The egg and larval stages of the Atlantic cod Gadus morhua and haddock Melanogrammus aeglefinus populations on Georges Bank, northeastern USA, were sampled monthly from February through July in 1995 and January through June in 1996 to 1999 as part of the US GLOBEC Georges Bank program. The eggs were staged by means of microscopic examination. Larvae were aged by otolith increment analysis. Seasonally averaged rates of egg mortality were estimated for both species and ranged from 9.9 to 20.4 % d -1 for cod and 7.8 to 13.4% d -1 for haddock. From the results of a simple drift model, the interannual variability in egg mortality rate is believed to be due largely to wind-driven transport off the southern side of the bank. The estimated number of hatched eggs is strongly correlated with the subsequent recruitment for both the Atlantic cod and haddock stocks. Mortality during the early larval period was estimated for 10 d cohorts within each year, based on the decrease in abundance from egg hatching to the first sampling of the cohort on a survey, when the larvae were on average about 15 d old. For both species, these rates were slowly varying between cohorts within a season, but showed large variation between years. For the 1995 to 1996 period, the annual average mortality rate was about 6.3% d -1 for cod and 10.1% d -1 for haddock, whereas in 1998 to 1999 the values were 3.9% d -1 for cod and 5.4% d -1 for haddock. The lower mortality rates in 1998 to 1999 are believed to be due to higher prey abundance for the larvae in those years. From the larval stage to stock recruitment, haddock appeared to have a survival rate (recruits per larvae) 3 times higher than that for Atlantic cod.
The eggs and larvae of developing year-classes for cod Gadus morhua and haddock Melogrammus aeglefinus on Georges Bank were sampled during monthly surveys in the winter and spring of 1995 and 1996. The distribution and abundance of the age groups or cohorts within the egg and larval populations were estimated during each survey. The displacement of the cohorts between surveys was largely consistent with advection by the climatological circulation pattern. Egg mortality rates, calculated from the decrease in seasonal total abundance of early and later stage eggs, were 14 and 12% d-1 for cod and 12 and 11% d-1 for haddock in 1995 and 1996, respectively. Larval mortality rates, calculated from the decrease in the abundance of larval cohorts between surveys, ranged from 3 to 9% d-1 for cod and 7 to 14% d-1 for haddock, with no evident difference in the rate between the 2 yr. Variability in the larval mortality rate estimates did not exhibit a temporal pattern, suggesting that mortality was chronic and not dominated by events that caused particularly high or low mortality.
We used a stepwise linear discriminant analysis to investigate the population structure of summer flounder, Paralichthys dentatus (Linnaeus). Analysis was based on 18 morphometric and meristic variables taken from 1,214 specimens collected in coastal waters between Montauk Point, New York and Cape Canaveral, Florida. Two populations were identified: one in the Middle Atlantic Bight, or between New York and Cape Hatteras, North Carolina; the other in the South Atlantic Bight, or between Cape Hatteras and Florida. Discriminant analysis coefficients, based on five morphometric variables taken from specimens collected at geographic extremes of the survey area, provide a mathematical means for classifying summer flounder into either the northern or southern population with an accuracy of 93%.
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