Perch pests culled

Red perch

In March, scientists from the University of Waikato and NIWA (National Institute of Water and Atmospheric Research) announced that their electro-fishing and netting operations have successfully reduced perch numbers in the lake at the Karori Wildlife Sanctuary. NIWA used an echo-sounder survey to confirm the results.

The problem with perch

Last year, the Science Learning Hub reported on the problem of high numbers of perch in the lake. Estimates indicated that there were anywhere between 20,000 and 30,000 red finned perch (Perca fluviatilis) in the 2.57 hectare reservoir. Scientists, including Associate Professor Brendan Hicks from the Biological Sciences Department at the University of Waikato, were fearful that smaller perch were contributing to algal blooms in the reservoir by eating pelagic zooplankton. In addition, middle-sized perch (30–80mm) can feed on benthic large invertebrates such as crayfish. Adult perch (up to 450mm long and weighing 2 kilograms) prey on smaller fish, including eels.

Over the last 3 years, University of Waikato researchers have removed a total of 14,892 perch from the reservoir. This is a significant number, but Prof Hicks warns that it takes an on-going, collaborative effort to research and control the perch numbers.

Karori Sanctuary

“Perch numbers have increased rapidly after earlier culls so it is important we keep on top of the problem. We can also clearly see the link between perch numbers and the health of the lake. 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 removed from the lake, water quality would improve permanently.”

“Some perch do remain, but the zooplankton communities are growing as a result of the latest cull. Perch removal should also promote survival of eels, banded kōkopu and beneficial invertebrates in the lake.”

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.