A sudden collapse in distribution of Pacific sardine (Sardinops sagax) off southwestern Australia enables an objective re-assessment of biomass estimates

Abstract

Fishery-independent estimates of spawning biomass (BSP) ofthe Pacific sardine (Sardinops sagax) on the south and lower west coasts of Western Australia (WA) were obtainedperiodically between 1991 and 1999 by using the daily egg production method (DEPM). Ichthyoplankton data collected during these surveys, specifically the presence or absenceof S. sagax eggs, were used to investigate trends in the spawning area of S. sagax within each of four regions. Theexpectation was that trends in BSP and spawning area were positively related. With the DEPM model, estimates of BSP will change proportionally with spawning area if all other variables remain constant. The proportion of positive stations (PPS), i.e., stations with nonzero egg counts — an objective estimator of spawning area — was high for all south coast regions during the early 1990s (a period when the estimated BSP was also high) and then decreased after the mid-1990s. There was a decrease in PPS from the mid-1990s to 1999. The particularly low estimates in 1999 followed a severe epidemic mass mortality of S. sagax throughout their range across southern Australia. Deviations from the expected relationship between BSP and PPS were used to identify uncertainty around estimates of BSP. Because estimation of spawning area is subject to less sampling bias than estimation of BSP, the deviation in therelation between the two provides an objective basis for adjusting some estimates of the latter. Such an approachis particularly useful for fisheries management purposes when sampling problems are suspected to be present. The analysis of PPS undertaken from the same set of samples from which the DEPM estimate is derived will help provide information for stock assessments and for the managementof purse-seine fisheries.