Water Velocity Tolerance in Tadpoles of the Foothill Yellow-legged Frog (Rana boylii): Swimming Performance, Growth, and Survival
We explored the effects of large magnitude flow fluctuations in rivers with dams, commonly referred to as pulsed flows, on tadpoles of the lotic-breeding Foothill Yellow-legged Frog, Rana boylii. We quantified the velocity conditions in habitats occupied by tadpoles and then conducted experiments to assess the tolerance to values at the upper limit of, and outside, the natural range. In laboratory flumes and field enclosures we mimicked the velocities observed during pulsed flows. In all experimental venues, the behavioral response of tadpoles was to seek refuge in the channel substrate when velocity increased. In a large laboratory flume, tadpoles moved freely at low water velocities (0–2 cm•s−1) and then sheltered among rocks when velocity increased. In a smaller scale laboratory flume, the median critical velocity was 20.1 cm•s−1. Critical velocity varied inversely with tadpole size, developmental stage, and proportion of time spent swimming. Velocities as low as 10 cm•s−1 caused tadpoles approaching metamorphosis to be displaced. In field mesocosm experiments, tadpoles exposed to repeated sub-critical velocity stress (5–10 cm•s−1) grew significantly less and experienced greater predation than tadpoles reared at ambient velocities. Responses to velocity manipulations were consistent among tadpoles from geographically distinct populations representing the three identified clades within R. boylii. The velocities associated with negative effects in these trials are less than typical velocity increases in near shore habitats when recreational flows for white water boating or peaking releases for hydroelectric power generation occur.
Discharge during the summer of 2005 depicts the range of flow conditions experienced by eggs and tadpoles of Rana boylii, the Foothill Yellow-legged Frog in six California locations. For the Middle Fork American River, the data gap indicates equipment failure.
Schematic diagram (A) of a rectangular re-circulating flume where the ability to withstand elevated flow velocities by tadpoles of Rana boylii was tested (Expt. II). Rocks, not drawn to scale, were embedded (black ellipses) or un-embedded (white ellipses) in a foam bed. Arrows indicate direction of current. Behavioral responses are activity (B) and emigration (C) from the simulated cobble bars to the downstream pools.
Flow conditions measured at Foothill Yellow-legged Frog, Rana boylii, egg masses and tadpoles in three California rivers. Exponential declines in velocity curves r2 values: the unregulated South Fork Eel River = 0.64, a free-flowing reach of Alameda Creek = 0.96, North Fork Feather River downstream of Poe Dam = 0.95. Horizontal reference lines indicate the range of mean velocities measured at breeding sites in the Feather River downstream of Cresta Dam during pulsed flows (17–35.4 m3·sec−1) above base discharge = 7 m3·sec−1.
The critical velocities (Ucrit) at which tadpoles of Rana boylii could no longer maintain position in a small flume (Brett Chamber Expt. III) as a function of size (A), developmental stage (B), and proportion of time spent swimming (C). Tadpoles with large tail area (stages 34–36, Expt. IV) relative to body mass withstood higher velocities (D).
Survival of Rana boylii tadpoles in stream mesocosm enclosures exposed to elevated flow velocity levels as hatchlings (A, Expt. VII), and over seven weeks in a factorial manipulation of velocity and predation (B, Expt. VIII). Presence of an insect predator is indicated by black and white hourglass filled symbols. Velocity conditions inside enclosures are indicated by bars, units on right axis. Tadpole snout–urostyle length (C) as a function of velocity (filled symbols = elevated conditions) and density in Expt. VIII (Alameda Creek, circles) and in Expt. VI (South Fork Eel River, triangles).
Contributor Notes
Associate Editor: M. J. Lannoo.