Evaluating the accuracy of amphibian monitoring designs in the context of hydroelectric dam relicensing: A retrospective analysis of a 15-year Rana boylii egg mass census

Over the next fifteen years the environmental impact of >150 California hydroelectric dams will be scrutinized prior to federal relicensing. Dam operators and regulators are faced with the task of monitoring the status of frogs, salmonids, and other indicator species over distances too great for comprehensive surveys. Here we evaluated the accuracy of assessing the site occupancy, abundance, and population trends through time for a declining river breeding frog, Rana boylii. Using egg mass surveys from a 15 year monitoring effort conducted over a continuous 5.2 km reach of the South Fork Eel river in northern California, we retrospectively evaluated spatial and temporal sub-sampling strategies that are currently being considered in relicensing efforts. Results of bootstrapping simulations indicate that the underlying spatial and temporal patterns in R. boylii breeding strongly influence the accuracy of monitoring because, 1) nearly 50% of the population breeds within one riffle-pool sequence of perennial tributary confluences; 2) the most populous breeding sites are also the most stable through time, and 3) fluctuations in the number of breeding females appears linked to the hydrologic conditions three years prior. Were monitoring efforts focused on tributary confluences, both the population size and stability through time would likely be overestimated. In the absence of a priori knowledge about the longitudinal distribution of individuals (e.g. a full-reach survey) and the lag time between hydrologic effects on embryos and the number of breeding females, it is unlikely that monitoring will accurately capture the spatial and temporal dynamics of R. boylii populations.