A New Framework to Evaluate and Avert Risks to Fragmented Fish Populations

Many populations of stream fish persist in remnant patches that are isolated by anthropogenic barriers. Risk-averse management of such populations could involve either of two contrasting strategies: (1) restoration of connectivity, which averts risks associated with small population size, or (2) maintenance of isolation, which averts risks associated with the mixing of divergent gene pools. The optimal strategy depends on the relative magnitudes of these "small-population" (SP) versus "outbreeding-depression" (OD) risks, which depend on the demographic and genetic characteristics of populations and evolutionary relationships between populations. We developed a framework to facilitate comparison of SP and OD risks based on published risk criteria and commonly available data. To illustrate how the framework could guide conservation choices, we applied it to seven isolated populations of Roanoke logperch (Percina rex), an endangered stream fish. Both risk types varied considerably among populations, suggesting that a uniform management strategy would be ineffective. Only one population exhibited low risk for both types, whereas four exhibited high SP (but not OD) risk and two exhibited moderate OD (but not SP) risk. Based on this analysis, four P. rex populations could benefit from restored connectivity, whereas two should remain isolated. Our approach could be used to develop transparent, science-based management strategies for P. rex, as well as other rare or imperiled taxa.