Exotic fish: a growing problem in New Zealand fresh waters

Dave Rowe

Evidence is mounting of the damage done to New Zealand freshwater habitats and native species by non-indigenous (exotic) fish.

Introduced fish such as perch, gambusia, rudd, catfish, koi carp, and tench continue to spread within New Zealand. Although some of these introductions are accidental, others are clearly deliberate, unapproved attempts to create new recreational fisheries. While these introductions may benefit some anglers, the environmental costs need to be weighed carefully. Evidently, some anglers don’t understand the environmental impacts of introducing exotic fish to New Zealand lakes or the risks they pose for native biodiversity.

Learning to recognise the problem

Research on impacts addresses this lack of understanding and will help underpin public relations campaigns to restrict the spread of these fish. It will also inform the development of management plans to control or eradicate pest fish, because not every occurrence of exotic fish poses a problem. Impacts by exotic fish are generally related to the development of large populations but this may not occur where key habitats are lacking or where there are natural predators. Some lakes don’t have large populations of exotic species, and control may be less urgent there or where exotic species have little effect. Therefore, one key objective for NIWA’s biosecurity research is understanding impacts.

Perch predation on small native fish

Perch were introduced to New Zealand by early European settlers to provide a food supply. Today, they are being deliberately (but unofficially) spread to small lakes and ponds to create new recreational fisheries. But their spread is a major concern as adult perch are carnivores, and feed mainly on small fish. Experimental research by Otago University has shown that perch can reduce populations of native common bullies in South Island lakes. Meanwhile, NIWA surveys in North Island lakes indicate that lakes with perch have fewer bullies, galaxiids and smelt compared with lakes without perch. It is becoming clear that perch reduce the abundance of small native fish in lakes, and we suspect this will affect other top predators, such as eels, and will also reduce native biodiversity. Trout have reduced galaxiid populations in most of the large, inland lakes. Perch now threaten them in many of the smaller lakes.

Are gambusia a threat to whitebait?

Perch. (Photo: S.C. Moore)


Gambusia. (Photo: R.M. McDowall)


Catfish. (Photo: R.M. McDowall)


Rudd. (Photo: D. Rowe)


Koi carp. (Photo: D. Rowe)

The recent spread of gambusia is also a concern. These small fish are stocked into ponds to control mosquitos and, although they may be effective at this, there is no evidence that they are any better than native fish! Meanwhile, we’re learning more about their negative impact on native fish. They have been implicated in the decline of the dune lakes inanga in 3 of the 13 lakes where this threatened species occurs. Gambusia are a highly aggressive fish and nip the fins of larger fish. Such aggression results in the exclusion and displacement of native fish from the shallow, food-rich, marginal habitats of lakes, to deeper waters. Gambusia can also reduce native fish abundance: mass mortalities of the dune lakes inanga have been caused by fin-nipping. (Read more about this behaviour in “Gambusia – a biodiversity threat?”.)

Recent research in New Zealand indicates that gambusia may also pose a threat to whitebait by excluding them from the shallow, still-water habitats of swamps, wetlands and stream margins where both species prefer to live. This is a real concern, because gambusia can spread very easily to these habitats. This species is a live-bearer and one pregnant female transferred to a new pond can start a new population. In Northland, a few gambusia in a cattle trough spread to a nearby stream and then colonised the water course downstream to the harbour. Here, this species quickly adapted to full-strength sea-water and colonised the shallow mangrove swamp habitats. Reports indicate that gambusia can then be spread within harbours, and even between rivers, by tides and currents. Thus, if gambusia is present somewhere in a river catchment, it can be expected to eventually spread to all suitable habitats in the lower reaches and affect the whitebait.

Catfish versus eels

Brown bullhead catfish is another species that is easily spread, probably by accident in eel nets and boat trailers. DOC is carefully monitoring its colonisation within Lake Taupo. It appears that the trout fishery here is safe because the main food of trout is smelt, whereas for adult catfish it is freshwater crayfish (koura). This is bad news for the koura in Taupo and for koura in the rivers and streams now colonised by catfish. Koura are the main prey for large eels in rivers and eel fishers tell us that eels have declined where catfish occur. Competition for food between large eels and catfish may therefore be an issue in some rivers.

Are rudd underwater possums?

Adult rudd are herbivorous and so may pose little direct threat to other fish. However, dense populations have been known to affect trout fisheries by taking lures before the trout do and thus discouraging anglers, who find they can only hook small, stunted, rudd. The browsing habits of rudd are also a concern and have been likened to those of deer and possums in native forests. By browsing new plant growth and new seedlings, rudd can prevent the regeneration of native plants in lakes and this may eventually result in the collapse of the macrophyte beds and reduce water quality.

Exotic fish communities and water quality

Like rudd, tench have no teeth and so pose no direct threat to other fish. However, they feed on plankton as well as on small benthic animals and often co-occur with rudd and perch. In fact, it is rare for just one exotic species to be present in a lake: usually there are two or more species, creating an exotic fish community. Recent research indicates that North Island lakes containing exotic fish communities have lower water clarity than comparable lakes without exotic fish populations. The juveniles of all the exotic species feed on zooplankton and high densities of planktivorous fish can increase planktonic algae in lakes. Meanwhile, adult koi carp, tench, and catfish, are bottom feeders and can disturb sediments, re-suspending both silt and nutrients in the water column. Other exotic species also feed on the bottom when other prey are scarce, and being relatively large fish, can also disturb sediments. Finally, herbivorous species such as rudd and koi carp can be expected to browse down aquatic plants, releasing the nutrients locked away in plant tissue back into the water. These processes all combine, depending on the exotic species present, to increase nutrient levels, planktonic algae and silt in lakes and to thereby reduce water clarity.

The problems and a way forward

Exotic fish, depending on the species, can reduce native fish populations, interfere with other fisheries, reduce native biodiversity, and degrade water quality. Although we cannot yet predict what impacts will occur in which lake or river, it is clear that the potential environmental effects of new introductions are serious. The costs of restoration where these impacts are unacceptable will be high and will fall on future generations. Further research to develop the necessary controls will be a key part of NIWA’s response to both the Biodiversity Strategy and Tiakina Aotearoa: Protect New Zealand – the Biosecurity Strategy for New Zealand 2003.

Teachers’ resource for NCEA AS: Biology 1.5, 2.9, 3.2; Science 2.2, 2.3. See other curriculum connections at www.niwa.co.nz/pubs/wa/resources