Bio acoustic Fish Fence installed
Hamish Stevens, Fish and Game Offcer.

The Rangitata Diversion Race (RDR) diverts about 30 m3/s from the Rangitata River for irrigation and power generation in mid-Canterbury. Along with the water so too are fish diverted from the river with most ending up on irrigated paddocks. A juvenile salmon trapping program over the 1998/99 irrigation season estimated that about 200,000 salmon smolt were lost from the Rangitata that season and represented up to 30% of the river’s total production of juvenile salmon. RDR management then went about finding a suitable screen design capable of screening a large volume of what is often laden with silt.

F & G Officer Mark Webb testing the RDR Baff system

The RDR fish screen is not a traditional mesh barrier type screen. Instead it is a Bio-acoustic Fish Fence (BAFF) designed by Fish Guidance Systems Ltd of Hampshire and will be the first of its type in Australasia. The fence has a combination of low-medium frequency sound within a range that repels fish with a swim bladder, such as trout and salmon, together with an air bubble curtain that concentrates the sound into a “wall of sound” about 1m across. The BAFF consists of 2.4 m-long modules fixed to the bed of the race near the sandtrap, about 2 km downstream from the intake, with a bypass back to the Rangitata. Overseas, BAFF systems have been installed in power station and water supply intakes with reported effiencies for diversion of atlantic salmon smolt of up to 90%.

One of the great advantages of the BAFF system is that it is unaffected by Didymo which was discovered in the RDR during the test runs. Mesh fish screens with holes no bigger than those in a whitebait net would be needed to prevent salmon fry passage and become almost unusable in Didymo infected rivers. The BAFF has no physical barrier on which Didymo can collect.

Test run of the RDR Baff system

Typically with new technology there will be a period of fine tuning of the BAFF and testing of its efficiency at keeping juvenile salmon where they belong. Test runs for the RDR Baff system were carried out early last week and it was hoped it would be up and running for the rest of the season. However Murphy decided to throw a spanner in the works with a troublesome compressor meaning a start date of early January is more likely. We are hopeful that at last a solution has been found to this loss of Rangitata salmon and the improvement in the salmon fishery will be immediate and impressive.

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