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:

Population Monitoring and Fish removal

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: Nick Ling (

Computer modelling

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 (

Food web studies

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 (, Michael Pingram (

Molecular biology

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 (, Ian Duggan (

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