An additional problem with application of the Mullin–Brooks calculation is that
most copepod populations never have steady-state population composition, as
required. Whether averaging stage abundances, stage ratios, or estimated mortality
rates over months of data, for example as done by Ohman et al. (2004) with the 5-year
January to June GLOBEC sampling program on Georges Bank, can compensate for
the generality of short-term violations of the assumptions is unclear. The two-weeks-
on, two-weeks-off sampling of Georges Bank must also have introduced gaps in the
series of stage-ratio estimates, gaps with at least some impact on the estimates, despite
the very large number (thousands) of samples evaluated. More positively, mortality-
rate estimates for newly spawned eggs to N3 of C. finmarchicus on Georges Bank by
a modified Mullin–Brooks method (Ohman et al. 2008) are likely of the right general
magnitude, about 20% per day in March, about 50% per day in May. Those authors
attribute a substantial share of that mortality to cannibalism by Calanus females.
Experiments in several laboratories (e.g. Basedow & Tande 2006) confirm such
feeding, at least by females cooped up with their eggs and nauplii in one-liter
containers.
(^) An alternative route to mortality-rate estimation is to model population stage
composition and abundance by fitting mortality rates to time series of field data for
stage abundances. The greater the number of population-dynamical variables that can
be approximated from field data, the more accurate the fitted mortality rates will be.
Egg production per female can be measured, and multiplied by female abundance to
obtain egg input. Stage duration is more difficult. Inverses of stage molting rates
(fraction molting per day among incubated samples from the ocean) would equal
stage durations at steady state, but again steady state does not develop. The oldest
individuals in a cohort that is just reaching a stage will have zero molting for a while.
Even the youngest individuals in a cohort just finishing a stage will have fractions
near 1.0 that molt in a day. Laboratory rearing data for effects of temperature and
nutrition can be applied, although characterizing available nutrition in the field is an
unsolved problem, and even the actually effective temperature can be difficult to