at Herald Bight. A 21 .5 m seine net was used to sample the creeks on high and low tides, and on the flats on high tides. Composite gill nets were employed at Herald Bight on the flats and in the creeks on high tides, as well as opportunistically from low to high tides, and high to low tides. Gill netting was not undertaken at Dubaut Creek due to the high prevalence of turtles.

A total of 55,781 fish, comprising 42 species representing 22 families, were caught using the 21.5 m seine net. The most abundant species were Craterocephalus pauciradiatus and Gerres subfasciatus, which contributed 52.5 and 15.0%, respectively, to the total number of fish. Amniataba caudovittatus, Acanthopagrus talus, and Atherinomorus ogilbyi contributed most to the biomass of fish, specifically 18.5, 16.6, and 15.2%, respectively. The fish assemblages were dominated by tropical species (59.5% of the number of species and 61 .2% of the total abundance), resident species (35.7% of the number of species and 58.6% of the total abundance), and a relatively small number of highly abundant zooplanktivorous species (11 .9% of the number of species and 59.0% of the total abundance). Circum-Australian species, benthic invertivores, and juvenile or juvenile-transient adult species also contributed significantly to the abundance of fish, i.e. ca 24.6, 24.9 and 37.5%, respectively.

A total of 740 fish, comprising 12 species representing 7 families, were caught using gill nets. The most abundant species were Amniataba caudavittatus, Mugil cephalus, and Va/amugil buchanani, which contributed 57 .8, 15.8, and 14. 7%, respectively, to the total number of fish. Tropical species (81 .2%), resident species (63.8%), and benthic invertivores (63.9%) dominated the abundance of fish caught by gill nets.

Analyses of the mean number of species, mean densities and mean biomasses of fish collected by the 21 .5 m seine net indicated that the fish assemblages were influenced strongly by site, followed by region and tide. Multivariate analyses indicated that site was more influential than tide on fish assemblages, and that regional differences in fish assemblages occurred in the creeks, but not on the flats.

The greater variety and number of species in the creeks, than on the flats, reflected the greater variety of substrates, shelter, food sources and water depth in the creeks. The greater biomass of fish, lower densities of fish, and differences in species composition at Dubaut Creek, compared to Herald Bight, reflected the differences in the structures of the creeks in each region, particularly the presence of rocky reef and sponges, and the greater depth of Dubaut Creek. However, the greater ease of capture of small fish at Herald Bight and large fish at Dubaut Creek may have exaggerated these differences.

The movements of fish in relation to tide, both within the mangrove areas and between mangroves and other habitats, resulted in differences in the fish assemblages on low and high tides. Small juvenile fish, such as Mugil cephalus, occurred in the creeks on low tides, but the absence of these fish on high tides suggested movement out from the creeks into the shallow, vegetated areas on high tides to avoid predation. Conversely, relatively large and transient species occurred in the creeks on high tides, but moved out from the mangroves into other habitats on low tides, except in Dubaut Creek, which remained relatively deep on low tides. However, certain long term transient species, such as sub-adult Acanthopagrus latus, remained in the creeks on low tides and were subsequently caught in higher densities than on high tides, due to the greater ease of capture of fast-swimming fish on low tides.

The fish assemblages in mangrove habitats underwent progressive seasonal changes. MOS ordinations revealed that the fish assemblages in the creeks and on the flats underwent cyclical changes over a 12 monthly period, and that these changes were most evident on low tides, due to the prevalence of small juvenile fish, and least evident in the creeks on high tides, due to the greater abundances of transient species. Seasonal change in the fish assemblages was driven by the recruitment of small juvenile fish and the subsequent migration of nursery species. The duration of residency of juvenile fish in the mangroves varied from months, e.g. Mugil cephalus, to years, e.g. SilIago schomburgkii.

The fish communities of mangrove habitats were distinctly different to those in sand or seagrass habitats in Shark Bay, indicating that the mangrove areas in Shark Bay provide a unique and important fish habitat. The mangrove areas were nursery grounds for commercially and recreationally important fish species, including Mugil cephalus, SilIago schomburgkii, Acanthopagrus latus, and Rhabdosargus sarba. Juveniles of the commercially fished SilIago analis also occurred solely in the mangrove habitat, where it completes its lifecycle. Furthermore, the compositions of nursery species in mangrove areas were distinctly different to those in other nearshore and shallow-water habitats in Shark Bay, indicating that the mangroves were unique nursery habitats.

Publication Type: Thesis (Honours)
Murdoch Affiliation: Freshwater Fish Group & Fish Health Unit
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: Potter, Ian
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