Cohort growth and survival of bay anchovy Anchoa mitchilli larvae in Biscayne Bay, Florida

Abstract

Egg and larval abundance of bay anchovy Anchoa mitchilh from 4 field experiments in Biscayne Bay, Florida, were analyzed to determine variability in growth and mortality rates of daily-spawned cohorts. Mean egg densities varied by more than an order of magn~tude in the 4 experiments. Otolith-aged larvae, validated by laboratory rearing, were used to construct the age-frequency distributions of 71 cohorts. Mean growth rate estimates ranged from 0.43 to 0.56 mm d-' and the means of cohort-specific instantaneous mortality rates ranged from 0.30 to 0.45 (26 to 36 % d-l). Egg mortality rates were generally higher and more variable than those for larvae. An approximate mean mstantane-ous mortality rate for eggs was 1.94 (85.6 % d-l). Mean larval growth and mortality rates differed significantly among experiments, although reasons for the lfferences were not identified. On average, relative s u ~ v a l s of cohorts varied by a factor of 20 at 20 d after hatching among the 4 experiments. Estimated mean abundances of larval cohorts at 20 d varied by a factor of 45, reflecting differences in in~tial abundances of eggs and the variable mortality rates. Models of bay anchovy growth and mortality in relation to prey concentration, developed in previous laboratory expenments, were used to partition field-expenment mortalities into nutrition-related and presumed predabon-related components. Mean daily losses of a cohort to predation (18 to 28 %) were 2 to 3 times higher than losses to starvation (10 to 11 %).