. The Eastern Bering Sea Shelf : oceanography and resources / edited by Donald W. Hood and John A. Calder. Oceanography Bering 534 Fisheries oceanography 10,000 p- -\ 1,000 (0 3 > c T- > o c (0 3 < 0) (0 jO O 0) O) 10 OVERALL SURVIVORSHIP (1973-1978) \ \ \ \ \. \ .\ \ \ \ZL 1 - 6 8 10 Age (yr) 12 14 Figure 33-10. Overall survivorship curve, based upon the data in Table 33-3. rates of in-migration to, and out-migration from, the index area were equal, and that: N2 -Nie-zt (1) where Ni N2 Z t e = the number of individuals at any moment, = the number of individuals at some later ti
. The Eastern Bering Sea Shelf : oceanography and resources / edited by Donald W. Hood and John A. Calder. Oceanography Bering 534 Fisheries oceanography 10,000 p- -\ 1,000 (0 3 > c T- > o c (0 3 < 0) (0 jO O 0) O) 10 OVERALL SURVIVORSHIP (1973-1978) \ \ \ \ \. \ .\ \ \ \ZL 1 - 6 8 10 Age (yr) 12 14 Figure 33-10. Overall survivorship curve, based upon the data in Table 33-3. rates of in-migration to, and out-migration from, the index area were equal, and that: N2 -Nie-zt (1) where Ni N2 Z t e = the number of individuals at any moment, = the number of individuals at some later time, - an instantaneous rate of total mortality, = the intervening time interval, and = the natural constant , (Z) was This rate corresponds to 51 percent of the pollock population dying each ye£ir. Rather than being constant, mortality was relatively high for ages 1, 4, 5, and 10 or more years (Fig. 33-11). Fac- tors that may have contributed to apparently higher MEAN AGE-SPECIFIC L MORTALITY ^ then the overall observed instantaneous mortality rate Figure 33-11. Age-specific instantaneous mortality rates, based upon the data in Table 33-3. mortality at ages four and five include the possibili- ties of increased selective removal by fisheries, increased migratory activities associated with onto- geny, and effects of reproductive stress. Senescence was presumably a major cause of higher mortality at ages 10 and older. Life table characteristics are summarized in Table 33-4. Contrary to the assumption of a closed population, or that in- and out-migrations were in equilibrium, the indices of abundance observed in 1975 for all age-classes except the age-one population were inconsistently low (relative to cohort abundances observed in 1976), apparently indicating a distribu- tional shift out of the index area during that survey period. Another interesting result was the indication of density-dependent mechanisms possibly regulating age-class siz
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