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Published: 2002-06-07 00:00:00

A unique fishing operation has helped remove hundreds of pest fish from a threatened Waikato peat lake.

Environment Waikato’s Environment Committee heard this week that the pest fish rudd was discovered in Lake Rotopiko’s northern lake last year. Freshwater ecologist Grant Barnes said the discovery set off alarm bells because the presence of these fish could destroy the lake’s unique native plant communities.

Working with the Department of Conservation and NIWA, Environment Waikato staff began an operation not attempted before in New Zealand on such a scale.

They went fishing in the early hours of the morning with gill nets and removed 671 fish in the first pre-spawning stage last September, and another 317 fish following spawning in March this year. The absence of any large fish indicated that the majority of breeding stock was removed.

The Waikato Region had almost 100 lakes, most of them shallow and less than 10 hectares in size, he said. Apart from a few lakes in native forest catchments, most were affected by vegetation clearing and land use practices. Many had degraded water quality, fewer native fish and plants and were invaded by plant and animal pests.

Fewer than 10 percent still had aquatic vegetation and only five were in their natural state. said it was important to protect the few that remained. Lake Rotopiko, now in three parts, was the remnant of a larger lake before drainage was carried out.

“The population has taken a hammering so it will take them a while to recover, and we intend to mop up remaining fish this September.”

Further work would also be done to capitalise on the benefits of removing the fish, which could include lake level setting, catchment management to reduce land runoff and drainage. Long term there was a need to increase the buffer area around the lakes.

Staff were working with the Federation of Coarse Anglers to ensure that the fish were kept out of the lake as there was a real risk the work would be undone if people returned fish to the lake.

The fish caught in the operation were not wasted. Some were kept to examine and improve understanding of their physiology, while the rest were fed to the otters at Hamilton Zoo.