An introduction to Australian fish
Fish are the most diverse vertebrate group, with more than 36,000 species known worldwide and ~350 – 400 new species described each year. There are more fish species in the world than birds, reptiles, amphibians and mammals combined. Freshwater fish comprise about 50 per cent of all known fish species but amazingly freshwater comprises <0.5 per cent of available global water.
Australia only has about 350 species of freshwater fish—a very small number for such a large continent. Many more Australian fish species remain to be scientifically described, and it has been estimated that only ~70% of Australia’s freshwater fish are currently formally named.
Fish in the Murray-Darling Basin
- The longest river system in Australia, the Murray-Darling has only 51 native freshwater fish species, (as of January 2023) whereas the Mississippi-Missouri system has around 170 native species and the Amazon Basin some 5000-8000 fish species described so far. The relatively low number of Australian freshwater fish species is related to this country's long isolation from other continents (freshwater fishes from other continents could not colonise), low rainfall and low surface runoff (evaporation is high).
- Although no Australian freshwater fish species is known to have become extinct since European settlement, it is certain that some have been lost before they were recognized as distinct species. Approximately 40 percent of the Australian freshwater fish fauna is now considered of conservation concern and is listed on National. State/Territory or non-statutory lists (such as that curated by the Australian Society for Fish Biology, and the IUCN Red List). Local or regional extinctions have occurred, and many species have either suffered a significant decline in distribution or are now found only in restricted areas.
- The Murray-Darling system has a high proportion of alien species — 13 of the 63 fish species are alien or translocated. Alien fish now make up about 70 per cent of the numbers and 80–90 per cent of the biomass of fish in many of the Basin’s rivers.
New and undescribed species
Genetic studies are increasingly recognising several previously undescribed fish species in the Basin. For instance, the Mountain galaxias was recently found to contain 15 separate species; and Australian smelt and the blackfish genus Gadopsis are now known to contain several previously undescribed species. Similarly, increasing survey effort has recently documented the presence of species not previously known from the Basin (e.g. Crucian carp and Silver tandan). Consequently, the number of species known from the Basin is expected to continue to rise into the future.
Twenty-four of the 51 native species in the Basin are recognised as rare or threatened on State, Territory, National or International listings. Previously, no fish species was known to have become extinct in the Basin, but the discovery in the early 2000s and then loss after the Millennium Drought of Yarra pygmy perch in the Lower Lakes has changed this. Several localised extinctions have occurred and many species and populations are at risk.
The increasing number of cryptic species currently being described from genetic investigations means it is almost certain that additional species will have been lost before they have been described, a sobering thought in a country that has a relatively depauperate freshwater fish community.
Several Basin fish communities are also listed as threatened. In NSW, three aquatic ecological communities are listed as endangered (the lower Murray, lower Darling-Baaka, and lower Lachlan), and in Victoria the Lowland Riverine Fish Community of the Southern Murray-Darling Basin is listed as threatened.