THE INEVITABILITY OF MORTALITY

Studies of rates of ingestion, particle selection, metabolism, growth, moulting, and egg production have dominated the marine zooplankton literature for many decades. It is clear, however, that the historical emphasis on processes of growth, development, and fecundity is too limited: much more work is required on mortality in natural populations. In particular, stage-specific patterns of mortality, their variability over time and space, and analysis of the underlying causal agents - predators, parasites, starvation, senescence, and advective losses - are essential elements of any quantitative description of population growth. Examples given illustrate the sensitivity of different zooplankton behaviours, population dynamic processes, and interactions with other elements of marine ecosystems to the characteristics of the mortality function. Realized fecundity is shown to depend on adult female mortality rates. Application of a new method for estimating mortality rates for stage-structured populations to the copepod Pseudocalanus newmani illustrates that temporal variations in per capita mortality rates can be several times the magnitude of temporal variations in per capita fecundity. This emphasizes the need for quantitative assessments of the loss terms for natural populations. (C) 1995 International Council for the Exploration of the Sea