How Many River Otters Inhabit Yellowstone Lake? An Assessment with Fecal and Hair DNA Analyses

River otters (Lontra canadensis) in Yellowstone Lake are different from most other otter populations in that they are heavily dependent on one prey species - Yellowstone cutthroat trout (Oncorhynchus clarki bouvieri; Crait and Ben-David 2006, Crait et al. 2006). Our 2002-2003 studies in Yellowstone Lake showed that cutthroat trout were the most common prey item in otter scats throughout the summer. Overall, trout occurred in 72% and longnose suckers (Catostomus catostomus) in 43% of otter feces, based on 515 samples identified to the family-level and 110 samples analyzed to the species-level. Suckers were more prevalent than trout in otter scats only on tributary streams, towards the end of the cutthroat trout spawning season. Introduced lake trout (Salvelinus namaycush), which inhabit deep water and are largely inaccessible to otters, occurred in less than 5% of otter scats. In addition, based on fecal deposition rates, we established that otters were more active on spawning streams and less active along the Yellowstone Lake shoreline during the height of the cutthroat trout spawning season, with a return to elevated activity on the lake after spawning had ended (Crait and Ben-David 2006). Therefore, because of the lack of a suitable alternative prey, it is likely that a decline in cutthroat trout (Koel et al. 2005) will translate into a decline in the abundance of otters in this system. Indeed, our 2002-2003 survey indicates the number of river otter latrine sites in the Yellowstone Lake ecosystem is lower than expected based on similar surveys in the Rocky Mountain region (Crait and Ben-David unpublished data).