Szuwalski, Cody; Britten, Gregory L.; Licandeo, Roberto; Amoroso, Ricardo O.; Hilborn, Ray; Walters, Carl J. 2019. Global forage fish recruitment dynamics: A comparison of methods, time-variation, and reverse causality. Fisheries Research. (Abstract) (Online version)
The population dynamics of forage fish are often â€˜boom or bustâ€™, and variation in recruitment may be a contributing factor to changes in abundance. Here we applied several methods for identifying stock recruit relationships (SRR) to 52 forage fish stocks: a time-invariant Ricker model and two time-varying methods (dynamic linear models and regime-based models). A positive relationship between spawning biomass and recruitment existed for 28 stocks, with 14 of those being best fit by a Ricker curve. Time-varying models were preferred for 26 stocks over a time-invariant Ricker curve. We also showed through simulation that spurious, yet significant stock-recruit relationships can be generated if recruitment was environmentally driven, the lag between recruitment and spawning biomass was small, and autocorrelation in recruitment was highâ€”a combination of characteristics that many forage fish stocks display. Based on our results, observed time-invariant SRRs can result from a reversed causality of â€˜boom and bustâ€™ dynamics in situations in which life histories are short (i.e., changes in recruitment are driving changes in spawning biomass). â€˜Hiddenâ€™ SRRs are also possible because of time-variation in SRR parameters. Understanding when each of these potential failings of fitting SRRs can occur is a key challenge in forage fish management.