Fishing a factor in collapse: scientist

Years of commercial trawling and dredging of the seabed are partly responsible for the collapse of the scallop fishery in Tasman Bay, says a Cawthron Institute marine scientist.

Dr Paul Gillespie reported his findings from a study of the causes of the disappearance of the shellfish from the bay at a meeting in Takaka on Friday night.

About 40 people attended the talk, which was organised by Forest and Bird to mark Seaweek.

Fears that the Golden Bay scallop fishery could go the same way prompted a call for a separate marine management plan for the bay by Alan Vaughan, a representative on the Fisheries Advisory Group, and Golden Bay Community Board chairman Joe Bell.

The meeting heard how the total scallop catch in Golden and Tasman bays and the Marlborough Sounds had dropped from 850 tonnes of meat weight in 1994 to just 69 tonnes in 2006.

Last season the total harvest was 105 tonnes of meat weight, of which more than 90 per cent was from Golden Bay.

Dr Gillespie's presentation centred on the collaborative research programme done in the Motueka River catchment and the freshwater inflow to Tasman Bay.

He said high levels of suspended sediment were discovered in the river plume just offshore, both at times of flood and during dry weather.

"Scallops are very sensitive to high concentrations of suspended sediments."

Dr Gillespie said scallops ate phytoplankton and diatoms floating in the water, and would not have enough food when sediment levels were too high in their feeding environment near the sea bottom.

Continual seabed disturbance due to commercial trawling and dredging was one of the factors causing increased levels of suspended sediment, he said.

There were also high levels of the metal contaminants nickel and chromium in the river plume, which had been flushed out to sea naturally from the Red Hills mineral belt, the meeting heard.

"Golden Bay has a similar shallow mud-dominated seabed and similar high levels of suspended sediment."

However, on the positive side, Golden Bay had higher levels of phytoplankton than Tasman Bay.

Dr Gillespie said the sediment problem could be mitigated by various means, including catchment land use management to lower the sediment discharge from rivers, the establishment of "no trawling or dredging" zones, and a management regime that gave areas time to recover from disturbance of the seabed from commercial dredging.

Following the presentation, a panel that included Tasman district councillor Stuart Borlase, Mr Bell and Mr Vaughan discussed the implications for the Golden Bay scallop fishery.

Advocating the need for a separate management plan for the bay, Mr Vaughan said the amount of space devoted to aquaculture in the bay was set to increase by 700 hectares over the next five to 15 years.

Comment from the Challenger Scallop Enhancement Company was not available in time for today's edition.

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