Cat-door for carp hits fish invaders



No getting away: Waikato University researchers, from left, Bernard Simmonds, Claire Taylor and Adam Daniel, with a catch of the koi carp pests at Lake Ohinewai.

Kiwi can-do finds a novel solution to the fish pest clogging our waterways, writes Susan Pepperell. 

A TINY Waikato lake is the focus of a world-first scientific experiment, using what is essentially a giant cat-door to get rid of pest fish.

Waikato University scientists last week installed the first fish pest barrier at Lake Ohinewai, near Huntly, in a bid to clean up the near-dead lake, which is clogged with algae and koi carp that have killed virtually all other animal life.

The introduced species operate as mud pumps, sucking plugs of mud off the bottoms of lakes and using their teeth to crush any bugs they find. They also breed prolifically, with one female able to produce 700,000 eggs a year.

Scientists have been trapping koi carp to reduce the population ahead of installing the barrier as a more permanent solution.

No one knows how koi carp got into our waterways but it’s thought they were imported as an ornamental fish and spread when ponds flooded. They have been in the Waikato River, its tributaries and surrounding lakes since the 1980s.

The Waikato University-based and government-funded Lakes Ecosystem Renewal New Zealand programme member Dr Adam Daniel said the Ohinewai trapping programme was so successful they were netting up to 350kg of koi carp a day, which had reduced the population from about 7000 to 2700.

Now the aim is to rid the lake of even more fish by using the carps’ natural instincts to look for better habitats.

“If fish are moving between lakes, they will do it when conditions are poor, such as drought,” says Daniel.

“This is about using their own behaviour. It works like a one-way cat door – they’re allowed out, but they can’t come back in.”

The barrier has gaps wide enough for native fish to pass through, but not the carp.

The project, part of a series to restore biodiversity in New Zealand lakes, is about using realistic tools to reduce the mass of fish pests, to improve water quality and encourage bird and plant life.

He was also concerned that koi carp were spreading into waterways outside the Waikato, and said climate change and some in the fishing community moving them from one catchment to another, made their spread inevitable.

The carp caught haven’t gone to waste. They’ve been ground up, turned into slurry and sprayed on to paddocks as an organic fertiliser.

Others have been used as berley by fishing guides.

Overseas, koi carp roe is used in caviar and their skins are processed into fashion accessories. In some countries they are eaten, but Daniel says they are a particularly bony species.

There is also an annual bow-fishing koi carp competition on Waikato river systems, which saw 3200 koi caught in 2004.

– Sunday Star Times

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