Effects on trout counts predicted

Expert recalled to give evidence

BY PENNY WARDLE

A river modelling expert yesterday told the Environment Court in Blenheim that he had enough information and knowledge about TrustPower's proposed Wairau River hydroelectricity scheme to predict the effects on trout.

This was despite a lack of confidence in a computer model produced by the company's expert witness, Henry Hudson, to predict the impacts of changed river flows on habitats for species living in and near the river.

Ian Jowett, an expert in modelling changes in river habitat when flow is altered, was a witness for the Marlborough District Council at the original hearing, where a panel it appointed granted TrustPower resource consent to build the scheme.

In week one of the Environment Court hearing, called after appeals against the original decision, Fish & Game lawyer Maree Baker objected that Mr Jowett had been listed to give evidence, then withdrawn.

His expertise was vital to the case, she said, and with support from the court, the council agreed to engage him.

Mr Jowett said TrustPower's proposed 15 cubic metres per second (cumec) minimum flow at Marchburn would result in trout losing 20 per cent of their food supplies while retaining 80 per cent of their habitat.

If the summer minimum flow at Marchburn was increased to 25 cumecs, there would be little, if any, effect on trout numbers.

Yesterday, Mr Jowett would not be drawn to recommend that errors in Dr Hudson's model meant the court should be conservative when setting flows that would meet TrustPower's commercial needs while protecting river species.

"I think there is just as much chance of the model under- as over-predicting," he said.

Save the Wairau lawyer Mike Hardy-Jones said engineers might add a 50 per cent safety margin to a bridge design to avoid the risk of it collapsing.

Similarly, if the court had evidence that birds were endangered by the Wairau scheme, it should be prompted to set conservative flow regimes.

Mr Jowett responded: "I can't see that a bird population would require more flow than a population of large trout.

The second-highest population of black-fronted term in the Ohau [River] is where the minimum flow is zero."

Based on food availability, there was no evidence of a critical relationship between flow and birdlife sustainability, he said. However, he had not considered nesting site impacts.

TrustPower lawyer Christian Whata asked whether the court should seek precise numbers showing the impact of various flows on habitat. Mr Jowett was "uncomfortable with very prescriptive approaches" and preferred to "look at the bigger picture".

The court would need to come up with a set of conditions for the operation of the power scheme, he acknowledged.

However, the information available was not sufficiently precise for a cumec either way to make any difference.

Mr Whata suggested that natural flows did not necessarily provide optimal trout habitat, and that a modified flow regime could improve conditions for trout.

Mr Jowett said that while on the surface, that conclusion might be drawn, in reality it was the balance between high flows, which provided food, and lower flows, which provided fish habitat, that mattered.

"It is the total food production in the river that counts, and if low flow persists for weeks or more, that could affect trout populations.

"All my studies have shown that trout are like the piggies of the river.

"They always congregate where the food is. Food supply is very important to maintaining population."

Asked whether he agreed with Fish & Game expert witness Dr Henry Hayes that it would take 15 to 20 years of monitoring to detect a 5 per cent decline in trout, Mr Jowett said detecting a change in the affected reach of the Wairau would be difficult because numbers fluctuated due to migration. However, "in five years I think we would know of material change".

With current counts of 11 to 25 trout per kilometre, if 10 to 20 trout were counted "we would say there was no significant change, but if it dropped to five, there would be", he said.

- The Marlborough Express