Where there's mullock there's gold: Long term changes in the distribution of biogenic reefs (mullock) and oysters (gold) in Foveaux Strait caused by dredging for oysters.

John Cranfield, Keith Michael and Miton Roderique

In Foveaux Strait, commercial densities of oysters are closely associated with bryozoa and other epifauna which is termed "mullock" by fishers. Fishers found mullock forms strips aligned with the tide through oyster beds. In 1979 a sidescan survey of oyster beds showed that these strips of mullock were extensive linear assemblages of bryozoan patch reefs.

Oysters have been exploited in Foveaux strait since 1863. The first oyster beds fished were small and were reduced below economic densities within a few years. There are no oysters on sites of these beds today. In 1888, fishing moved to a more extensive bed found in Eastern Foveaux Strait (the East Bed). This became the principal bed fished until the 1950's when oysters could no longer be caught there economically. There are no oysters on the site of this bed today. By 1996, mortality from fishing and Bonamia sp. had reduced oysters in eastern Foveaux Strait to below economic densities and there is concern whether beds in this area will recover at these low densities.

Dredging removes bryozoa as well as oysters and fishing has progressively destroyed biogenic reefs. Removal of reefs was slow in the early days of the fishery when dredges were light and vessels few and not powerful. It took 60 years for all the reefs and oysters to be removed from the East bed. The seafloor in that area Is now very mobile and has no macro-epifauna. The records of fishers show that the rate of disappearance of biogenic reefs increased when all vessels started using double-ring-bag, heavier dredges that allowed more aggressive dredging practises.

Infection of Bonamia sp. started in 1985 in central Foveaux Strait where there was no bryzoa and spread from there to reach the periphery of the fishery in the south and the northwest. The epizootic ceased in 1992 as the wave of infection reached the areas with bryzoa. After the epizootic, recruitment and survival of oysters have been higher in areas with bryzoa and oyster populations there have had consistently large numbers of juveniles while areas without bryzoa have had few.

Blue cod spawn on the biogenic reefs which provide juvenile habitat. The reduction in reefs by oyster dredging has also had a long term effects on the distribution and probably on the size of the blue cod population in Foveaux Strait.

NIWA is commencing a research programme to describe biogenic reefs, to quantify changes in their distribution with dredging and establish the impact of these changes on the oyster and blue cod fisheries of Foveaux Strait. The long term aims of the research are to mitigate the damage caused by dredging by investigating use of less destructive dredges and investigating specific harvest regimes that can protect and rehabilitate parts of the reef habitat of Foveaux Strait.

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