Biological Oceanography

(^) The horizontal method has been applied also to copepod populations, with stage
distinctions used as an approximation of age. Johnson (1981) estimated stage
abundances twice per week through the growing season for the Acartia californiensis
population active in summer in warm upstream waters of Oregon’s Yaquina Bay
(USA). Nauplii, unfortunately, could not be distinguished from those of the co-
occurring Acartia clausi, so they were not counted (also saving labor). The data are
shown for 1973 (Fig. 8.9). From fecundity estimates, females present in the plankton
on any given day were assumed to spawn 20 eggs each, giving a very rough egg
number to start each cohort. Stage-to-stage survivorships (Fig. 8.10) were then
calculated as reductions in the number of “copepodite-days” on the graph for each of
the year’s cohorts (distinguished similarly to Landry’s work in Jakle’s Lagoon,
Chapter 7). Clearly, there must be distinct mortality stanzas in the life history of
copepods as well as euphausiids. Very large numbers of eggs and nauplii die at high
rates, leaving a few copepodites, which then have relatively high survivorship to
adulthood. Earlier cohorts with low abundance have better naupliar survival than later,
more abundant cohorts. Similar estimates were made by Landry (1978) for Acartia
clausi, Twombly for diaptomids in lakes, and others. Eventually, it was realized
(Hairston & Twombly 1985) that changes in copepodite-days occupied by a cohort are
not dependent upon mortality alone, but on development rates. If development is
nearly isochronal, that is each stage passes in equal time, then the procedure isn’t too
bad. That is true of well-fed Acartia, so the results of Johnson and Landry are
probably useful. But, when stage durations vary, a more complex model is required.