Source: Radio New Zealand
Usually, there are only calm currents at Auckland’s Western Springs Lake.
But this week, electrocurrents are being used to stun pest fish and turtles so they can be scooped up, brought to land and killed.
Associate Professor in Biodiversity and Ecology at the University of Waikato’s School of Science, Nicolas Ling, is one of the specialists scooping up hundreds of koi carp, including goldfish and brown bullhead catfish, on New Zealand’s only electrofishing boat.
He said no native species would be harmed by the electrofishing process.
“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 catfish were most likely released into the lake more than a hundred years ago. Koi were believed to have been introduced into New Zealand in the 1960s.
But he said most of the goldfish and turtles in the lake were people’s unwanted pets.
“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 said they were also removing red-eared slider turtles from the lake.
“Again, these are pets that people have released when they no longer want them. The problem with the turtle is, if you take that on as a pet, it’s a 50-year commitment. It’s a multi-generational pet. They get huge and it becomes very expensive to provide the habitat for an adult turtle. And so people just go and release them, which is a sad thing.”
Associate Professor in Biodiversity and Ecology at the University of Waikato’s School of Science Nicolas Ling. Nick Monro
All of the pests collected were brought back to shore to be killed and then taken off-site to be turned into compost or rendered down to produce natural gas.
While killing the pests was not a pretty sight, Ling said it was necessary.
“These particular species are known to cause negative impacts on water quality. The lake should be nice and clean, but it’s not, and the fish are definitely contributing to that. They stir up the bottom sediments, and that resuspends nutrients back into the water column, which can cause increases in algal growth and bacterial growth.
“The koi and the goldfish eat the plants in the lake, which also take out nutrients.
“Those challenges with water quality mean that it makes it potentially unsafe for recreation around the lake. You don’t want to be touching the water.
“There’s fantastic bird life around here. When the water quality is really poor, they can suffer from a disease called avian botulism.
“And of course, they’re competing with the native species as well. There are lots of eels in the lake, and that’s good to see. There are also īnanga, which are key whitebait species. There’s common smelt in the lake as well.”
The lake was home to three native eel species: the short-finned eel, the long-finned eel, and the Australian long-finned eel, and there are plans to declare the lake an eel sanctuary.
The Waikato University specialists would be at the lake for one week ending on Friday, 13 November, with the council saying there were plans for them to return in the near future to continue the pest removal operation.
Auckland Council Senior Freshwater Advisor Matthew Bloxham. Nick Monro
Auckland Council Senior Freshwater Advisor, Matthew Bloxham, said the council had previously tried other techniques to remove pests from the lake, but this was the first time they had brought in an electrofishing boat.
He said so far it had been a success.
The team at Western Springs had caught Koi weighing up to 14kg. The largest Koi ever caught in New Zealand weighed 15kg.
Invasive fish are being caught and eliminated in Western Springs. Nick Monro
“Not many people realise that they get so large,” Bloxham said.
We’ve found quite a few diseased goldfish in here recently. So, putting them in here doesn’t necessarily give them a better life. It’s actually passing on the problem to somebody else, in this case the environment.
“It’s far better to repurpose that goldfish or re-home it, reach out and see whether anybody’s prepared to take it on and there will be people out there, you know, people love goldfish, they are attractive things after all, but they’re a nuisance when they’re released into the wild.”
It was costing the council $20,000 to rent the electric fishing boat from Waikato University.
“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.”
“It only takes two fish, a male and a female, to breed up and to produce the sorts of volumes that we’re seeing here now. We’ve been fishing all week, and so far we’re just under 300 kilograms of fish. That’s hundreds of fish.”
Auckland Council Senior Freshwater Advisor Matthew Bloxham says eradication is the goal, but that won’t happen if people keep putting their goldfish in the lake. Nick Monro
Bloxham said eradication was the goal, but that wouldn’t happen if people kept putting their goldfish in the lake.
“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 gold fish a favour and then just quietly, surreptitiously emptying them into the lake, and suddenly we’re back to square one.
“The big message that we’re trying to get out is that if you’re contemplating, you’ve reached the end of the year, and you’ve got a pet, you don’t know what to do with it, don’t release it into your waterway.”
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– Published by EveningReport.nz and AsiaPacificReport.nz, see: MIL OSI in partnership with Radio New Zealand
