Auckland Lake Undergoes Pest Fish Cull to Restore Ecosystem
Auckland’s Western Springs Lake is undergoing a major operation to remove invasive pest fish, including goldfish, koi carp and brown bullhead catfish, in an effort to improve water quality and protect native species. The cull, which began this week, utilizes electrocurrents to stun the fish, allowing specialists to collect them for humane euthanization.
The operation is being led by specialists from the University of Waikato, utilizing New Zealand’s only electrofishing boat. Associate Professor in Biodiversity and Ecology, Nicolas Ling, explained that the process is designed to avoid harming native fish. “It puts a pulsed electric current into the water, and it temporarily stuns the fish, which means we can recover them. The native species, we can recover those, and we can put them back in the lake unharmed, and the pest species, we can humanely euthanise them,” Ling said.
While catfish have been present in the lake for over a century, many of the goldfish and turtles being removed are reportedly unwanted pets released by owners. Ling emphasized the detrimental impact of releasing pets into the wild. “People think they’re doing the right thing, you know, when they don’t want their pet anymore, they go and release it into the local lake. And it’s actually the worst thing you can do. If you want to take on a pet, then take it on for the course of its natural life.” He noted that turtles, in particular, represent a long-term commitment, often spanning 50 years.
The invasive species are causing significant damage to the lake’s ecosystem. Koi carp stir up sediment, clouding the water and promoting algal blooms. Catfish prey on native fish and kōura (freshwater crayfish), competing with them for food. The lake is also a regionally significant habitat for three species of eel, and Auckland Council is considering designating it an eel sanctuary.
Auckland Council Senior Freshwater Advisor, Matthew Bloxham, explained that the pest fish negatively impact water quality, making it potentially unsafe for recreation and harming birdlife through diseases like avian botulism. “Reducing pest fish numbers is a proven way to help lakes recover,” Bloxham stated.
The council has previously attempted other removal techniques, but this is the first time an electrofishing boat has been employed. The operation has already yielded impressive results, with the team catching Koi weighing up to 14kg – nearing the New Zealand record of 15kg. As of this week, nearly 300 kilograms of fish have been collected.
The cost of renting the electrofishing boat from Waikato University is $20,000. While acknowledging the expense, Bloxham emphasized the necessity of the operation. “The cost of this operation is not cheap. We’d prefer not to be spending the targeted rate on controlling pests. We’d rather spend it on direct biodiversity outcomes, such as planting trees, but it’s a necessary evil. We have to maintain these fish at really low numbers.”
Bloxham also reiterated the importance of responsible pet ownership, urging people not to release unwanted animals into waterways. “If ever we do achieve eradication, and we’d really like to, that is our end goal, it’s so easily undone by somebody otherwise well-mannered person who thinks they’re doing their goldfish a favour and then just quietly, surreptitiously emptying them into the lake, and suddenly we’re back to square one.”
The specialists are scheduled to continue the pest removal operation until Friday, November 13th, with plans for future returns to maintain the lake’s improved ecological balance.
