UW Aquatic & Fishery Sciences Quantitative Seminar
Ken Haste Andersen
Professor in theoretical marine ecology at AQUA, Danish Technical University, Deputy Chair in the Centre for Ocean Life
The theory behind fisheries reference points
Calculation of reference points for fisheries, in particular Fmsy and the minimum size limit, is customarily done using age-structured Beverton-Holt population theory. Here I develop a size-based version of the Beverton-Holt theory and use it to calculate fisheries reference points. The theory confirms some classic results and while also unveiling new surprises. 1) Formulating the theory using body size leads to a prediction of the “slope” parameter in the stock-recruitment relationship. This reveals how larger species (large asymptotic size) show stronger density-dependent regulation than smaller species. 2) The fishing mortality leading to maximum sustainable yield (Fmsy) is roughly independent of asymptotic size. Smaller species (such as forage fish), though very productive, are therefore less resilient towards fishing that one would expect from simple metabolic scaling rules. 3) Allowing density-dependent regulation by other mechanisms than a stock-recruitment relationship (i.e. through competition and cannibalism) show that in many cases fishing juvenile fish will maximize fisheries yield. This result challenges classic theory stating that yield is maximized by targeting only adults, but is in accordance with empirical and theoretical results from lake studies. The importance of this result for fisheries management based on mesh-size regulations can hardly be overstated. I will outline cases where stock-recruitment based theory may still apply as well as cases where it may fail.