Predicting fish population dynamics: compensation and the importance of site-specific considerations

The magnitude of the compensatory response inherent to fish population seems to be questioned with each new generation of scientists and regulators. The major reason for this is the demand by some that compensation be completely understood at the process level before compensation can be factored into population predictions. We first discuss several lines of evidence for the existence of compensation, and the reasons why compensation is so difficult to measure at the process level. We then offer life history theory as a general framework for improving our understanding of the likely magnitude of compensation in fish population dynamics. We use detailed, individual-based models and field data for yellow perch-walleye, striped bass, and a lake community to illustrate the complexity of compensation, and to show that predictions for a particular population require site-specific information. Analyses show that the life stage and magnitude of density-dependence can vary for different populations of the same species, and that population responses can depend on the arrangement of the food web. We must stop reinventing the density-dependence wheel; too many resources (both natural and monetary) are at stake. We suggest using three approaches for addressing the compensation issue: (1) life history theory for providing a sound basis for qualitatively understanding fish population dynamics and the magnitude of compensation; (2) age- and stage-structured matrix models for screening-level analyses to identify populations that may be at unacceptable risks of decline and therefore require additional analyses; and (3) individual-based models for better understanding the complexities of compensation at the process level and for impact assessment of at-risk, well-studied populations.