Invasive Fish

Over one-quarter of the freshwater fish species present in New Zealand have been introduced from overseas. Like all introduced species, they have some impact on native ecosystems, but some cause more problems than others. These invasive fish are most widespread and abundant in the northern half of the North Island where they can have a range of potential adverse effects on freshwater ecosystems. These effects are likely to become more significant and widespread in the face of human population growth and climate change which both have the potential to increase the distribution of these species.

Our research aims to:

  1. Understand the biology and abundance of invasive fish in New Zealand
  2. Quantify their ecological impacts on freshwater ecosystems, and
  3. Develop methods to assist with their control and monitoring.

  • Invasive species
  • Population estimates
  • Ecological effects
  • Control and monitoring methods
  • Management implications

Ecological impacts

The larvae of all invasive fish eat zooplankton and high numbers can potentially compete for food with larvae of native fish species. As adults invasive fish can have a range of impacts when present in high numbers:

  • Koi carp - Feeding method stirs up sediment reducing water clarity, and uproots aquatic plants reducing native biodiversity. Stirring up sediments also releases nutrients which can lead to algal blooms.
  • Gambusia - Become highly aggressive to other fish species when in large numbers. They also out-compete native fish species for food such as zooplankton and larger invertebrates.
  • Catfish - Eat invertebrates, including crayfish/k?ura, and potentially compete with eels/tuna for food. They may also degrade water quality through their feeding activities.
  • Goldfish - Goldfish feed on a wide range of things from bottom habitats, and in so doing stir up sediment and nutrients affecting water clarity.
  • Rudd - Adults browse on aquatic plants and can prevent their re-growth in lakes.

Our research is quantifying these ecological impacts by:

  • Conducting fish removal and exclusion experiments and monitoring ecological changes...

LERNZ has undertaken to reduce pest fish populations in Lake Ohinewai through fishing pressure and exclusion. Lake Ohinewai is a small (16.8 ha) riverine lake on the lower Waikato River floodplain. Populations of koi carp, brown bullhead catfish, rudd and goldfish have contributed to the loss of submerged aquatic plants, resulting in a decline in water quality and frequent algal blooms. The goal is to reduce the total pest fish population to below 100 kg/ha to encourage recovery of lake vegetation. Fishing pressure and a one-way exclusion barrier on the lake outflow has reduced the pest fish population from 289 kg/ha in February 2011 to less than 50 kg/ha in December 2013. This has resulted in an increase in water clarity from 0.45 m Secchi depth in 2011 to 0.90 m in 2013. Monitoring of the pest fish populations and aquatic vegetation recovery is continuing.

Contact: Grant Tempero ([email protected])

  • Modelling contributions of invasive fish to lake nutrient budgets...

LERNZ is investigating how pest fish contribute to overall nutrient cycling at an ecosystem scale. A hydrodynamic-ecologically coupled model (DYRESM CAEDYM) has been created to model the impact of carp in Lake Ohinewai.Initial results estimate that, at a density of 250 kg ha-1, carp contribute significantly to nutrient recycling from lake sediments, via excretion. Carp are also responsible for significant sediment resuspension due to feeding activity. Future work will include simulating different 'bottom up' control and 'top down' control scenarios for lake water quality management, such as carp removal, and land use changes. In addition, pest fish modeling will be investigated for other fish species and other lakes.

Contact: Mat Allan ([email protected])

  • Determining effects of pest fish on lake food-webs...

LERNZ is working with Waikato Regional Council to understand how invasive fish affect the flow of energy through lake food-webs. This project is using gut content and stable isotope analyses to investigate the diet of fish and incorporation of carbon from various sources into different trophic levels within aquatic food-webs in several lower Waikato lakes. The lakes being studied have different invasive fish communities, and some have been subject to pest fish control. This work will help predict the effects of species invasions on lake food-webs, and also the effects of implementation or removal of pest fish control on other species.

Contact: Kevin Collier ([email protected]), Michael Pingram ([email protected])

  • Providing a molecular tool for the rapid assessment of zooplankton communities...

