Growth and mortality of Atlantic cod Gadus morhua and haddock Melanogrammus aeglefinus eggs and larvae on Georges Bank, 1995 to 1999

摘要

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.