October 10, 2008

Grass Carp Rid Lake of ‘Environmental Cancer’

By SHARPE, Marty

FOUR hundred grass carp introduced to a Hawke's Bay lake have chewed their way through an aquatic weed called an "aggressive environmental cancer".

The eradication of hydrilla from Lake Eland, 45 kilometres west of Napier, bodes well for Biosecurity New Zealand's plans to release hundreds more fish into nearby lakes that are also infested with the weed.

The carp were put into the four hectare man-made Lake Eland in 1986 in the hope they would control hydrilla -- one of the world's most invasive waterweeds.

But the fish have done better than that -- they have eradicated the weed, which in New Zealand is found only in Hawke's Bay.

"This really is a great result," Biosecurity NZ senior adviser Victoria Lamb said yesterday. "Lake Eland was a mess. Algal blooms were common and the water got so bad cattle stopped drinking from it."

Grass carp had succeeded in ridding other New Zealand lakes of weeds, but not hydrilla. There were a few lakes in the United States where it was believed to have happened, but it was not an easy weed to fight, Ms Lamb said.

"It's like an aggressive cancer. We're very lucky it's never spread from Hawke's Bay. We've had US experts come out and tell us if we can't get rid of it, we'd be better just filling the lakes with concrete."

Grass carp cannot breed in New Zealand conditions. Their average life expectancy is 12 years and they can grow to 18 kilograms. There are about 30 fish left in the lake.

Biosecurity NZ has asked the conservation minister for permission to release grass carp into lakes Tutira, Waikopiro and Opouahi, which are also infested. The proposal is open to public submission and if approved, fish will be put into the lakes this summer.

It is believed hydrilla may have been introduced by someone "liberating" a goldfish in Lake Tutira in the late 1950s. Thick mats of the weed were discovered in 1963. By then it covered nearly a fifth of the 174-hectare lake. It smothers other aquatic plants, including natives, and can remain dormant for up to 10 years, waiting for the right conditions to grow.

The weed is native to Asia and northern Australia and is widely used as an oxygen weed in pet fish tanks and ponds. It is banned in New Zealand.


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