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Evaluation of Three Different Artificial Habitats for Marron (Cherax tenuimanus) and Biodiversity Enhancement in Habitat-Limited Freshwater Impoundments in South-Western Australia

Brett Molonyand Chris Bird

Department of Fisheries Western Australia, NORTH BEACH, AUSTRALIA

THEME: ASFB

Many of the natural valleys and basins of the south-west of Western Australia (WA) have be dammed to provide water for irrigation of broad acre crops and livestock and potable water. In order to minimise the risk of outlet blockages, basins have been cleared of vegetation during the construction process. Further, many of the rocks within the basins have been used during the construction of the dam wall. The result is that most dams of the south-west are “clean-bottomed”; that is very little structure (aquatic habitat) remains. Although individual dams may have old tree stumps, recent maintenance to these basins have resulted in even these being removed. Thus the basins are severely habitat-limited. Nonetheless, native crayfishes (especially marron, Cherax tenuimanus), a species of native fish (the catfish, Tandanus bostocki) and three introduced species of fishes (rainbow trout, Oncorhynchus mykiss; brown trout, Salmo trutta; and redfin perch Perca, fluviatilis) form the major part of the inland recreational fisheries of the south-west and these irrigation dams receive high levels of effort. Recently, it has been determined that one of the introduced predatory fishes, the European redfin perch (Perca fluviatilis) can impose extremely high mortality on juvenile marron in these clean-bottomed dams, leading to a decline in the numbers of marron. In order to address this issue, three types of artificial habitats were placed into one irrigation dam and the use of these habitats by marron and other species assessed. Due to the life-history of marron (brooders with most females within a population releasing young within several weeks) the effects of marron-biomass concentration by the artificial habitats and increase in survivorship (and productivity) could be identified. The results clearly showed that the artificial habitats concentrated older marron and that young marron were more likely to be found near artificial habitats, as compared to control sites (i.e. without habitat). Although marron take at least two years to grow to legal-size (76 mm carapace length) and thus the effects on the fishery of artificial habitats has not been assessed, the results indicate that increased survivorship is possible with relatively cheap, inert structures. Thus, the addition of habitat may increase the suitability of an impoundment for marron and increase the sustainability of the recreational marron fishery. Further, native fishes with a high conservation value (e.g. Galaxias species) appeared to be closely associated with these artificial structures. Artificial habitats are about to be installed into two much larger dams (in excess of 150 ha) to increase recreational fishing and conservation values. Further, the installation of artificial habitats in “closed” waters (i.e. potable-water dams) is an ecologically-viable, cost-effective option to increase the conservation value of these areas, especially where redfin perch populations have become established.