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