Interseasonal variation in the transport of pilchard eggs and larvae off southern Western Australia

The distribution and relative abundance of pilchard Sardinops sagax neopilchardus eggs and larvae along the south coast of Western Australia were determined in July and December 1991. In July, there were 2 sites of spawning activity; large numbers of Day 1 eggs were found along the inner shelf region, 2 to 6 km from the coast off the King George Sound region of Albany, with a further peak in the inner and mid shelf regions off Bremer Bay, 150 km east of Albany. Almost no eggs were found in the area west of Albany where the seasonally strong, eastwards-flowing Leeuwin Current was located close to the coastline. The peak in the concentration of Day 2 eggs was located some 40 km east of Albany, with peaks in the abundance of the yolk-sac and post-larvae found 100 and 150 km east of Albany, respectively. This variation in the location of the different pilchard stages was consistent with an eastwards transport rate of 0.5 to 1.0 knots. By contrast, in December, when the Leeuwin Current was weak, there was no evidence of unidirectional transport with the peaks in the abundance of all pilchard stages located in the Albany region. The influences of the variation in transport between spawning seasons on the structure of pilchard stocks in this area and the implications for the management of the fisheries in this region are discussed. The value of using several boats for simultaneous sampling to provide a 'snapshot picture' is demonstrated. A comparison of plankton towing methods showed higher relative catch rates for Day 1 pilchard eggs using vertical compared with oblique tows, with the reverse found for Day 2 eggs.