Mortality estimation for planktonic copepods: Pseudocalanus newmani in a temperate fjord

Abstract

The population surface method for estimating mortality was modified to incorporate diapause of copepodid stage 5 and missing naupliar stages and applied to a 2‐yr time series of the planktonic copepod Pseudocalanus newmani in a temperate fjord. The fitted model captures nearly all of the observed dynamics of the population and yields age‐, stage‐, and time‐specific measures of mortality along with confidence limits. Average mortality rates for eggs and adult females were quite similar, as would be expected for species that carry eggs until hatching. Instantaneous mortality rates of adult males were on average 11–12 times higher than those for adult females. Average mortality rates were high for copepodid 1 (C1), declined in stages C2 through C4, and increased again at C5. This bimodal pattern of mortality with developmental stage probably results from changing susceptibility to different guilds of predators with ontogeny. Mortality rates for C5 and adult females were not significantly different between the overwintering period and the active growth season. Mortality rates of adult female P. newmani varied in a nonlinear manner with the abundance of predators as a result of predator avoidance behavior, underscoring the importance of zooplankton behavior in interpreting population dynamics.