AUSTRALASIAN PILCHARD MORTALITIES

A large-scale epizootic occurred in the Australasian pilchard Sardinop s sagax between March and September 1995. The deaths occurred along 50 00 km of the Australian coastline and 500 km of the New Zealand coastl ine. Affected fish died within a few minutes of clinical signs of resp iratory distress and death was associated with hypoxaemia and hypercap nia. Significant lesions were confined to the gills and comprised acut e to subacute inflammation followed by epithelial hypertrophy and hype rplasia. The lesions were initially focal but progressed to become gen eralized over about 4 days. Only a herpesvirus was consistently presen t in gills of affected fish and absent from unaffected pilchards. Ther e was no correlation with oceanographic conditions or the presence of plankton. The rate of spread of the mortality front (approximately 30 km/day) in relation to the migration rate of pilchards and prevailing currents suggested that a vector might be involved. The disease may be the result of a virus introduced into Australian waters, or of a newl y emerged virus.