Scientists wage war on invasive fish to save Auckland lake

Scientists are trawling for hundreds of pest fish as part of a mass cull to save what was once one of Auckland's most pristine and biodiverse freshwater lakes.

Lake Tomarata, on Auckland's northern border, was a haven for mussels and native freshwater plants, until an English immigrant named Stewart Smith introduced rudd and tench into the lake in 1971.

He also released thousands of invasive fish into New Zealand's lakes, rivers, and streams in the hope of establishing a population for coarse fishing.

"We now have this 50- to 60-year legacy of his activities with lots of different species being introduced all over the place," said Waikato University associate professor Nicolas Ling.

The introduction of rudd and tench have caused native populations to crash due to predation.

Freshwater plants have all but disappeared in the past 15 years and the pests stir up sediment which causes algal blooms.

Since the project began to remove the pest fish in 2022, a collaborative team of Auckland Council ecologists and biodiversity researchers have laid over 35 kilometres of gill net in Lake Tomarata.

Ling said the goal of the project was to "fish the population to extinction".

The team caught around 900 pest fish in its first outing, and this year they've caught around 80.

"Our modelling suggests we've removed more than 95% of the adult population."

Auckland Council freshwater biodiversity advisor Belinda Studholme said there were two "key species" the project was looking to protect: "The kakahi freshwater mussel and the native aquatic plants that should be growing here."

Once caught, the pest fish are taken to an organic farm where they are turned into compost.

The team will return to Lake Tomarata in spring to trawl again.

"I would hope that within two years, we're getting zero catches," Ling said.