Population Survey for River Otters in the Rocky Mountain National Park

Ben-David, Merav; Herreman, Jason; Boyd, Jessica

Recently, reintroduction of river otters (Lontra canadensis) to the Grand Canyon was suggested. This, however, could result in the genetic swamping of endangered L. c. sonora in adjacent watersheds. A quantitative spatial model evaluating the potential of natural recolonization of the Grand Canyon by otters assumes dispersal of animals from a viable source population in the upper reaches of the Colorado River. We initiated population surveys of river otters in the Rocky Mountain National Park (RMNP) to identify latrine sites, monitor scat deposition as an index of population size, and evaluate seasonal changes in latrine use to determine preferred sampling periods. Our data indicated that use of latrines and latrine site selection by river otters in RMNP changed both seasonally and annually. A sharp decline in number of latrines per km of stream between spring and fall surveys occurred in all years, suggesting that otter behavior changes seasonally (2001: spring - 2.43 sites/km of river, fall - 1.41 sites/km; 2002: spring - 2.3 sites/km of river, fall - 0.6 sites/km; 2003: spring - .86 sites/km of river, fall - 0.22 sites/km; 2004: spring – .65 sites/km of river, fall – 0.12 sites/km). In 2004, latrine densities were significantly lower than in previous years in both spring and fall. The number of feces per site, a measure of otter activity, declined through time and between spring and fall in 2001 to 2003 (2001: spring - 10.4 feces per site, fall - 4.75; 2002: spring - 6.1 feces per site, fall - 1.9; 2003: spring - 4.4 feces per site, fall - 0.8), but was not different in spring and fall 2004 (spring - 4.08, fall - 4.33). Latrine density was unrelated to stream flow in the preceding month but exponentially declined with the cumulative stream flow in the preceding 3 months. It is possible that stream flow in the preceding 3 months influences the abundance, distribution, and availability of fish in the stream, thereby affecting otter use of the river. Future surveys should also include an evaluation of fish abundance and distribution in the Colorado River.