Stock-recruitment relationships for life cycles that exhibit concurrent density dependence

This study provides theoretical stock-recruitment relationships for life cycles in which multiple, density-dependent mechanisms stemming from different periods during the life cycle act concurrently on a single demographic transition. Using graphical examples and analytical derivations, it is demonstrated that overcompensatory density dependence emerges from such life cycles despite the initial assumption that density-dependent mechanisms follow simple compensatory Beverton-Holt dynamics. These results indicate that concurrent demographic effects of temporally distinct density-dependent mechanisms provide a biologically plausible basis for empirically derived, three-parameter stock-recruitment models. This theory is inspired by, and may be most applicable to, spawner-recruit relationships in anadromous salmonids but may also inform analysis of stock and recruitment data for other taxa that putatively compete for both food and spawning space. Application of this theory will require the estimation of additional parameters from stock-recruitment data. Such parameters, however, have clear biological meaning and, at least theoretically, are accessible to empirical measurement. Stock-recruitment relationships analogous to those presented here may therefore facilitate the construction of models that incorporate independent empirical data and environmental covariates for populations that are currently better described by phenomenological equations and represent an important step towards models that incorporate spatial structure in populations.