Cautionary calls follow Tasman Bay shellfish collapse
The scallop and oyster fisheries in Tasman Bay have collapsed. Despite the area's closure to commercial scallop boats for the past two years, entire beds of adult scallops have perished and juvenile scallops (spat) have failed to survive after seeding.
Cawthron Institute marine scientist Dr Paul Gillespie said that Tasman Bay's filter-feeding oysters and scallops were dying in the turbulent fine silt covering the seabed. The silt is the result of land development and many years of commercial dredging and trawling and although the situation was slightly better in Golden Bay, he did not rule out a similar fate for shellfish here.
As there has been less intensive land development in Golden Bay than in the Motueka region, more phytoplankton has been available for shellfish to feed on, and productivity was therefore raised at the moment, Dr Gillespie explained. However, seabed disturbance from trawling and dredging contributes to higher levels of suspended sediment. "If the seabed is continually disturbed it becomes less stable, and if it is turbulent, filtering the water for food becomes more difficult for the scallops and if the scallops cannot feed they die. The juvenile scallops would only last a week without food."
"Golden Bay has the same muddy seabed and is in an area which is under the influence of fairly vigorous river plumes and suspended sediments. It is a soft seabed with input from major rivers over many years, which puts the seabed environment on edge for scallops. If there was an increase of silt, and we had quite a lot of rainfall and sediments going into the coastal environment, it could put the environment over the top in terms of sediments," Dr Gillespie said.
The solution, he believes, is to leave areas undisturbed and extend rotation periods. "There are two scenarios. It may be just a long-term climatic cycle and things may come right naturally, or MFish and Challenger make an effort to co-ordinate fishing."
The Cawthron Institute will continue to monitor the situation in Golden Bay until funding runs out in 2010.
The Challenger Scallop Enhancement Company also keeps an eye on current shellfish developments, as it manages the rotational seeding and commercial harvesting of scallops. All spat seeded through Tasman Bay's beds in 2004, 2005 and 2007 perished, said operational manager Mitch Campbell.
Tasman Bay alone brings in one third of the upper South Island's multimillion-dollar scallop business, which, depending on seasonal fluctuations, is worth between $1.5 million and $50 million per annum.
Mr Campbell said he was well aware of the state of the marine environment and was observing the situation in Golden Bay with caution. If there were reasons calling for a closure of the scallop fisheries in Golden Bay, his company would not hesitate to take the necessary steps, he added.
"In Golden Bay, this season in particular has been relatively productive for the commercial fishermen, but we manage Golden Bay very carefully and fish on a rotational basis."
In Mr Campbell's view, tidal movements rather than dredging were responsible for the decline in scallop harvesting in Tasman Bay. Although dredging for scallops had stopped there six years ago, there had been no change he said.
Pakawau resident Alan Vaughan, the Forest & Bird representative on the Challenger Inshore Finfish Management Committee, said he had observed a steady decline in the scallop harvest over the last eight years in Golden Bay.
"There has been the odd spike in yield which was then followed by a decline. If the biomass of any fish species drops in total, that species is in danger of total collapse unless it is left completely alone to recover."
Mr Vaughan concluded that due to dredging year after year in combination with contaminants and sediments being washed into the sea, "...you don't get a very rosy picture of the marine environment," and a cautionary approach appeared appropriate.
"We need a moratorium on scallop dredging. I'd like to see the commercial and the recreational fishermen call for a stop in scallop dredging for a period of time in the hope of getting some sort of recovery."
In response to the crisis, the region's iwi have called for an urgent investigation into the disappearance of Tasman Bay's shellfish. The disappearance of the once-abundant kaimoana impedes traditional harvesting rights and contravenes the Treaty of Waitangi, and iwi have the full support of the Waitangi Tribunal.
Barney Thomas, chairman of Nelson's Tiakina te Taiao, which represents six upper South Island iwi, said Golden Bay's Manawhenua Ki Mohua was equally concerned about the situation and he welcomed their support. Local iwi representatives could not be reached for comment.
Ina Holst
