Estimating cladoceran birth rates: The importance of egg mortality and the egg age distribution1

Abstract

Three models of egg hatching in zooplankton populations gave significant differences depending on whether egg mortality was included. The differences were due to the effect of adult mortality on reducing the probability that eggs survive from the time of laying until they are released as live young. Adult mortality results in a population of eggs disproportionately dominated by younger stages, with resulting recruitment rates being lower than expected from development times alone. A technique based on the relative duration of various developmental stages was used to determine the age of eggs from two natural populations during periods of decline. The observed tendency for the age distribution of eggs to be dominated by younger eggs was as expected from models of egg hatching which included mortality but was not as expected from models of declining populations which did not incorporate egg mortality.