Biology and fishery of pilchard, Sardinops sagax (Clupeidae), within a large south-eastern Australian bay

Length-frequency and maturity data of pilchards (Sardinops sagax) are described from monthly purse-seine commercial catch samples obtained in Port Phillip Bay (Victoria) between December 1994 and January 1997. These data, together with findings of a 12-month ichthyoplankton bay survey from September 1995 to August 1996, were used to determine the size at which pilchards recruit to the bay fishery and whether they spawn within this system. Monthly pilchard catch rates between January 1990 and June 1996 are also described and analysed in terms of environmental variables during that period. Results show that pilchards do not generally attain sexual maturity or spawn within the bay but use it as a nursery area, entering this system mostly as 0+ to 1+ year-old juveniles (4-12 cm fork length, FL) in late spring-early summer and returning to sea the following winter. This migration is supported by the marked seasonality in catch rates, which each year peak in March-May and are lowest in August-October. The seasonality was adequately explained by temperature lagged 2 months in a multivariate time-series model. Port Phillip Bay appears to be the only semi-enclosed, shallow marine embayment in temperate Australia that supports a substantial pilchard fishery that, in addition, is based predominantly on juveniles.