The broad and fine scale habitat associations of the fish fauna of the Blackwood River and it's tributaries

To aid in the reintroduction/recolonisation of native fish into degraded systems, an investigation of their habitat associations is paramount. This study investigated both large scale, i.e. faunal assemblage versus land use (agricultural, plantation, pristine), and fine scale, i.e. faunal assemblage versus instream habitat (snags, riffle, pool etc.), habitat associations of the fish species of the Blackwood River system. A total of 30 sites in the main channel of the Blackwood River were sampled for fish using seine nets, electrofishing equipment and scoop nets, with a further 45 sites sampled in it's tributaries running through agricultural land (15 sites), pine plantation (14) and natural vegetation (16) using these methods. Of the 9990 fish captured, the introduced Gambusia holbrooki and the estuarine Leptatherina wallacei accounted for approximately 50 and 27%, respectively of all fish caught. While G. holbrooki was most abundant in the main channel of the Blackwood River and agricultural sites, L. wallacei was found exclusively in the main channel, as were the other estuarine affiliated species, namely Pseudogobius olorum and Mugil cephalus. These latter species accounted for only 2 and 0.1 % of all fish captured, respectively. The other species confined to the main channel was Tandanus bostocki, which was only captured using gill nets. In contrast, the endemic freshwater species, Edelia vittata and Bostockia porosa were most abundant in the naturally vegetated sites where conductivity was low. The endemic Galaxias occidentalis, however, was found in the majority of sites sampled over a wide range of salinities. The only other endemic species captured during this study was Galaxiella munda, which was captured in low numbers at only three sites. In addition to these species, a further four species, all introduced, were captured, namely Oncorhynchus mykiss, Sa/mo trutta, Perea jluviatilis, and Carassius auratus. These species were only found in low numbers. Of those species caught in sufficient numbers in the tributary sites to allow estimates of fine scale habitat association to be made, B. porosa and E. vittata showed strong affiliations to structure (e.g. snags, macrophytes etc), whereas G. occidentalis showed no preference for structure being consistently found in all habitat types. Each of these species was most commonly found in slow to moderate flows. Although being present in the majority of habitat types, the introduced G. holbrooki appeared to show preference for cover in the form of algae and grass. Furthermore, this species tended to be restricted to areas of very low flow rate. The results of this study show that the salinisation of water bodies in south-western Australia is likely to reduce the abundance of several of the endemic freshwater species, such as £. vittata and B. porosa. While salinity reduction is probably not feasible in many areas of south-western Australia, the simple practice of re-snagging habitats in freshwater systems could ensure the successful re-establishment of native fishes in otherwise degraded areas