Threat of Invasive Species in the Murray-Darling
This is likely a result of significant changes in water flow, thermal (cold water) pollution, instream habitat degradation, and barriers to fish passage which have fostered conditions favourable to invasive species over native fish populations.
The European Carp
In 30 years since its introduction, however, the European Carp has become the predominant biomass in the Murray- Darling. At many sites, carp account for an estimated 60-90% of the total fish biomass, with densities as high as one carp per square meter.
Due to the high level of water regulation and fragmentation in the Murray-Darling, carp’s ability to breed in turbid water in the absence of the natural flood and drought cycle, give it a biological advantage over native fish species.
Irrigated agriculture accounts for 95% of water diversion in the Murray, covers almost 1.5 million ha in the Murray-Darling basin, and has severely damaged the rivers' ecology.
Extensive dam and weir development for agriculture creates barriers to native fish migration, extracts half of the annual stream flow in the Murray, and increases periods of low flow.
Such development also causes permanent flooding and high water in some areas, increases sedimentation, and reverses the seasonality of natural flows. In addition, carp change the natural habitat by uprooting the vegetation upon which native fish depend for habitat and food.
Carp also muddy the water in which they feed, which blocks the photosynthetic growth of native aquatic plants. Unlike native fish, these invasives have fleshy barbs which are well-adapted for searching for food in murky waters.
Compounding this damage, periodic unnaturally cold water released from dams in the headwaters of all but three river tributaries prevent 'warm water' native fish from breeding for around 300 Km downstream.
For example, localized extinction of the Trout Cod, Murray Cod and Macquarie Perch has occurred 100 Km downstream following completion of the Dartmouth Dam, the last major dam built in the basin.
Removing 'snags' (fallen trees and branches) from the watercourse also reduces the habitat quality and breeding success of native fish while increasing the competitive advantage of invasives. Carp also spawn in the many vegetated irrigation channel systems in the Murray-Darling River system.
The Mosquito Fish or Plague Minnow
The Mosquito Fish or Plague Minnow is another serious threat to native fish in the Murray-Darling. An aquarium fish that was introduced in the 1920's to prey on mosquitoes as it did in the Rio Grande, this species has had no impact on mosquito prevalence, but attacks, injures and preys on native fish.
It nips the fins of other fish, leaving open sores which spread pathogens among fish, and competes with them for food and habitat. The Mosquito Fish also feeds on native fish fry at the water's surface and preys on the eggs and attacks the tadpoles of native frogs.
Speaking of aquarium fish released into the Murray-Darling, this is now the largest source of new feral freshwater fish in Australia. Since 1990, the number of exotic fish in Australia's waters overall jumped from 22 to 34, and all except for one of these introduced species originated from the aquarium trade.
Compounding damage to the Murray-Darling, are invasive plants including water plants released from aquariums and ponds, riparian trees introduced for aesthetic purposes, and a variety of plants introduced for agricultural and ornamental purposes that are invading floodplains and other wetlands.
One example is Lippia, introduced as an ornamental 'no mow' lawn. This unpalatable herb is smothering the river system’s floodplain. These plants reduce the natural productivity of the floodplain, river and other wetland habitats, further depleting natural wildlife populations.
These invasive species reflect an ongoing governance failure common to most countries. While the Australian Government has long had some quarantine controls, they have not adequately excluded new introductions of dangerous species.
Most importantly, Australian governments have failed to adequately screen the many exotic species already in the country - legally and illegally - and undertake 'incursion management', to kill dangerous species while their populations are still low.
Also better 'vector controls' to manage the sources of these exotic species are missing, such as programmes directed at aquaculture and at the aquarium trade to regulate and prevent release of alien fish and plants. Instead most state (provincial) governments have focused on ineffective and expensive 'control' programmes, when these alien species have invaded too thoroughly to be eradicated.
At least 11 introduced fish species make up one quarter of the basin’s total number of fish species, including the Brown Trout, Rainbow Trout, Redfin Perch, Gambusia, and Goldfish.