Scheme's effects minimal - scientist

TrustPower's proposed Wairau hydroelectric power scheme could be designed, built and managed to have minimal effects on the river's ecology, water quality and aquatic life, aquatic scientist Gregory Ryder told the Environment Court in Blenheim last week.

In 2008, a Marlborough District Council-appointed panel granted TrustPower resource consent to build the power scheme. That decision was appealed, leading to the Environment Court hearing now under way.

A development of this scale on the Wairau River could have significant impact on the environment, said Dr Ryder, an expert witness for TrustPower. However, conditions of consent as well as monitoring and management plans should identify and address any unexpected environmental effects.

Dr Ryder and other ecologists contracted by TrustPower calculated that the proposed power scheme would sustain about 30 per cent of habitat for fish and the macroinvertebrates which provided food for birds. The remaining 70 per cent would maintain existing aquatic communities, water quality and ecosystems, they said.

However, lawyers for Fish & Game and Save the Wairau, Maree Baker and Mike Hardy-Jones, said this could have unacceptable impacts on trout, salmon, native fish, macroinvertebrates and black-fronted terns.

Ms Baker quoted research findings that invertebrate biomass was the single most important factor determining the abundance of brown trout in New Zealand rivers. While Dr Ryder did not dispute this, he said there was no research demonstrating whether this applied to the Wairau.

Available habitat was also important, he said. A stream with plenty of invertebrate biomass but a depth of 10 centimetres would not be ideal for trout, he said.

The months from December to April were the most important for trout to have good access to food, to ensure successful spawning, said Ms Baker. The bigger the fish, the larger the number of eggs they produced.

In this summer period, the impact of a 20 per cent loss of habitat for trout could be more than minor, she said. Dr Ryder agreed that this could be the case during extended low flows, but said over the length of river involved, the loss would not be significant.

Ms Baker said TrustPower studies highlighted that most juvenile trout and salmon were found in or near tertiary channels and seepages of the river. During times of low flow, these area would be the first to dry up, she said.

This could force the young fish to move into the main channel which was undesirable due to greater flow velocity and higher predator numbers.

Minor braids were also a nucleus for recolonisation after floods.

Ms Baker highlighted the importance of trout and salmon moving freely up and down the Wairau River and its tributaries, not just for spawning but also to feed and seek refuge from unfavourable conditions such as high water temperatures. Juveniles started swimming downstream from September onwards.

At times of low flow, connectivity with tributaries could be disrupted which would be an issue for spawning adults and migrating juveniles, she said.

Dr Ryder said he was not aware of connectivity issues under normal climatic conditions as juvenile fish tended to move during freshes.

Fish screens were a topic of debate, with Dr Ryder recommending a screen with a 5mm gap be installed and monitored; one of the finest meshes recommended for any hydro scheme in the country, he said.

However, Fish & Game expert witness Davor Bejakovich recommended a 2-3mm gap which would exclude close to 100 per cent of salmon and trout and a significant proportion of other fish species.

This was a highly conservative approach given that a reasonable proportion of small fish were likely to survive through the turbines and find their way back to the river, said Dr Ryder. Questioned by commissioner Helen Beaumont, he said 10 per cent of fish would survive going through a turbine. The figures might not be so good after they had been through five sets, he acknowledged.

Ms Baker said Fish & Game was concerned that a proportion of juvenile fish diverted into the scheme could become resident and lost to fishing.

Questioned by Save the Wairau lawyer Mike Hardy-Jones, Dr Ryder said he had seen no evidence that intake into hydro schemes was a potential problem for juvenile eels.

Asked by commissioner John Mills why there was opposition to the finer screens recommended by Mr Bejakovich, Dr Ryder said their $1 plus purchase cost, plus potentially higher maintenance costs, could be issues.

- The Marlborough Express