First acoustic fish fence installed

FISH FENCE: These pipes will create a wall of sound and bubbles in the Rangitata Diversion Race. This will encourage salmon smolt to enter the 250m bypass, to the right of the gabion basket, heading back to the river. Pictured from left are Central South Island Fish and Game officer Mark Webb and RDR manager John Young.

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New Zealand's first acoustic fish fence, designed to stop salmon smolt being lost in irrigation water, was installed and tested yesterday.

At a cost of around $1 million the Rangitata Diversion Race Management Limited has installed the 58m bubble and sound system designed to discourage salmon and divert them into a bypass back to the Rangitata River.

In the late 90s, research found that 200,000 smolt went down the system and while 20,000 crossed Mid Canterbury and entered the Rakaia River, the rest were lost to the fishery.

As a resource consent condition this had to be mitigated.

RDR manager John Young said metal screens were soon shown to be ineffective. Algae quickly built up and silt just stuck to it.

With the arrival of didymo in the Rangitata the choice of an acoustic fence was fortuitous.

Central South Island Fish and Game officer Mark Webb said stopping smolt from ending up on paddocks was something anglers had been hoping for about 60 years.

He said bioacoustic fish fence technology worked well overseas and could divert 80 to 95 per cent of target fish. However, there were unknown factors. The noisy Rangitata Gorge may affect the smolts' response to the noise created by vibrating plastic blocks and air bubbles driven by compressed air.

When not required the acoustic fence will not operate.

The system has to be proven to work and there is a computer controlled monitoring system so that when a fish passes through the bypass it triggers a video recording.

The acoustic fence will be fully operational by winter when the salmon eggs laid this summer develop into smolt and run to the ocean.

- © Fairfax NZ News