Cohort Variation

As the size of the Año Nuevo colony increased more than fivefold from 1971 to 1988 (an increase similar to that of the entire population; see Stewart et al., this volume), one might expect lower survivorship values in the later years due to increased competition for resources either on the rookery or on the foraging grounds. Pup mortality on the island prior to weaning increased from a low of 14.5% of pups born in 1971 to a high of 70% of pups born in 1983; preweaning pup mortality is density dependent, and there is a significant interaction with weather (Le Boeuf and Briggs 1977; Le Boeuf and Reiter 1991). There was no tendency for survivorship to age 1 to decrease with time and increasing density (fig. 7.3a); however, survivorship to age 4 decreased with time (the regression of y on x = 90.1 – 0.94x; r = 0.81).

An indication of the relative long-term strength of a cohort is the percentage decrease in the juvenile survival rate from year 1 to year 4. The percent decrease in survivorship from age 1 to age 4 increased over the study period (fig. 7.3b); that is, the mortality rate over the juvenile years increased with time.

The mean percentage decrease in survivorship from age 1 to 4 (fig. 7.3b)

Fig. 7.3
(A)  Survivorship to age 1 (open circles) and to age 4 (closed
circles) as a function of cohort year. The regression equation
for the latter is y = 90.1 – 0.94x; r = .81.  (B)  The percent
decrease in survivorship from age 1 to age 4 as a function
of cohort year. The regression equation is
y = –111 + 2.1x; r = .69.

for all 18 cohorts was –55.2 ± 14.1%. The three strongest cohorts, those with the lowest percentage decline over the three-year period, were 1978 (–26.2 ± 3%), 1973 (–31.3%), and 1972 (–32.3%). The three weakest cohorts were 1983 (–74.4%), 1985 (–74.4%), and 1988 (–66.9%).