population survivorship. In the same vein, of course, a horizontal life table can trace
the fate of a cohort that is unusual and can be of no general application to populations
of the species. Nobody who knows about it will tell you that ecology is easy.
Estimation of mortality rates has seldom been attempted for plankton other than
crustacea, so our examples will come from that group.
(^) The vertical method can be used in a number of ways to obtain stage-specific
mortalities. One is to sample repeatedly and do lots of averaging between generations,
years, etc. Brinton (1976a & b) used the massive sample set of the CalCOFI
investigations in the Southern California Bight area to study the age structure
(actually the size structure) of Euphausia pacifica. If you simply add up all of the
size–frequency distributions over hundreds of samples from many years and seasons,
you get a sort of average vertical survivorship curve (Fig. 8.8). There are three or four
stanzas of roughly constant mortality per millimeter of body length: larval, early
juvenile, adult, and senescent phases. This survivorship pattern is plotted on a semi-
logarithmic scale and appears roughly linear in successive growth stanzas, suggesting
that exponential mortality with age is an appropriate model. Rates (–m, mm−1) have
been calculated and are shown along the graph. Brinton (1976) dissected the data-set
into cohort patterns (figures 9 and 10 in his paper) and derived more information
about mortality patterns, achieving a horizontal approach for some of the better-
defined age groups that moved through the population from 1953 to 1956. For
example, there are differences in reproduction timing and growth rates among seasons
that recur from year to year. Close study of this landmark paper will show you why
such studies are so very rare: the labor is enormous.
Fig. 8.8 Long-term averages for numbers of Euphausia pacifica in the Southern
California Bight by length category. Abundances (1000 m^3 )−1 are plotted for larvae
and immature specimens on a log scale above and for adults on a linear scale below.
This generates a survivorship curve according to size. Mortality has three stanzas with
relatively constant rates: a larval rate from 3 to 6 mm, a lower juvenile rate to 12 mm,
and an adult rate slightly higher again from 12 to 20 mm. Mortality rates certainly
change, but the slope changes in the progressive decline of abundance with size are
also affected by changes in growth rate and net avoidance capability.
(^) (After Brinton 1976.)