Environment Waikato’s Operations Committee wants urgent action taken to destroy koi carp threatening to infiltrate the Waihou River – but the job will be difficult and costly.
The pest fish, which can undermine river banks and make water turbid, have just been spotted in the Hikutaia Cut, part of the Waihou Valley Scheme. Staff are concerned they will infest the Waihou River in the first flood of the winter season.
The Committee was told the noxious fish were spotted during repairs to the floodgate outlet of the Cut in mid-February. The area is used extensively for water skiing and coarse fishing, and while it is not directly open to the river, the Cut has spillways at each end which operate during floods.
Koi carp are large ornamental fish which were deliberately released in the 1960s into the Waikato River system. They suck sediment from banks and river bottoms, expelling the unwanted parts and undermining banks as well as making water turbid. Females can lay up to 1.5 million eggs which can attach themselves to nets and boats.
Asian communities considered them good luck, and gold ones were highly valued. The fish grew to about 10kg.
Freshwater ecologist David Speirs said the fish were likely to have been deliberately spread as they had been in the past.
It is illegal to have possession of live fish and there are strong penalties for possessing, and spreading them, with control responsibilities spread between Department of Conservation, and to a lesser extent the Ministry of Fisheries and Regional Councils.
Destroying them was difficult, as drainage and removal was only possible in small ponds. The use of a poison, rotenone, had been experimented with in Nelson. The poison had a temporary narcotic effect on all types of fish, floating them to the surface where the carp could be removed and the other species left to recover when the effects wore off. Other poisons were not selective and had other side effects for waterways.
Cr Jim Howland said he was horrified that the fish could pollute the Waihou River.
“I don’t believe as a nation that we are taking this seriously enough. We have seen what possums have done and there should be a national approach.”
Cr Evan Penny said an attempt must be made to eradicate the pest as the consequences of doing nothing did not bear thinking about.
“We have got to do this with some considerable speed before the next flood, when they will be out in the river for sure. We could sit and analyse, report, monitor and liaise until it’s too late. If we do nothing they will be all over the river system.”
The Committee recommended working with DoC to determine the best eradication method.