A graphic review of the growth and decay of population cohorts

Abstract

A series of growth and decay curves have been studied to interpret the effects of changing natural mortality and growth on the time at which a cohort will maximize the amount of material produced and the expected length of the time envelope. A graphic approach is used to familiarize marine biologists and students of fish population dynamics of factors governing the growth and decay of fish cohorts. Equations have been established to compute the age or time a cohort will achieve its maximum weight, the maximum expected weight of a cohort per given input, the weight of a unit number of animals at the critical size; and an equation has been established for forecasting the critical size for various values of M and K. If the coupled M, K1 values for any curve are equal and W∞1 is held constant, the maximum biomass generated by such cohorts will be constant, regardless of the coupled M, K values. Similarly, for all curves in which M equals K and W∞ is held constant, the critical size for all such curves will be identical. It was also noted that the maximum biomass generated per given input would be the same for all curves for which the M, K ratios were equal (assuming W∞, is held constant). The authors finally propose a method of making a first approximation of natural mortality based on the time a cohort will maximize its aggregate weight and reach its “theoretical” age maximum. © 1975 Oxford University Press.