Why are Quantitative Relationships between Environmental Quality and Fish Populations so Elusive?

Despite the ecological and economic importance of fish, fisheries management has generally failed to achieve its principal goal of sustainability. Management is hindered because most exploited fish are long-lived species that utilize a variety of habitats and exhibit high interannual fluctuations in abundance. Effective management requires that we understand how natural and anthropogenic sources of variability in abiotic variables (termed environmental quality [EQ]) affect fish population dynamics. Quantifying the effects of anthropogenic changes in EQ on fish populations has remained elusive and controversial. I illustrate, with examples, six issues related to quantifying EQ effects on fish populations. These examples also serve as demonstrations of how modeling can be used to address these issues. The six issues are: (1) detectability-high interannual variation and interaction effects among climatic variables that affect population dynamics make isolating effects of individual stressors difficult; (2) complex habitat and nonintuitive responses-spatial heterogeneity in habitat can result in population responses that are disproportionate to the changes in EQ; (3) regional predictions-biological realism is often sacrificed unnecessarily when broad spatial scale predictions are needed; (4) community interactions-too little attention is paid to how community-level interactions can affect population-based analyses; (5) sublethal effects-sublethal effects are often ignored but can have large effects on population dynamics; and (6) cumulative effects-the combined effect of multiple stressors can be much different than expected from the sum of their individual effects. Examples include a variety of freshwater and marine species. Quantifying EQ effects on fish populations can be improved by considering these issues in analyses, and by taking a true multidisciplinary approach that combines individual-based modeling and life history theory.