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Disease caused by the haplosporidian parasite, Bonamia exitiosa, swept through the dredge oyster (Ostrea chilensis) population of Foveaux Strait between 1986 and 1992, with consequent mortality reducing the population to 9% of the pre-disease level. Dead and dying oysters were first seen by fishers in far western Foveaux Strait in 1985 and more were found further east in 1986. Infection spread slowly through Foveaux Strait so the progress of the epizootic can be described from population surveys. A wave of infection radiated through the oyster population from the epicentre of infection in central western Foveaux Strait, and was followed by a wave of mortality. The epizootic ceased in oyster beds around the margins of oyster distribution in 1992. Infective particles released by diseased oysters spread through the water to infect other oysters directly. The epizootic broadly fitted a simple deterministic epizootic model and suggested that both diffusion and turbulent processes were important in transmission of infection. Bonamia exitiosa was also present in oysters at the end of an epizootic in 1964 and was probably the cause of that epizootic. Bonamiasis appears to be an endemic disease in Foveaux Strait. The high mortality in the 1986–1992 epizootic was like that caused by a newly introduced disease in an immunologically naïve population. We propose that other stressors have increased the susceptibility of oysters to this disease. Mechanical disturbance of oysters by increasingly intense dredging appears to be a major source of stress, as does the increasing scale of modification of benthic habitat by fishing. Recovery of the oyster population after the epizootic is closely linked to regeneration of habitat. The prognosis for the fishery could be improved by mitigating mechanical disturbance during dredging by use of lighter dredges and less damaging towing strategies, as well as pursuing rotational fishing strategies that allow benthic habitat to regenerate in undisturbed areas.

Bonamia exitiosa epizootic in Ostrea chilensis from Foveaux Strait, southern New Zealand between 1986 and 1992