An age-structured simulation model to investigate species replacement between pilchard and anchovy populations in the southern Benguela

Abstract

Clupeoids form the basis of important commercial fisheries off Namibia and South Africa’s Western Cape. Studies on the abundance of pilchard (sardine) and anchovy populations subject to heavy exploitation have revealed the possible succession of sardine by anchovy as the dominant species. In investigating the evidence for species replacement, a simple, age-structured computer simulation model was developed, which assumes that the system is initially in a steady state. The model’s sensitivity to starting biomass, natural mortality, harvesting, the level at which density-dependence is initiated and environmental perturbation is considered. Important factors determining the results are the present estimates of natural mortality and the level at which density-dependence is initiated. A lower natural mortality in pilchard results in dominance by anchovy. High threshold values of density-dependence produce dynamic oscillation in the model populations. Harvesting affects the natural balance between populations in a system at steady-state, causing the dominant stock to decline, resulting in species replacement. This situation is particularly evident in scenarios employing harvesting numbers-at-age. The intensity and the duration of harvesting determines whether replacement is temporary or permanent. The model populations show resistance toward environmental perturbation; indeed, in scenarios including it, species replacement is delayed. © 1992 Taylor & Francis Group, LLC.