New modelling study challenges the current assumption that density dependence always occurs early in life in marine fish and that young fish therefore should not be exploited
Fisheries advice is based on demographic calculations, which assume that density dependent processes regulating recruitment occur only in early life in marine fish stocks. This assumption has been challenged by laboratory and lake studies and some recent indications from marine systems that demonstrate density dependent regulation late in life.
Empirical knowledge about density dependent regulation of animal populations is exceedingly difficult to obtain and interpret. But in the new paper “When in life does density dependence occur in fish populations?”published in Fish and Fisheries, researchers at DTU Aqua develop the missing theoretic understanding about density dependent regulation in fish populations.
"Our results challenge the current assumption that density dependence always occurs early in life in marine fish and highlights the need for an increased understanding of density dependent processes. This can only come about by a change of focus from determining stock-recruitment relationships towards understanding when and how density dependent regulation occurs in nature", states Professor Ken Haste Andersen, DTU Aqua.
Implications for fisheries management
By accounting for spatial dynamics of a population—something that has previously been ignored in models of fish—the researchers at DTU Aqua show that density dependent regulation is determined by the size of the habitat: in small habitats, e.g. small lakes, regulation occurs late in life while it can occur early in large habitats.
A correct understanding of density dependence is crucial for a central instrument in Maximum Sustainable Yield fisheries management, the minimum landing size regulations, that are used to avoid landing young fish by fisheries. When density dependent regulation happens late in life, fisheries yield could be maximized by exploitation of mainly juvenile fish, while exploiting mature fish maximizes yield if regulation happens early.
The potential implication of these results for current mesh size regulations should motivate investigations into how and when density dependent regulation occurs in natural fish stocks. A complete understanding of the population dynamics and density dependence of a specific fish stock requires that more detail about the stock in question, e.g. the presence of life history change such as settling, is explicitly resolved.
Download the paper
Ken H. Andersen, Nis S. Jacobsen, Teunis Jansen and Jan E. Beyer: When in life does density dependence occur in fish populations?, Fish and Fisheries, 18 November 2016.
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