The Environment Agency (EA) will put a ‘piscicide’ into the ponds and moat of Clissold Park in the east London borough of Hackney, to kill off topmouth gudgeon, a non-native fish species from Asia which it says could have "a disastrous effect on the native fish population and wider ecology".
The topmouth gudgeon is one of the most invasive species of fish and poses a risk to the biodiversity of the park and the wider area. It is banned from sale in England and Wales.
The authorities are so keen to get rid of it before it spreads they are willing to kill the rest of the fish as well. The move is part of a five-year nationwide programme to eradicate the fish.
Originally imported from Asia into the UK as an ornamental species, the 8-10cm long silver and purple fish eat the eggs of other fish, breed rapidly and out-compete native fish for food and habitat. It is thought they ended up in the park ponds through human intervention.
An organic ‘piscicide’ called rotenone, which is selective to fish, will be used in the cull.
Mammals and birds will not be affected but invertebrates may be - however they will recover quickly, the council has said.
Cabinet member for health, social care and culture councillor Jonathan McShane said: "We fully support the Environment Agency in their eradication work. The topmouth gudgeon poses a significant threat to the biodiversity of Clissold Park and, if left, could result in serious environmental problems on a national scale.
"This process is necessary to ensure the future health of not only Clissold Park’s waterways, but of rivers and ponds across the country."
This intervention is the latest in an EA eradication programme which has been going on since 2004, when there were 23 confirmed sites in the UK. A spokeswoman said there were now just three, all in south east England.
She said that in some cases other fish were rescued before the piscicide was added but in Clissold Park any other fish in the ponds had been illegally dumped there and may not be native species. They may also be infected by the topmouth gudgeon, which were successful carriers of parasites and disease, breed quickly and are highly effective at thriving in unpleasant circumstances.
EA staff will carry out the operation wearing protective equipment, although studies show that rotenone does not pose a hazard to human health.
It says that once the rotenone has degraded and broken down there will be no toxic chemicals remaining.