Atlantic cod and haddock eggs on Georges Bank, 1995 to 1997;
interannual variation in distribution, abundance and mortality rates
P. Berrien and J. Sibunka
GLOBEC Broadscale Surveys on Georges Bank were implemented
to assess bank-wide features on a monthly basis during the first
half of the calendar year. These surveys provide information on
distribution and abundance and on biological rates such as
mortality/survival and growth of planktonic life stages of target
species. This bank-wide approach provides some of the 'big
picture' examination of potential factors in the determination of
recruitment rates, as compared to the more focused approach of
Analysis of egg collections of the two targeted fish
species, Atlantic cod and haddock, provides the starting point in
a 'life table' assessment. It is assumed that egg production rate
parameters can be linked with those of larvae, and with the adult
stocks that produced them. Additionally, because eggs are passive
constituents of the plankton their fate can be strongly
influenced by circulation patterns on the Bank.
Data / Procedures
- Monthly Broadscale surveys were conducted from January to June (February
Eggs were identified, staged and counted, from quantitative
Counts of staged eggs were normalized to no./10m²/day (at
ambient water temperature).
Egg densities were averaged by survey (delta-mean).
Survey means were expanded by the number of days represented by
each survey and accumulated over the season.
Instantaneous daily mortality rate (Z) was calculated for an
assumed exponential decline in numbers at stage over time.
The mortality rate was applied to early-stage densities to
estimate spawned numbers of eggs, and to late-stage densities to
estimate hatched numbers.
Spawned (and hatched) densities were averaged (delta-mean) by
survey and accumulated over the season to estimate total eggs
spawned (and hatched).
- Instantaneous daily mortality rates during 1995 to 1997 ranged
from 0.094 to 0.195 for Atlantic cod eggs and from 0.082 to 0.109
Percent survival ranged from 1.5 to 10.6 for Atlantic cod and 8.0
to 13.7 for haddock.
Numbers of eggs spawned by both species have increased to the
highest levels observed since the early 1980's.
The upward trend in numbers of eggs spawned is in general
agreement with that of the "Spawning Stock Biomass" estimates
However, the numbers of eggs hatched have not exhibited a similar
increase. The cause of this disparity between trends in spawning
and hatching rates could be endemic (due to egg condition or
viability) or environmental (due to predation or to water mass
properties) and should be pursued.