Effects of aseasonal pulsed flows on the Foothill Yellow-legged Frog (Rana boylii) in California rivers

The foothill yellow-legged frog (Rana boylii) is a river breeding frog whose life cycle is synchronized with the seasonality of floods and low flows typical in a Mediterranean climate. Analysis of time-series data from 5 dammed and 7 un-regulated California rivers indicates that un-regulated river populations are much more robust. We hypothesize that recruitment limitation contributes to this pattern. R. boylii’s life stages occur along a continuum of susceptibility to flow fluctuation: immobile eggs, moderately mobile tadpoles, and highly mobile adults. Dam spills and manufactured pulses can subject vulnerable early life stages to relatively larger magnitude and higher frequency flow fluctuations causing greater scouring and stranding than is observed in unregulated systems. Censuses in the unregulated Eel River (1992-2006) and the regulated Feather River (2001-2006), indicate a three year lag between unfavorable hydrologic conditions (i.e. late spring rains or summer white water boating pulses) and declines in adult numbers. At the Eel River, egg season rainfall was negatively correlated with λ, R= -0.53, p=.05. To assess the effects of summer flows on tadpoles, we manipulated velocity and substrate in laboratory flumes and field enclosures.  Across all experimental venues, tadpoles decreased activity and sought refuge in substrate as velocity increased. In a Brett chamber, tadpoles could no longer swim or maintain position under rocks at a mean critical velocity of 20.9 ± 1.6 cm/sec. Critical velocity varied negatively with tadpole size and developmental stage. Sub-critical velocities had energetic and predation costs. In field rearing experiments, velocities from 8-10 cm/sec increased the risk of predation and slowed tadpole growth. During these investigations we observed 7.9% prevalence of parasitic copepods, Lernaea cyprinacea, along with limb deformities (2.4%), on tadpoles and metamorphs (n=331).