Mosquito fish

While most people know about the European carp and the damage it does to the ecologies of our lakes and rivers, few have heard of the mosquito fish (Gambusia holbrooki), which is common in our waterways and also does a great deal of ecological damage.

The mosquito fish is a small fish (males are around three centimetres in length and females six centimetres). They were introduced into Australian waterways in 1925 to control mosquitos and quickly spread across much of Australia, including our district. So how can such a small fish that goes largely unnoticed impact upon the ecology of our waterways?

For a start, the mosquito fish did not control the mosquitos and mosquito larvae any better than the native species. However, they did have a voracious appetite for fish eggs and fish larvae as well as frogs’ eggs and newly hatched tadpoles. They also nip the fins of native fish, leaving them vulnerable to infection and less capable of swimming after prey. Mosquito fish are very aggressive and will attack and harass larger species. They will eat anything that will fit into their mouths, meaning they are is in direct competition with native species that also eat small aquatic creatures. All across the mosquito fish’s range the populations of native fish and amphibians have fallen dramatically.

Mosquito fish are prolific breeders and can out-breed native species. They give birth to live young three or four times a year. The young attain breeding age in only four to six weeks. This puts enormous pressure on the native species.

Mosquito fish are very hardy and can survive in a wide range of water temperatures, in water with low levels of oxygen and in turbid and polluted waters. It is a tough little survivor that can weather droughts with its ability to survive in small, muddy remnant waters.

One of the native fish most affected by the mosquito fish are the various rainbow fish species (genus Melanotaenia). These are small fish with horizontal stripes, often like a rainbow, along their sides. Many water catchments have their own unique species of rainbow fish. Some species from small, de-graded catchments are particularly vulnerable with a number of species being listed as “Critically Endangered” and others as “Endangered” or “Threatened”. Luckily, they do well as aquarium fish and can be saved. Another endangered fish species is the purple spotted gudgeon (Mogurnda adspersa). It also does well in aquariums and captive bred gudgeons have been reintroduced to several water catchments. The mosquito fish has also had an impact upon ten frog species including several species of bellfrog.

In short, the mosquito fish is the fish from hell. The only thing good about it is that it seems to be able to eat cane toad eggs. In Victoria it has now been declared a restricted noxious fish. It is illegal to keep the fish or to release it into a waterway (even if it was just caught moments before). If the fish is sighted it should be reported to the Fisheries Victoria Customer Service Centre on 136 186.