RECRUITMENT, GROWTH, MORTALITY, AND BIOMASS PRODUCTION OF LARVAL AND EARLY JUVENILE ATLANTIC TOMCOD IN THE HUDSON RIVER ESTUARY

We report on the dynamics of a closed population of larval and early juvenile Atlantic tomcod Microgadus tomcod in the lower Hudson River estuary. Previous Hudson River studies of this species were conducted farther upriver where the population is open, influenced by large-scale, undetected migrations. We determined the date of peak hatching for Hudson River Atlantic tomcod to be 9-11 March in 1975 and 1976, after an egg incubation period estimated to be 61-70 d. The average length of larvae with full yolk sacs (assumed to be newly hatched) was 7.0 mm, and yolk absorption took 2-3 weeks. Our estimates for egg incubation period, larval length at hatching, and duration of the yolk-sac stage were considerably higher than the values typically cited for Hudson River Atlantic tomcod. Early growth of this coldwater species was enhanced by higher spring temperatures, but growth declined significantly from 2.9%/d to 1.3%/d as water temperature rose above 13-degrees-C in tate May. Recruitment each year was estimated at 2-3 x 10(9) larvae, and daily mortality rates (Z) of 0.0503-0.0823 reduced the 1976 cohort by about 99% within 10-11 weeks from peak hatch. Cumulative biomass production during March-May was 130,000 kg for the 1976 cohort. Prey potential, in the form of cohort biomass, peaked at 36,000 kg in late May, prior to the midsummer recruitment of other juvenile prey species.