Freshwater zooplankton respond rapidly to environmental changes in lakes, including changes to invasive fish recruitment and nutrient loads. We have developed the Zooplankton Molecular-Based Assessment (ZooMBA) tool utilising DNA sequencing and an online library of DNA 'barcodes' to enable rapid assessment of zooplankton communities without the need for specialist taxonomic skills. Ongoing developments of this method are focussing on next generation sequencing to allow for analysis of entire environmental samples to characterise community composition without the need for sorting.

Contact: Ian Hogg ([email protected]), Ian Duggan ([email protected])

Learn more...

Control and monitoring methods

  • Electric fishing boat

LERNZ has New Zealand's first electrofishing boat designed specifically to fish non-wadeable habitats. The ability to catch pest fish in fresh water that is too deep for wading (1-3 m) is the key to determining the abundance, impacts, and biological effects of pest fish. A major advantage of boat electrofishing is its time efficiency, with the ability to fish kilometres of habitat in a single day without the need for repeat visits. With our on-board GPS unit we estimate the distance and area fished, and can then calculate density and biomass per unit area. Since its launch in July 2003 we have was used in over 100 locations in the North Island of New Zealand to catch over 20,000 fish (total weight over 5 tonnes). We have developed multiple-pass boat electrofishing to give removal population estimates (Hicks et al. 2006 ).

Contact: Brendan Hicks ([email protected])

  • Pod traps

Pod traps are pyramid-shaped nets equipped with an automated wildlife feeder that frequently adds fresh bait to the trap to attract fish. Baits lose most of their attraction properties within an hour in the water, so by adding fresh bait trapping rates are greatly improved. Once inside the pod trap one-way doors keep fish within the trap until emptied. Pod traps are particularly effective at trapping koi carp, and have been shown to improve catch rates compared with other types of nets. Baited traps such as the pod trap lose their effectiveness after the bulk of the population has been removed because food becomes plentiful, making bait less attractive to fish, but they are nevertheless as useful method for monitoring koi carp populations.

  • Poison baits

Baits laced with toxins can be used to reduce carp numbers, but flavouring is often necessary to mask their unpleasant taste. LERNZ researchers have investigated the effectiveness of floating baits made mainly from brewer's yeast and grain laced with 'bold' flavours, such as vanilla or strawberry essence. Fish were given the choice of different bait flavours in tanks at the University of Waikato fish lab. All flavours were readily consumed indicating that any of the formulations could be used with equal success. One advantage of using floating baits is that unconsumed pellets can be removed from the water surface before they sink and toxins become available to native species feeding at night or on the bottom, such as eels. LERNZ researchers have also investigated the use of cube root powder and sodium nitrite as toxins in floating baits. The dose required to kill half the test animals (LD50) was 136 mg/kg for cube root powder and 122 mg/kg for sodium nitrite, indicating they were viable for inclusion in poison baits for koi carp.

Learn more...

  • Barriers

Exclusion barriers provide opportunities to prevent adult invasive fish from breeding in lakes or wetlands where they are not currently present or where they have been removed as part of an eradication programme. They typically comprise metal bars spaced to allow access to native species but not large pest fish. The bars swing one way to allow access downstream (i.e., out of the water body) but not upstream, and they are placed at constrictions on outlet streams. One such trap was installed by the Department of Conservation, with the assistance of LERNZ researchers, on a culvert at the Lake Ohinewai outlet following koi carp removal. It has proven effective at excluding breeding koi carp while requiring little ongoing maintenance.

  • Bottlenecks & spawning aggregations

This joint project with Waikato Regional Council is building on previous LERNZ research that documented the migratory behaviour of koi carp from river systems into lakes and wetlands to breed. We are determining the environmental cues initiating these migrations and determining key bottleneck' locations in the lower Waikato River floodplain where carp concentrate to support the deployment of mobile cages for carp trapping. In addition, we are investigating the use of unmanned aerial vehicles equipped with high resolution cameras and photo recognition software to survey lake margins for spawning aggregations.

Contact: Kevin Collier ([email protected]), Bruno David ([email protected])

  • Environmental DNA

Sensitive monitoring tools are needed to determine the success of invasive fish eradication or provide early warnings of invasion into new areas. LERNZ researchers are working with the Department of Conservation and the Nature Conservancy (Indiana, USA) to develop the use of ....

Contact: Jonathan Banks ([email protected])