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Marine Ecology Progress Series

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MEPS 212:247-263 (2001)  -  doi:10.3354/meps212247

Diversity, density and community structure of the demersal fish fauna of the continental slope off western Australia (20 to 35°S)

Alan Williams*, J. Anthony Koslow, Peter R. Last

CSIRO Marine Research, GPO Box 1538, Hobart, Tasmania 7001, Australia

ABSTRACT: The first survey of the continental-slope demersal fish fauna off the west coast of Australia ‹ the region between latitudes 20 to 35°S in depths from ~200 to 1500 m ‹ was undertaken in 1991. Most species were caught rarely and only 14 species were represented by >1% of individuals in the total catch. Collectively, the catches were dominated numerically by the Acropomatidae and Chlorophthalmidae at shelf-break and upper slope depths (~200 to 600 m), and the Macrouridae, Bathygadidae, Synaphobranchidae, Alepocephalidae and Oreosomatidae at greater depths. Overall, the Macrouridae was both the most abundant and most species-rich family. The fauna appears to be richer (388 species from 109 families) than the slope faunas of the more intensively sampled North Atlantic and northern Pacific; its richness may be attributable to the overlap of ancient and extensive Indo-West Pacific and temperate Australasian faunas that extend from shelf to mid-slope depths. These faunas are maintained in the region by a variety of near-surface and intermediate-depth ocean currents that bathe the western slope. Seven distinct fish community types were defined by bathymetric and latitudinal boundaries: a northern and a southern shelf-break community; 2 depth-stratified communities on the upper-slope, and 3 communities defined by depth and latitude on the mid-slope. Ecotones between these communities at the 250 to 350 m and 700 to 800 m depth intervals coincide, respectively, with the lower limits of the near-surface Leeuwin Current and the upper extent of Antarctic Intermediate Water. The composition of the mid-slope fauna in the southern part of the study area suggests it is part of a wide-ranging Australasian mid-slope community shared with the Great Australian Bight, southeastern Australia and New Zealand. There is progressive replacement by a northern fauna at all depths as latitude decreases along the west coast. The western slope fauna is characterised by low density as well as high diversity. Fish density was estimated to be several times lower than on the continental slope off southeastern Australia, and also lower than North Atlantic and northern Pacific slope regions. Low density is likely to be related to low overlying productivity due to the absence of upwelling associated with the near-surface, southward-flowing Leeuwin Current. Low overlying productivity also appears to be linked to the ecological composition of the demersal fauna; it is characterised by many relatively small, benthic species, non-vertically migrating species, and a lack of aggregated large commercial fishes.

KEY WORDS: Demersal fish · Continental slope · Western Australia · Diversity · Species richness · Density · Community structure · Productivity

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