Salinization of southwestern Western Australian rivers and the implications for the inland fish fauna - the Blackwood River, a case study

Increasing salinities throughout southwestern Western Australia, facilitated by extensive land clearing, have compromised the region's highly endemic freshwater fishes. Salinization of the Blackwood River has resulted in the main channel and upper cleared catchment being dominated by estuarine and halotolerant teleosts. The non-halotolerant species are restricted to the forested non-saline tributaries of the lower catchment. Of the 12 943 fish (13 species) captured in 113 sites, the halotolerant introduced Mosquitofish Gambusia holbrooki was widespread and the most abundant, representing almost 52% of fish caught. The estuarine Western Hardyhead Leptatherina wallacei, which was also widespread throughout the main channel and upper catchment, was the next most abundant, representing ca. 24% of fish caught. Freshwater endemics represented ca. 23% of captures, with the Western Minnow Galaxias occidentalis and Western Pygmy Perch Edelia vittata accounting for most (i.e., ca. 20%). There were significant differences in teleost communities among the naturally vegetated, low salinity tributaries of the river compared with the main channel and upper cleared catchment. While the forested tributaries still contain populations of E. vittata, Nightfish Bostockia porosa and Mud Minnow Galaxiella munda, the elevated salinities in the upper reaches of the Blackwood River system appear to have caused a massive decline in, or extinction of, populations of these species. The protection of the region's unique freshwater teleosts relies on the preservation of their remaining habitat in both the uncleared catchments of the region and in the low salinity forested tributaries within largely cleared systems, such as those in the Blackwood River catchment.