Winterkill in Lotic Systems May Be an Important Driver of Amphibian Population Declines

Studies of frogs overwintering in ice-covered ponds and lakes have documented large winterkills, suggesting that winter can be a time of great stress and may be a serious threat to rare and endangered frog species that overwinter in these systems. Lotic systems are assumed to be better oxygenated and therefore less subject to overwinter mortality than lentic systems, but few studies of the winter ecology of ranids in stream ecosystems exist. We investigated the habitat use and survival of Columbia Spotted Frogs (Rana luteiventris) overwintering in a stream system in the Toiyabe Mountains of Nevada. We radio-tracked 13 Spotted Frogs from October 2000 to March 2001 to locate hibernacula and document winter habitat use. During the winter, frogs moved up to 191 m under the ice, often upstream against the current. We marked 1,763 frogs from 2000 to 2001 and found evidence of winterkill (population decreases of 66% to 86.5%) at four out of five sites while the fifth site showed no evidence of winterkill. This corresponded to an actual observation of winterkill of 88% of the frogs at one of the sites, making this one of a few studies directly tying observed wintertime mortality to mark–recapture estimates of decreases in ranid populations and the first to do so for amphibians overwintering in a lotic system. Amphibian winter ecology in lotic systems, the severity and frequency of winterkill events in these systems, and their effect on population size and structure is in need of further study over a wide range of species and should be considered in conservation plans for ranids in lotic systems.