Distribution, abundance and mortality of cod and haddock eggs and larvae on Georges Bank in 1995 and 1996

Abstract

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.

Hatch date distributions of young-of-year haddock Melanogrammus aeglefinus in the Gulf of Maine/ Georges Bank region: implications for recruitment

“…This was the conservative approach since any correction for mortality would have moved the distributions toward the left (earlier in the year), resulting in greater separation between HDFs of juveniles and eggs. For comparison with the HDF of eggs (Mountain et al 2003), juvenile hatch dates were summed over 10 d intervals. To estimate relative survival, the HDFs of eggs and juveniles were first converted to relative HDFs by dividing the number in each interval by the total number in the sample.…”

“…While the relatively small sample size in some years limited the degree of resolution possible, the considerably larger sample of the 1998 year class revealed a broad and relatively smooth distribution of birth dates when binned into 10 d intervals. HDFs based on late egg stage abundance sampled as part of the GLOBEC broad-scale cruises between January and June of 1995 to 1999, indicated that haddock hatched between January and May, with a pronounced peak in the first half (Mountain et al 2003) and on YOY haddock with relative survival (relative HDF juveniles/ relative HDF eggs) for (a) 1995, 1996, 1997 and 1999 combined and for (b) 1998 of April in most years (Mountain et al 2003, D. Mountain pers. comm.).…”

“…We used HDFs generated from late-stage egg abundance on GB (Mountain et al 2003) together with our HDFs for Georges Bank YOY juveniles to generate relative survivorship curves for each of the 5 years and a composite of all years combined. Due to the relatively small sample size in most years, we show 2 survivorship curves, one with all years combined except 1998 (Fig.…”

Seasonal trends in mortality and growth of cod and haddock larvae result in an optimal window for survival

“…Also, at about half of the stations, a 10 m 2 MOCNESS (3 mm mesh) sampled older fish larvae and other planktonic organisms. The full sampling plan and methodology can be found in Mountain et al (2003Mountain et al ( , 2008. The density and total abundance of hatching larvae were estimated from the distribution of late stage eggs corrected for estimated egg mortality.…”

“…Here we combined data on the seasonal and inter-annual variability in mortality rates of Atlantic cod and haddock obtained from 24 cruises (Mountain et al 2003(Mountain et al , 2008 with comparable data on the seasonal and ontogenetic trends in larval growth rates (Buckley & Durbin 2006. We examined models relating M to hatch date, photoperiod, and water temperature.…”