Foothill yellow-legged frog breeding biology in a semiregulated river, Humboldt County, CA

Abstract

River-breeding foothill yellow-legged frogs (Rana boylii) are endemic to California and Oregon. Across this wide geographic range, many populations have declined due habitat loss, non-native competitors and predators (e.g., American bullfrogs [Lithobates catesbeianus], Centrarchid fish), and disrupted water flow due to dams. Even when flow conditions are not extensively regulated, managers still require basic and region-specific information about the breeding biology of this species to prevent further decline. To document spatiotemporal dynamics of reproductive output during drought and high flow years, we surveyed a 13.5 km reach of the lower Mad River, Humboldt County, CA approximately 70 km downstream of Matthews Dam. We found relatively high densities of egg masses (39 to 59 masses / km). Egg masses were generally laid on small cobbles (mean ±SE diameter = 11 ± 0.24 cm) at depths between 0 and 20 cm, and 95% of egg masses were laid within 6 m of the wetted edge. Egg masses were disproportionately found in the tailouts of fast runs and glides, and found less often than expected in side arms, runs, and riffles than would be expected by chance. Breeding timing appeared to be more related to rapid decreases in stream flow variance than air temperature. Taken with previous information about the species, our results suggest that R. boylii rely on multiple cues to initiate breeding. Our results can be used to help inform breeding timing and habitat use by R. boylii breeding under natural flow regimes in Northern California. Our recommendations for future research include further investigating upland habitat use by post-metamorphic life stages factors that influence breeding site selection.

Identifying drivers of population dynamics for a stream breeding amphibian using time series of egg mass counts

“…Time series of egg mass counts were not continuous for some streams, but state‐space models that underlie MPVA (see below) allow for the use of incomplete time series by inferring the abundance of egg masses during years without surveys. Egg mass surveys came from a variety of sources, including long‐term studies of the ecology of R. boylii (e.g., Kupferberg et al, 2012; Wheeler & Welsh, 2008), monitoring programs for populations affected by dams (e.g., Pacific Gas and Electric Company, 2020; Snover & Adams, 2016), and focused studies on the effects of environmental conditions on breeding (e.g., Gonsolin, 2010; Lind et al, 1996; van Hattem et al, 2021; Wheeler et al, 2015). We treated reaches of the same stream as a single population if they were directly connected without any barrier (e.g., a dam, lake, or reservoir) between the reaches and were surveyed using the same methods by the same research group.…”

“…In unregulated streams, adult frogs migrate from overwintering sites in tributary streams, seeps, springs, and under terrestrial cover to breeding reaches after the cessation of high winter and spring flows where females deposit egg masses in shallow, low‐velocity sites with boulder or cobble substrate. In unregulated streams, eggs and tadpoles develop through late spring and summer as the stream flow slowly recedes (Kupferberg, 1996; van Hattem et al, 2021). In regulated rivers where dams alter flow regimes, aseasonal flow fluctuations can create high‐velocity conditions or rapid stage changes that reduce recruitment by scouring or stranding early life stages (Kupferberg et al, 2012; Lind et al, 1996).…”