Influence of Age-Specific Mortality on the Life History Traits of Two Estuarine Copepods

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Abstract
Life history traits, including day of 1st reproduction, lifetime reproductive output and total liefspan, were determined in the laboratory for 2 estuarine copepods: Scottolana canadensis (Willey) and Oithona colcarva Bowman. The day of 1st reproduction was earlier for O. colcarva than for S. canadensis, but the latter had a much longer reproductive lifespan and a greater reproductive output in terms of nauplii produced per female. The copepods exhibited intrinsic rates of increase ranging from 0.01-0.19/day for O. colcarva (15.degree.-25.degree. C) and 0.05-0.15/day for S. canadensis (20.degree.-25.degree. C). A comparison of life history traits of S. canadensis and O. colcarva with the copepod Acartia tonsa Dana revealed differences which may reflect variation in age-specific patterns of mortality of the field populations. For S. canadensis, juvenile mortality (specifically of planktonic nauplii) may be higher and more variable than mortality of epibenthic adults. Mortality of planktonic A. tonsa adults may be high and more variable than their juvenile stages. The reproductive strategy of S. canadensis is characterized by a later day of 1st reproduction, more discrete egg production and a longer lifespan compared to A. tonsa. Life history traits and variable age-specific mortality patterns of these species are consistent with current theory which predicts that variation in age-specific mortality acts as a selective force on the reproductive patterns of organisms.

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