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Rock snot fish impact confirmed

NZ Newswire logoNZ Newswire 16/08/2016

Scientists have confirmed that didymo, or "rock snot", in South Island rivers can hurt fish populations - particularly brown trout.

"The results of our study are of particular concern," says University of Canterbury ecologist Professor Jon Harding after he and NIWA scientist Dr Phil Jellyman surveyed 20 South Island waterways.

"We have assumed for some time that didymo will have an impact on fish, but these results show both native fish and introduced sports fish are all being affected by didymo."

Didymo was first spotted in 2004 and has now spread into more than 150 South Island waterways, although it hasn't yet been seen in the North Island.

In extreme cases it forms extensive thick mats, smothering the river bed and changing the habitat.

Fish prey insects such as mayflies, stoneflies and caddis flies do very poorly in heavily didymo-infected rivers, the scientists say.

Streams and rivers covered in didymo can still have numerous fish species, but the abundances and biomass (or size) of fish changed markedly.

"We showed that fish biomass in didymo-affected rivers can be decreased by 70 per cent, in particular brown trout were strongly affected," Prof Harding said.

Rivers with didymo were liable to have much smaller trout and other fish.

Trout had been recorded in all 20 rivers in the study but now were absent in 60 per cent of the rivers with high didymo biomass.

"The consequences for our international trout fishery are obvious," Prof Harding said.

The study emphasised the need to limit the spread of didymo to other rivers and for water users to adhere to the check, clean, dry procedures aimed at preventing the spread.

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