Impact of mortality, possibly due to herpesvirus, on pilchard Sardinops sagax stocks along the south coast of Western Australia in 1998-99

During progression of a mass mortality of Australian pilchards in late 1998 and early 1999, quantities of dead pilchards on the sea-surface, sea-floor and along beaches were estimated in three regions along southern Western A ustralia (WA) by use of transects. Total mortality was estimated at 17590, 11 193 and 144.4 t for Esperance, Bremer Bay and Albany respectively. Morta lity rates at Esperance and Bremer Bay were similar at 74.5% and 64.7% resp ectively, with a mean of 69.6%. In contrast, estimated mortality at Albany was only 2.4%. Although the difference in total mortality between regions i s probably related to differences in stock size, as determined by simulatio n models, the much lower estimate for Albany is probably an artefact of an over-estimated pilchard biomass and not due to large differences in actual mortality rates. Variability in estimates of both pilchard biomass and quan tities killed resulted in a wide range of estimated mortality rates, with l ower estimates for Esperance and Bremer Bay of 28.0% and 22.9% respectively . This represents a significant decline in the breeding stock of WA pilchar ds. If the impact was closer to the mean (69.6%), then pilchard stocks in W A are severely depressed.