female abundance. Egg mortality is relatively low and about the same as for females,
presumably because eggs are carried until hatching by the females. Male mortality,
not estimated, must be very high, since males in Pseudocalanus are very few
compared to females. This could also result from disproportionate maturation of C5s
as females, not males. There are strong hints that adult sex in many copepods is
determined by environmental conditions experienced by late copepodites. Elevated
mortality rate of fifth copepodites is attributed to growing into the size range of
interest to abundant predators, while reduced female mortality is attributed to their
adoption in summer of reverse vertical migration (down at dusk, up at dawn; Ohman
et al. 1983) to avoid predators which migrate normally (see below) and respond to
vibrational stimuli from prey. Every feature of such results can be explained
somehow; testing the explanations is another matter.
Fig. 8.11 Stage-by-stage instantaneous mortality rates of Pseudocalanus newmani in
Dabob Bay, Washington.
(^) Rates estimated from weekly data by the statistical procedure of Wood (1994).
The symbols • and represent data from two years.
(After Ohman & Wood 1996.)
A simple vertical method devised by Mullin and Brooks (1970) has also been
applied to the Dabob Bay data for P. newmani by Aksnes and Ohman (1996). Assume
that mortality rates will be very close for adjacent stages, the younger with abundance
X and the older with abundance Y. If the duration of both is a days, and input and
output are roughly steady, then Y < X, and: