Models based on full representation of population-dynamical processes are
developing rapidly, and estimates of mortality rates can be expected to shift up and
down for some years to come. For example, Neuheimer et al. (2009) have reworked
the C. finmarchicus over Georges Bank problem again, noting that the numbers of
nauplii estimated by the model of Li et al. (2006) for May (recruited from the last
major reproductive cycle) fall one to 15 standard deviations outside the data. They
propose varying the naupliar mortality rates more radically in both time and space
(much greater in late April and May, greatest over the bank crest). Seasonal and
spatial variations of mortality rates are not a particularly surprising conclusion.
Neuheimer et al. (2010) also propose an individual-based modeling approach with a
mechanism for tuning mortality rates to match time series of stage-specific abundance
estimates. Application to more studies will show whether such methods produce
reliable mortality rates.
(^) Apart from euphausiids and copepods, explicit estimates of mortality rates in
zooplankton have not been attempted.
Causes of Mortality
(^) Death results from predation, starvation, disease, and old age. We don’t have good
aging methods for any full-grown or adult plankton, so we don’t know how many
reach the ages to which we can hold them in the laboratory. Very few female
copepods or euphausiids that appear to be reproductively spent are collected in the
upper water column. It is likely that the daily risk of predation is so great that few
reach such an age. There are masses and masses of gut-content data showing that
zooplankton are eaten by other zooplankton, by larval fish, adult fish, and whales.
Being eaten is the fate of practically all individual zooplankters. Since the risk is
continuous and high, virtually every species in every group has a repertoire of
adaptations to foil predators. Pelagic habitats lack hillocks, trees, shrubs, and rocks to