THE 1995 MASS MORTALITY OF PILCHARD - NO ROLE FOUND FOR PHYSICAL OR BIOLOGICAL OCEANOGRAPHIC FACTORS IN AUSTRALIA

An unprecedented mass mortality of pilchard, Sardinops sagax, occurred in Australia in 1995, spreading east and west from the Great Australi an Eight at approximately 0.5 m s(-1) and 0.3 m s(-1) respectively to span the 6000-km range of the species from Noosa, Queensland, to Geral dton, Western Australia. Mortalities with the same clinical signs of h ypoxia also occurred in New Zealand. Upwelling and phytoplankton bloom s preceded the first mortalities, leading to widely publicized specula tion that environmental stress caused the mortalities. However, upwell ings as strong as in February 1995 off Eyre Peninsula occur as often a s once every three or four years, and environmental conditions surroun ding mortalities elsewhere were normal. Phytoplankton blooms were abse nt through much of the range; where they did accompany mortalities the y were of widely differing species. Hence, the hypothesis that environ mental stress caused the mortalities is quite confidently rejected. Th e hypothesis that ocean currents were a vector of an aeteological agen t is also rejected, since the Leeuwin and East Australian currents wer e both flowing strongly against the spread of mortalities, Other poten tial vectors exist, however, so the hypothesis that an introduced path ogen was responsible cannot be rejected.