KEY POINTS:
Waikato University scientists who have spent much of the past week killing thousands of pest fish at Wellington's Karori Wildlife Sanctuary hope the cull will provide clues to controlling algal blooms.
Red-finned European perch have been living in the sanctuary's lower lake for nearly 130 years and are so numerous that the young perch eat most of the microscopic animals - zooplankton - which would normally control blue-green algae blooms that turn the lake into a thick algal soup.
The lower lake has had toxic algal blooms most summers in the past decade. In 2004 experiments by a Victoria University student, Kirsty Smith, revealed perch were a bigger part of the problem than the usual suspects, nitrogen and phosphates leaching into the waterway.
Perch are now so dominant in the lower lake that they have effectively created a single-species ecosystem, excluding native fish such as kokopu and bullies, according to Karori Wildlife Sanctuary Trust chief executive Nancy McIntosh-Ward.
She said the lake's algal booms were providing biologists from Waikato University and Nelson's Cawthron Institute with an outdoor laboratory to study the effects of a high density of juvenile perch.
Waikato University is managing $10 million in taxpayer funding for researching lake restoration, with emphasis on the risks from weeds, pest fish and toxic algal blooms that have recently closed water supplies and killed cattle.
Some of its lake management specialists have been in Wellington since last week using the nation's only purpose-built electric fishing boat to stun thousands of the perch.
The 1000-volt charges delivered by the boat stunned 2000 perch in just the first two days - and an eel and a trout.
Based on these results, the researchers have estimated an extraordinary concentration of more than 30,000 perch in the tiny 2.5ha lake.
The dead perch will be dissected to collect information on their age and diet.
"The ecology of the sanctuary's lower lake provides us with a unique case study" said Associate Professor Brendan Hicks.
"The lake is a rare example of a totally controlled environment.
"The human impact on the lake ecosystem here is closely regulated and the water quality has been carefully monitored for over 10 years, giving us the ideal conditions to study the effects of exotic fish."
- NZPA
