Why invasive fish are bad

All five of these fish (Gambusia, Catfish, Koi Carp, Rudd and Tench) are labelled as pests as they are damaging freshwater habitats and competing with native fish.

Gambusia give birth to live babies and a single female can produce up to 50 offspring in a year. Because of this, Gambusia out-compete native fish for food and habitat. They are also known to attack native fish by nipping their eyes and fins. Mudfish and Inanga are particularly vulnerable.

Catfish are known to eat native fish, fish eggs and koura (native freshwater crayfish). Their feeding habits often stir up sediment reducing water quality.

Koi Carp stir up the bottom of ponds, lakes and rivers when they feed, muddying the water and destroying native plant, bird and fish habitat. They eat a wide range of food, including native insects, fish eggs, juvenile fish of other species and a diverse range of plants.

Adult Rudd mainly feed on native aquatic plants (macrophytes) and are referred to as the ‘possum of the waterways’. This type of feeding reduces the number of native macrophytes which limits habitat and food for native fish and macroinvertebrates. It also stirs up mud from the bottom of ponds and streams reducing water quality. Juvenile Rudd feed on zooplankton (tiny aquatic animals) which can endanger native fish stocks as well as trout and salmon fisheries. It can also lead to an explosion of phytoplankton (algae) populations.