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 'process studies'.

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 to July)
• Eggs were identified, staged and counted, from quantitative Bongo-net hauls.
• 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 for haddock.
• 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 from NEFSC.
• 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.