Exceptionally strong year classes in plaice Pleuronectes platessa: are they generated during the pelagic stage only, or also in the juvenile stage?

The exceptionally strong 1996 year class of plaice Pleuronectes platessa L. at the Balgzand in the western Dutch Wadden Sea offered the opportunity to study whether such a strong year class is generated and determined only du ring the pelagic stage, or also in the juvenile stage. This was done by com paring the 1996 year class with other years (1993 to 1999), including a wea k year class (1995). Larval immigration in 1996 was delayed by about 1 mo, compared to 1995. A significant relationship existed between reconstructed water temperature during developmental drift and the abundance of settling larvae. Estimation of the hatch date distribution of the larvae by means of otolith microstructure analysis showed that in 1996 the settling flatfish mainly originated from eggs hatched in February and March, while in the wea k 1995 year class the larvae originated from eggs hatched in January and Fe bruary. Mortality during the first months after settlement was density-depe ndent; however, in 1996 this relationship broke down and mortality was much lower than expected. After the severe winter of 1995-96, predatory crustac eans were absent in the nursery and as a consequence the already extremely strong year class of 1996 passed through this period of mortality relativel y unharmed. These results confirmed earlier observations that when an extre mely strong year class is generated after a very cold winter, the regulativ e mechanism of density-dependent predation in the nursery-which normally da mpens interannual variability in abundance-may fail due to low abundance of predatory crustaceans.