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Characteristics of the fish assemblages of soft substrata and reefs in northwestern Australia and their relationships with latitude and various factors

Travers, Michael (2016) Characteristics of the fish assemblages of soft substrata and reefs in northwestern Australia and their relationships with latitude and various factors. PhD thesis, Murdoch University.

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The overarching aim of this thesis was to provide the first quantitative account of the characteristics of the ichthyofaunas over the soft substrata and reefs of inshore waters along a continuous tropical coastline, which covers a substantial latitudinal range and three diverse marine bioregions, and to elucidate the factors that influence those characteristics. Fishes were thus sampled over both habitat types in deep (x = 22 m) and shallow (x = 12 m) inshore waters during both the dry and wet seasons at seven regularly spaced locations along the ~1,500 km coastline of tropical north-western Australia (NWA). The fishes over soft substrata were sampled by otter trawling during the day, while those over adjacent reefs were sampled using traps during both the day and night.

The 110,521 fishes sampled represented 361 species, 197 genera and 85 families. 229 species were caught exclusively over soft substrata and 76 solely over reefs, with only 56 species recorded in both habitat types. The Leiognathidae, Carangidae, Terapontidae and Mullidae collectively contributed 69% to catches over soft substrata, while the Lethrinidae and Lutjanidae dominated catches over reefs (82%). Most species had an affinity with the faunas of the Pacific Ocean, reflecting the connective influence of the polewards-flowing Indonesian Throughflow current (ITF) to the NWA coast.

Species composition over soft substrata and reefs were both influenced more by location than by season and water depth and also than by day vs night in the case of reefs. While ichthyofaunal compositions over both habitat types change progressively with decreasing latitude/water temperature, they differ among the three bioregions, reflecting marked differences in environmental characteristics. Tidal range and turbidity decline 2 markedly with latitude. Rivers and mangrove forests are numerous in the Kimberley Bioregion in the north, but restricted in the Pilbara Bioregion in the south and, to an even greater extent, in the intermediate Canning Bioregion. The fish fauna of the Kimberley is particularly distinctive, containing several species belonging to families adapted for visual acuity and recognition in turbid waters, e.g. Leiognathidae and Lutjanidae.

Species richness and abundance of fishes over soft substrata in deep and shallow waters in the environmentally ‘stable’ dry season peaked at the northern and southern locations (Kimberley and Pilbara), whereas they essentially declined with latitude during the wet season. In contrast, those biotic variables over reefs rose to a peak in the central locations (Canning Bioregion) and then declined. It is thus relevant that the waters in the central bioregion are relatively low in productivity, a characteristic associated with good reef development, but less so for sustaining the substantial benthic macro-invertebrate fauna that constitutes the prey of many of the fish species found over soft substrata.

Analyses of the composite dataset produced for the ichthyofaunas of the NWA coast during this thesis have led to a greater understanding of the ecology of this important, but relatively little studied, coastline. Furthermore, the thesis has produced the type of crucial baseline data that will enable, in the future, any faunal changes brought about by climate change, increases in fishing pressure and other anthropogenic effects to be detected.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Veterinary and Life Sciences
Supervisor(s): Potter, Ian and Newman, Steve
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