Regulation of fish stocks without stock-recruitment relationships: The case of small pelagic fish

Small pelagic fish lack clear stock-recruitment relationships. This is a problem because such relationships are taken to be the primary descriptors of density dependence, responsible for regulating population density. In this paper, we show that small pelagic fish species, anchovy (Engraulisspp., Engraulidae), living in a stochastic environment, can be strongly regulated without a stock-recruitment relationship emerging. This is done through numerical analysis of a size-spectrum model, in which fish grow by eating and die in part from being eaten, with the result that birth, growth and death are all density-dependent. The model includes cannibalism, and growth-dependent larval mortality, both of which have been suggested as regulatory mechanisms in anchovy, together with growth and reproduction later in life. Despite the lack of a clear stock-recruitment relationship in the presence of stochasticity, signals of density dependence in the vital rates remain clear, suggesting that they might prove to be better indicators of density dependence than stock-recruitment relationships in small pelagic fish.