University of Waikato and NIWA: Karori perch pests successfully culled


In a collaborative study, echosounder surveys of the lower reservoir at ZEALANDIA, the Karori Wildlife Sanctuary, show that electro-fishing and netting have successfully reduced perch numbers in the conservation safe haven.

Echogram image taken after the cull. The number of fish is significantly reduced. [Credit: NIWA]

Perch were first introduced to the reservoir in 1878, but since then numbers have increased to damaging levels in the scenic lake.

The study, initiated by Associate Professor Brendan Hicks of the University of Waikato, aims to establish whether the perch are contributing to algal blooms in the reservoir by eating zooplankton, which help to control these algal blooms.

In total, over three years, University of Waikato researchers have removed 14,892 perch from the reservoir, but it takes an ongoing, collaborative effort to research and control the perch numbers.

In 2007, boat electrofishing and netting showed there may have been at least 20,000 perch in the 2.57 ha reservoir – a potentially serious threat to the ecosystem of the waterway. In February last year (2008), after more perch were removed, a NIWA survey indicated that more than 3,000 perch still remained in the limnetic zone of the reservoir.

In February this year (2009), NIWA scientists again surveyed the reservoir using state-of-the-art echosounder technology. They found that the perch population has continued to increase.

NIWA fisheries scientist Dr Stéphan Gauthier says “the survey revealed far more perch echoes than we observed last year following perch removal, suggesting rapid population growth.”

Scientists from the University of Waikato then removed 5,436 perch.

A second NIWA survey (after the perch removal) has now shown the fishing to be a huge success.

Raewyn Empson, Conservation Manager at Karori Sanctuary Trust, which manages ZEALANDIA, says the survey confirmed the decision to cull perch numbers.

“Restoration of the valley’s streams and lakes remains a key goal of the Trust, because native fish, birds, and freshwater invertebrates have been severely impacted by the loss of wetland and lake habitats in the Wellington region. Research in our streams and lakes confirms the exotic fish are problem species that will need to be removed to allow the freshwater koura and other invertebrates to thrive, and native fish to be reintroduced.”

Dr Gauthier of NIWA says: “We estimate numbers have dropped significantly as a result of the fishing. Although some perch remain in the reservoir, densities are much lower. This can be clearly seen in the pre-cull and post-cull survey echograms.”

“This should have a meaningful impact on the health of the lake by allowing zooplankton communities to thrive and by promoting the establishment of native fish species.”

Associate Professor Hicks agrees: “Perch removals in 2007 and 2008 were followed by a dramatic increase in zooplankton and decrease in the algal bloom. These studies suggest that if perch were eradicated from the lake, water quality would improve permanently.”

The survey is part of a collaborative project involving the University of Waikato, NIWA, Cawthron Institute, and Karori Sanctuary Trust. Cawthron Institute has been monitoring algal blooms, and the University of Waikato has been monitoring zooplankton.

For more information, please contact:

Dr Stéphan Gauthier, NIWA

Associate Professor Brendan Hicks, University of Waikato

Raewyn Empson, Karori Sanctuary Trust


Red finned (or European) perch – Perca fluviatilis

  • Are one of at least 20 introduced freshwater species in New Zealand including brown and rainbow trout, koi carp, rudd, and goldfish.
  • Were first introduced in 1868 as an angling fish.
  • Belong to the Percidae family, a family of about 60 species that are native to the Northern Hemisphere. Only one species of perch exists in New Zealand.
  • Are found throughout New Zealand, including Northland, Auckland, Taranaki, Wellington, Otago, and Southland. Prefer slow-flowing and still habitats.
  • Grow up to 400-450 mm long and usually weigh between 1-2 kg
  • Larval perch (5-30mm) feed on pelagic zooplankton. At intermediate sizes (30-80mm) perch often feed on benthic macroinvertebrates including crayfish. Adult perch may eat other fish, including eels, and can also be cannibals, eating smaller perch.
  • Are classified as a sports fish in some areas, but many regional councils now looking at ways to reduce and control perch numbers because of the significant impact they can have on freshwater ecology, directly affecting zooplankton, macroinvertebrate, and fish populations.
  • Knowingly propagating, breeding or releasing perch into New Zealand waterways is an offence under the Biosecurity Act


Bottom-dwelling; describing the flora and fauna found on the bottom of a sea or lake

Limnetic zone
The well-lit, open surface waters in a lake, away from the shore

[Of fish] Living in the upper layers of the open sea or lake

Perch. [NIWA]


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