Australasian pilchard mortalities

A large-scale epizootic occurred in the Australasian pilchard Sardinops sagax between March and September 1995. The deaths occurred along 5000 km of the Australian coastline and 500 km of the New Zealand coastline. Affected fish died within a few minutes of clinical signs of respiratory distress and death was associated with hypoxaemia and hypercapnia. Significant lesions were confined to the gills and comprised acute to subacute inflammation followed by epithelial hypertrophy and hyperplasia. The lesions were initially focal but progressed to become generalized over about 4 days. Only a herpesvirus was consistently present in gills of affected fish and absent from unaffected pilchards. There 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 newly emerged virus.