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Published: 2003-09-09 00:00:00

Pest fish in Lake Rotopiko (Serpentine) near Ohaupo will get the shock of their lives later this month.

Environment Waikato and the Department of Conservation are going fishing with an electrofishing boat to remove populations of rudd and other pest fish such as goldfish and catfish, which are damaging aquatic vegetation by grazing in the Waikato’s shallow lakes.

The ‘e-boat’, recently purchased by Waikato University, sends an electric shock into the water to stun fish, which are then scooped up, measured and weighed and the pest fish removed while native fish are returned to the water unharmed.

It will be the first time the e-boat has been used commercially in New Zealand and the operation is planned to capture fish which have avoided nets used in previous fishing operations. The e-boat will be lifted into the water and transferred between the two sections of the lake by helicopter.

Freshwater scientist Grant Barnes said the electro method, which had been used in Australia and the United States, is another tool available in the fight against pest fish.

A full census of fish populations in the two lakes is being done to assess the success of previous netting operations. The team hopes not to catch many rudd, confirming the success of earlier control work.

The Council has been working with the Department of Conservation to remove rudd from the lakes for the past two years using mist nets, and fish numbers are declining. Rudd are a threat to significant native plant communities which are rapidly declining in shallow lakes in the North Island.

Major fishing operations had been undertaken, with staff using gill nets to remove about 700 fish in the pre-spawning stage in September 2001, and another 300 fish following spawning in March last year. No large fish were caught, indicating that the majority of breeding stock was removed, he said.

The Waikato Region has almost 100 small shallow lakes. Apart from a few lakes in native forest catchments, most are affected by vegetation clearing and land use practices. Many have been invaded by plant and animal pests, have poor water quality and fewer native fish and plants.

Fewer than 10 percent still have aquatic vegetation and only five are in their natural state, so it is important to protect the few that remain, he said.

“If this is successful in New Zealand, the e-boat offers another 'tool' to control pest fish. This is good news for lakes and waterways and bad news for those pest fish that threaten our Region's special areas.”