Scientists have developed what is essentially a "grass carp lawnmower" to tackle one of New Zealand's worst fresh water weeds.
Rather than releasing hundreds of grass carp into a lake, Niwa scientists found that by sitting an enclosed pen on the bottom of Lake Karapiro, five fish were able to raze most of the hornwort.
It wasn't the fastest of lawnmowers, as it took nine weeks for the caged carp to eat all the hornwort, but the trial was so successful the technique has been "added to the toolbox" to be used to combat newly discovered hornwort invasions.
The introduced hornwort spreads rapidly and can grow as high as 10 metres and forms a dense bed on the surface. It has invaded lakes around the North Island and there are fears it could ruin pristine South Island lakes.
Niwa's John Clayton says the study scientists built six large enclosures, 2.5m deep and 6m wide, and placed them within hornwort beds in Lake Karapiro.
"Three to five fish per enclosure were found to be effective and very fast, with weed in the enclosed areas removed in nine weeks."
Designing enclosures that were escape-proof was particularly challenging, but Dr Clayton said the end result was an innovative design that had animal ethics committee approval.
Grass carp have been used successfully for the control of large infestations of other invasive weeds in New Zealand lakes.
Unlike the pest koi carp, they are unable to breed in the wild in New Zealand.
However, when they are released in large numbers they eat indiscriminately and stay there until they die.
With the new technique, the fatter caged grass carp can be removed from the lake when they have eaten all the hornwort.