The 1995 mass mortality of pilchard: no role found for physical or biological oceanographic factors in Australia

Abstract

An unprecedented mass mortality of pilchard, Sardinops sagax, occurred in Australia in 1995, spreading east and west from the Great Australian Bight at approximately 0á5 m s^-1 and 0·3 m s^-1 respectively to span the 6000-km range of the species from Noosa, Queensland, to Geraldton, Western Australia. Mortalities with the same clinical signs of hypoxia also occurred in New Zealand. Upwelling and phytoplankton blooms preceded the first mortalities, leading to widely publicized speculation that environmental stress caused the mortalities. However, upwellings as strong as in February 1995 off Eyre Peninsula occur as often as once every three or four years, and environmental conditions surrounding mortalities elsewhere were normal. Phytoplankton blooms were absent through much of the range; where they did accompany mortalities they were of widely differing species. Hence, the hypothesis that environmental stress caused the mortalities is quite confidently rejected. The hypothesis that ocean currents were a vector of an aeteological agent is also rejected, since the Leeuwin and East Australian currents were both flowing strongly against the spread of mortalities. Other potential vectors exist, however, so the hypothesis that an introduced pathogen was responsible cannot be rejected.