Pest fish in our freshwater
Science Centres: Aquatic Biodiversity and Biosecurity
Rudd ruined a trout fishery in a New Zealand lake, and may prevent the growth of plant seedlings.
Koi carp are prolific breeders and can ruin lakes by removing vegetation and increasing turbidity.
The European perch can reduce populations of small native fish in lakes (photo: Stephen Moore).
There is a growing awareness worldwide that some species of freshwater fish are serious pests which can damage both the biodiversity and quality of our aquatic environments. Common carp have a reputation for turning pristine lakes, ponds, and rivers turbid, raising concerns about koi carp in New Zealand. However, new research shows that other exotic fish, including mosquitofish, rudd, perch, and catfish are also affecting New Zealand waterways and reducing indigenous biodiversity.
Mosquitofish were spread around the top half of the North Island to control mosquitoes in ponds, and their recent spread reflects the increase in irrigation ponds built for horticulture. Mosquitofish reproduce in ponds, and their offspring move downstream to colonise other suitable freshwater habitats. They can also colonise mangrove swamps in harbours because of their tolerance of salt water, and in time they will probably spread around the shallow margins of these harbours, swimming upstream into small tributary streams. Because mosquitofish displace whitebait in lakes, it is feared they could decimate whitebait stocks in northern harbours.
Perch prey on other fish and can drastically reduce the number of common bullies in ponds and lakes. They may also reduce the recruitment of other native fish. For example, juvenile whitebait and eels often migrate through lakes to tributary streams, and if there were perch in these lakes, the native fish would be vulnerable. We found almost no banded kokopu in one of the lake tributaries we surveyed, but there were many in a control stream just below the lake. This suggests that perch may cause problems for whitebait and bully populations in lakes.
We are just beginning to understand the extent of the problems caused by pest fish in New Zealand waters – and their impacts on native fish are not the only concern. For example, koi carp can reduce water quality in lakes; benthic-feeding fish (e.g., catfish and tench) can disturb lake sediments; and other herbivorous species (e.g., rudd and goldfish) can prevent the regeneration of aquatic plants.
We need better public education programmes and greater control over the introduction and spread of pest fish. DOC, MAF, MFish, and regional councils are working on this as part of the Biosecurity Act. Therefore, it is likely that we will see more emphasis placed on the control of pest fish in lakes and rivers to preserve or restore the quality and biodiversity of these environments.
We cannot keep pest fish out of our lakes and rivers because they are already found in many of them, especially in the North Island. If we cannot eradicate them, then we will need to control them – but only in waters where they cause problems. We are currently identifying these problems and trying to predict where they will occur so we can develop control strategies. Low-tech, inexpensive eradication and control methods are clearly needed, and we expect this area of research to increase in the near future.