Fish blamed for water pollution



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A Waikato University scientist believes that by diminishing koi carp numbers, the quality of water in Waikato lakes and rivers can improve without forcing farmers to reduce their fertiliser use.

Dr Adam Daniel, who has been been researching carp populations in the Waikato since moving here from the United States in 2006, has recently installed a barrier on an outlet stream coming from Lake Ohinewai near Huntly.

The barrier prevents the pest fish from returning to the lake once they have left, helping reduce the lake’s koi population from 200 kg per ha, the Waikato average to less than 100.

Around 7000 carp were estimated to live in the lake before the barrier was installed, and trapping began, in January.

Dr Daniel, who recently published a paper on carp management in the Waikato, said koi carp have played a key role in the collapse of aquatic plants, loss of native biodiversity, algal blooms and reduced waterfowl production and believes that by removing the fish, the water quality may improve. Over-use of fertiliser wasn’t the only reason for over-polluted waterways, he said.

”It’s all about trying to reduce the bio-mass of the carp while getting the level of the lake’s biochemistry to improve,” he said.

Koi carp populations in the Waikato exploded in the early 80s after a pond by the Waipa River, filled with the fish by a private landowner, flooded.

Since then, lakes and waterways across the region have been infested by them. It is thought around 72 million tonnes of carp can be found on the Waikato River itself.

Koi carp are prolific breeders, with a two to three kilogram female capable of producing a quarter of a million eggs. A large 10kg fish may produce a million.

In addition to their large bio-mass in a lake, koi carp feed by sucking holes in the bottom of waterways, from which they extract nutrients.

Dr Daniel said ”thousands of holes” have been sucked in the mud from koi feeding on the bed of Lake Ohinewai, estimating that the visible damage would equal more than 3000sqm of sediment displaced.
The scientist admitted that while eradication of the fish was ”now impossible” population levels could ”be reduced to a level where they are manageable”.

”No-one has taken on a big lake (anywhere) and taken all the carp out,” he said. ”The last 10 per cent of any lake-based population are extremely costly to eradicate. But if we can take as much carp out as we can and see how the health (of the lake) changes it could be very interesting.” Dr Daniel used the example of Lake Crescent in Tasmania, where environmentalists are still battling to eradicate the carp population, which now sits at around a tenth of its original number.

In 2007, Dr Daniel used acoustic tags to track carp movement in the Waikato River in more detail.
The acoustic tags each emit a unique sound signal which is picked up by a network of 19 receiving stations placed at strategic points along the river.

Dr Daniel is currently in talks with the Fisheries Ministry to produce a low-cost net trap for farmers to catch koi carp. The trap was likely to be available at hunting and fishing stores within six months, the scientist predicted.

Meanwhile the carp are also being seen as a replacement fertilizer by some Waikato farmers, with Ohinewhiro drystock farmer Rick Muir selling a carp-based fertiliser from fish trapped on the river.

Mr Muir got the idea after noticing how many mullet were in the Waikato River three years ago.

”Then I saw the carp, and thought ‘perfect’. They’re a pest fish and turn them into fert,” he said.
Mr Muir minces the fish, and spreads them as a cold slurry. He said there was no processing, and no smell to the fertiliser, which he sells at $2 a kilo, spread.

Mr Muir has been experimenting with different rates per hectare, with 50 kg per ha emerging as ”the right amount.” ”You think the amount you put on is absolutely nothing, but you’ve got to wait a few years to see results.”

– Waikato Times

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