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Characteristics of cyclic fluctuations generated by stock-recruit systems. Basasibwaki , Pereti

Abstract

Ricker (1954) derived a stock recruitment relationship from assumptions involving cannibalism or other compensatory density dependent mechanisms. His relationship indicates a decline in reproduction at high stock densities of spawners, the result of which may give a population a tendency to oscillate in numbers. The object of this study was to examine, using Ricker’s stock-recruitment model, the period and amplitude of cyclical fluctuations in numbers of animals as they are related to intrinsic factors such as age of first maturity, number of generations in the spawning stock and the shape of the reproduction curve, and extrinsic factors such as exploitation and random fluctuating environment. In lightly exploited populations the period of oscillation is dependent on age of first maturity and number of generations in the spawning stock, the period being equal to twice the mean age of maturity. In heavily exploited populations, the period of oscillation is dependent on age of first maturity but independent of the number of generations in the spawning stock, the period being equal to approximately twice the age of first maturity. When random fluctuations are added to the system, cyclic changes are maintained at low effects of random factors but they introduce instability in the oscillation and variability in the period of oscillation as they become more effective. The shape of a reproduction curve does not influence the period of population oscillation but the steeper the right hand limb of the curve the greater is the amplitude of the fluctuation. The amplitude of oscillation in numbers is basically determined by the shape of a reproduction curve and it increases with delayed maturity while it decreases with increasing number of ages spawning and increasing exploitation. It was considered doubtful whether the observed high reduction in the period of oscillation would every be big enough to notice, in natural populations, before exploitation removed oscillations completely and stabilised population abundances.

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