Juvenile cod (Gadus morhua) mortality and the importance of bottom sediment type to recruitment on Georges Bank

The demersal settlement of pelagic juvenile fish has been considered a critical period in which the final adjustment is made to the size of a year class. Distribution patterns of pelagic and recently settled juvenile cod (Gadus morhua) were examined from nine surveys on Georges Bank during the summer over 5 years, 1984-1989, to relate juvenile survival to the sedimentary environment. Pelagic juveniles were widespread across Georges Bank in June, and by mid-July they occurred on all bottom types from sand to gravel on eastern Georges Bank. However, by late July-early August they were mostly abundant on the northeastern edge gravel deposit, which with its complex relief, provides abundant prey and refuge from predators. A bank-wide estimate of pelagic juvenile abundance in 1986 and 1987 was used to assess mortality of the recently settled juveniles and to evaluate the relative importance of survivors from the northeastern edge gravel area to recruitment of the Georges Bank population. Settlement mortality rates over 1-2 months on the northeastern gravel area ranged from 3 to 8% day-1, which compared reasonably with other studies. The seasonal abundance of the pelagic juveniles was almost an order of magnitude higher in 1987 than 1986; however, recruitment at age 1 was similar, indicating that a high mortality of the demersal juveniles occurred in 1987. The limited northeastern gravel area on Georges Bank may represent a survival bottleneck depending on the variability in the distribution and abundance of juvenile cod settlement in relation to that of their predators.