Downstream thermal effects of hydroelectric dams: Growth, development, and susceptibility to parasites of larval Foothill Yellow-legged frogs (Rana boylii)

Background/Question/Methods

When utilities re-license their hydro-electric projects with the Federal Energy and Regulatory Commission, the implications of various flow proposals to downstream thermal conditions are evaluated. Dam operations can either raise or lower water temperatures compared to free-flowing conditions. Retention of water can result in artificially low discharge, causing unnaturally warm temperatures. Alternatively, if base flows are maintained with releases from a reservoir’s hypolimnion, downstream temperatures are un-naturally cold.  Rana boylii, the Foothill Yellow-legged Frog, has become a focal species during the re-licensing process in California.  To understand the thermal consequences on recruitment to frog populations, we reared tadpoles under a range of temperature regimes in flow-through enclosures at the Angelo Coast Range Reserve (Mendocino Co., CA). These thermal regimes mimic the conditions predicted by temperature models typically used during re-licensing.
Results/Conclusions

Maximum weekly average temperatures were 17.3, 18.2, 20.9, and 22.7°C in four study streams. Survival of tadpoles to metamorphosis was highest at intermediate temperatures and was significantly influenced by food quality. When we supplemented the ambient periphyton with macroalgae epiphytized by nitrogen-fixing diatoms, survival was 46.7% vs. 26.8% for ambient periphyton. Supplementation allowed metamorphosis at the colder temperatures, but development was slow: 111 and 118 days in the cooler streams vs. 69 days post spawning at the warmer sites. Contrary to expectation, tadpoles grew to larger size at warmer temperatures. Periods of warm temperatures were associated with outbreaks of the parasitic copepod Lernaea cyprinacea in R. boylii. Infestation varied spatially in the watershed with prevalence increasing concomitantly with temperature along a 5.2 km longitudinal transect.  Copepods were absent from upstream sites and infested up to 28.6% of individuals at downstream locations. Our results suggest that thermal regime should be considered when weighing the risks and benefits of alternative flow proposals to frog populations.