Population Survey for River Otters in Rocky Mountain National Park
By Jason Herreman and Merav Ben-David
A spatially explicit, individual-based model evaluating the potential of natural recolonization of the Grand Canyon by river otters from upper reaches of the Colorado River (see this issue) assumes a viable source population in Rocky Mountain National Park (RMNP). Judy Berg conducted surveys of river otters in the upper Colorado River between 1992-1997 and determined that the region was at carrying capacity (i.e., had the highest possible number of otters given the amount of food and habitat available) and that successful reproduction was occurring. Because this spatial model assumes continuous reproduction and dispersal from the source population, it is important to establish that the trends in population levels of river otters in RMNP observed by Berg have not changed.
Thus, we initiated population surveys of river otters in the RMNP with the assistance of the University of Wyoming Student Chapter of the Wildlife Society (UW-SCTWS). The goals of the surveys were to identify river otter latrine sites along the upper reaches of the Colorado River, monitor scat deposition as an index of population size, and evaluate seasonal changes in latrine use to determine the preferred sampling period. Surveys were conducted during two different sampling periods, spring (April 28-29) and fall (September 22-23) of 2001. We surveyed 7.41 km of stream in spring and 8.49 km in fall. We identified activity sites of otters by trails entering the water, tracks, and feces, and determined their location using handheld GPS units. We then characterized each site with respect to topography, composition of terrestrial vegetation, composition of river substrate, and presence of otter feces.
During the spring survey we identified 18 different river otter activity sites along 7.41 km of stream (i.e. 2.4 activity sites/km of stream). In contrast, we identified only 12 activity sites during the fall survey along 8.49 km of stream (i.e. 1.4 activity sites/km of stream). Additionally, the average number of feces per activity site was higher during the spring than fall (10.4 and 4.6 respectively). We suspect that the higher level of activity was a result of river otter behavior rather than a change in density. Our spring survey may have occurred during the mating period when both males and females likely advertise their reproductive status. Nonetheless, we do not have a complete knowledge and understanding of the life-history strategies of river otters in the intermountain west and realize that additional studies are necessary if we want to be able to predict the relation between otter behavior and the signs they leave behind. If our observations are indeed consistent among years (something we hope to establish with future surveys), we recommend that river otter surveys should be conducted during late spring rather than fall.
Unfortunately, we are unable to accurately estimate numbers of river otters in RMNP using our study design. At best, such surveys can give us an index of population status. We can say "we have seen a lot more sign this year than last, so there must be more otters this year" but not much more. There are methods to accurately estimate wildlife populations but most of them are not applicable to studying river otters because these critters are hard to catch and are nearly immune to re-captures (which are pre-requisites of sound population estimates). Although we are developing a method that will enable us to estimate otter numbers from DNA fingerprinting of feces (by identifying individuals from the DNA fingerprint of the feces), we haven't applied this method in RMNP yet. So, what can we say about otter numbers in RMNP from our survey? The highest density observed in freshwater systems was one otter per 2.7 km of stream observed by Wayne Melquist in Idaho. By comparing Melquist's report with our observations it appears that we have as much sign of activity as he observed in his study. By applying the Idaho density to the section of the Colorado River we surveyed we estimate that a minimum of three river otters inhabit this stretch of stream. If we extrapolate this density to the entire length of the Colorado River within the boundaries of RMNP, we get an estimate of 18 river otters. These values were used in the spatially explicit individual-based model.
Some interesting results we obtained from our measurements of habitat variables. Using some complicated statistics (logistic-regression models) we found that brush, cobbles, and stream shading were the variables most significant in discriminating between sites used in spring and fall by river otters in RMNP. Our analysis revealed that during fall otters used sites with higher stream shading, more cobbles, and less brush than in the spring. We interpret these results to mean that in fall, river otters chose areas with higher stream shading, which may produce colder water temperatures that would be favored by fish. During spring when water temperatures are colder due to snowmelt this variable may not be as important. The reason otters prefer areas with cobbles in the fall rather than spring may also relate to fish distribution. Trout, the most abundant fish we saw during our surveys, require areas with gravel beds in spring for spawning while in the fall they seek the protection provided by cobbles. Indeed, our data indicate that river otters selected for gravel in spring and cobbles in fall.
Selection of dense brush during spring, however, may be related to protection from predators. During the fall, brush is covered in leaves, which are absent during the spring. Thus, otters may require denser brush in the spring, a season when females raise young, to compensate for lack of leaves and to obtain higher safety from predators. Why are these findings important? They may give us some insights into the requirements of otters in different seasons and may help us assess habitat quality for otters along lower reaches of the Colorado River. This will be essential when we start exploring the availability of dispersal corridors for river otters into the Grand Canyon.
What are we planning to do next? We hope to continue our
surveys in RMNP for several more years, to identify individuals from
DNA fingerprints of feces to obtain more reliable population estimates,
and extend our surveys to other tributaries of the Colorado River. Our
first priority will be the Green River in Wyoming, but we hope that
with the help of other student chapters of The Wildlife Society we'll
be able to cover larger areas and longer stretches of stream.