Species diversity and spatial variation in fish assemblage structure of streams in connected tropical catchments in northern Australia with reference to the occurrence of translocated and exotic species

Abstract

The freshwater piscifauna of the watercourses of the Barron and Mitchell systems, two northern Australian catchments with adjacent headwaters, was surveyed. Fifty-eight species of fish from 26 families, including several marine vagrants, were sampled from the freshwater reaches of the Barron River, compared to 28 species from 15 families from the Mitchell River. Species diversity and richness was higher in the coastal section of the Barron catchment owing to the influence of marine vagrants and species that were estuarine dependent for part of their life cycle. Detrended correspondence analysis of fish abundance data showed that sites in the coastal Barron catchment and the Mitchell catchment were widely separated in ordination space. Historically, barriers to fish movement determined species composition in both catchments, but there have been major changes in species composition as a result of extensive translocations of native fish into the Barron catchment over the last 50 years. Five species of exotic fish, including the cichlids Tilapia mariae and Oreochromis mossambicus, were established in the Barron system and one species in the Mitchell catchment. Cherax quadricarinatus, a crustacean endemic to western drainages, has been translocated into the Barron catchment and has since become widely established. This has implications for the interbasin transfer of irrigation water between the Barron and the Mitchell catchments potentially providing a mechanism for fish, including the exotic species O. mossambicus, to spread across northern Australia.