Georges Bank Cod (Gadus morhua) egg and larval abundance and mortality rates during 1995 and 1996

D. Mountain, P. Berrien, S. Brownell, J. Green, D. Johnson, R. Jones and J. Sibunka

Monthly surveys (January - June) of Georges Bank were conducted each year during the GLOBEC program. Cod (Gadus morhua) eggs and larvae were collected with MOCNESS and bongo nets at 40-80 sites on each survey. Eggs were staged and then aged through use of temperature mediated incubation rate functions. Annual, bank-wide mortality rates were calculated and applied to sampled egg densities to derive estimates of spawning and hatching densities. These station densities were, in turn, summed across all locations and surveys to calculate annual production 'curves' for eggs spawned and hatched. Egg mortality rates of 10.5 and 9.0 % per day were calculated for 1995 and 1996, respectively. Larvae were aged by analysis of their otoliths to determine birth date. The distribution and abundance of larval cohorts, defined by birth dates in 10 day intervals, were determined for each survey. Cohort mortality rates were determined by the change in abundance between surveys. The mortality rates were fairly constant through the season and across cohorts, with values of about 6% per day. Episodic events did not appear to contribute significantly to the population mortality. The centers of the cohort distributions moved around the Bank between surveys in general agreement with the climatological drift rates determined by the GLOBEC circulation models