Study links runoff to smaller trout

New research appears to confirm what anglers have said for years – that nitrates from farm runoff mean trout in some rivers are smaller than they used to be.

The findings have been made by scientists studying water quality in Hawke's Bay's Taharua and Mohaka rivers.

The study started last year as part of a Hawke's Bay Regional Council move to improve the two rivers' water quality, following concerns about the effects of intensified dairying at the head of the Taharua Valley, southwest of Taupo. Several farms – including one formerly owned by the Crafar family – have started in the valley since the 1990s. There are now about 9000 dairy cows in the area.

A report by the Cawthron Institute is due to be handed to the council next month.

Council environment manager Graham Sevicke-Jones said it confirmed findings of a limited report by the institute last year suggesting dairying activities in the area affected the rivers' food value and water clarity, resulting in smaller trout.

Councils could now start implementing nutrient limits in catchments based on scientific fact rather than anecdote. "There have been a lot of studies done on [trout] abundance, but not much has been done on how big they are and how that has changed over time," Mr Sevicke-Jones said.

"They [trout] are smaller. They can actually be more abundant. They are just not reaching optimum size any more due to the food available to them. Now this work has been done we can say `OK, all those arguments we've been having, we no longer need to have again'."

The algae may also affect native species such as eels but this had not been studied yet, he said.

Fish and Game resource management co-ordinator Neil Deans said the findings would strengthen the attitude of all councils toward farm runoff.

Intensification of land use had occurred without regard to the effect until it took place. "It's a big issue across the country as a whole. This confirms what anglers have long been saying. International anglers come here because New Zealand is known for its large trout. This is very much the value-added end of tourism for us."

The Taharua flows into the Mohaka, a nationally prized trout fishery and recreational waterway and is one of just 16 waterways protected by a water conservation order – considered the equivalent of national park status for waterways.

A council proposal for further detailed monitoring of the Taharua land use and its relationship to water quality has been sent to Niwa for review. The council expects to start discussing nutrient limits for acceptable algal growth later this month.

A paper before the council said relationships with landowners and farm managers in the area had been constructive despite instability over the Crafar receivership

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