NIWA helps Sanctuary in the great perch search

Scientists from NIWA will be using state-of-the-art echosounder technology to get to the bottom of a perch problem that has been plaguing Wellington's conservation safe haven, Karori Sanctuary.

Red finned (or European) perch (Perca fluviatilis) were introduced in the lower Karori reservoir in the early 1900s. The perch are thought to be contributing to algal blooms in the reservoir (now one of the Sanctuary's two scenic lakes) because small perch eat zooplankton, which are important to control algal blooms in the reservoir.

"This 'top-down' effect may be one of the reasons why water clarity in the reservoir is so poor" said NIWA fisheries scientist Dr Stephane Gauthier, who is leading the survey.

In order to assess and potentially control the population of perch, it is crucial to estimate their numbers. NIWA is employing state-of-the-art echosounder technologies to sample the lake. Using approaches designed for surveying shallow waters, NIWA scientists hope to get accurateestimates of the perch population inhabiting the open-water portion ofthe reservoir. The technique could also have applications for surveying invasive fish populations in other parts of the country.

This survey, combined with estimates of perch abundance within the littoral zone (close to shore) performed by the University of Waikato, will provide a complete picture of the population in the reservoir. These earlier audits have shown the red-finned perch population at the conservation safe haven to be between 20,000 and 30,000 , however today's findings put this number at the upper end of the scale.

"We based our original estimates on the assumption that perch only lived in the oxygen-rich top layers of the lake" said Sanctuary spokesperson Keith Calder.

"However, the NIWA survey has indicated the presence of large numbers of perch living below the standard oxygenated water layer, suggesting our perch population may be far higher than previously thought."

Dr Gauthier and NIWA fisheries acoustics expert Dr Adam Dunford will be conducting the survey from Sanctuary's electric boat Ara Kawau. The boat is more usually used to ferry visitors up the lake from the Victorian Valve Tower.

The survey is being funded by NIWA's Capability Fund as part of a wider collaboration with the University of Waikato and Karori Sanctuary.

For further information, please contact:

Mr Alan Dicks
Marketing Co-ordinator
Karori Sanctuary