Assessing the Impotance of Cutthroat Trout to River Otters in Yellowstone Lake

Recently-introduced exotic stressors, lake trout (Salvelinus namaycush) and whirling disease (caused by the Myxobolus cerebralis parasite), threaten the native cutthroat trout (Oncorhynchus clarki bouvieri) population in Yellowstone Lake. A reduction in cutthroat-availability may affect fish predators negatively, specially during cutthroat spawning. In this study we examined the importance of spawning cutthroat trout to a fish specialist - the river otter (Lontra canadensis). We measured scent­marking rates at 87 otter latrine sites on Yellowstone Lake and its tributary streams throughout the cutthroat spawning period and identified prey items in scats collected at these sites. Based on fecal deposition rates, otters were more active on spawning streams and less active on Yellowstone Lake during the height of the cutthroat spawning season, with a return to elevated activity on the lake after spawning had ended. Cutthroat trout appeared to be the most common prey item in otter scats throughout the study, based on 515 samples identified to the family­level and 110 samples analyzed to the species-level. Overall, trout occurred in 72% and longnose suckers (Catostomus catostomus) in 43% of otter feces. Suckers were more prevalent than trout in otter scats only on tributary streams, towards the end of the cutthroat spawning run. Introduced lake trout, which inhabit deep water and are largely inaccessible to otters, occurred in less than 5% of otter scats. River otters in the Yellowstone Lake system appear to be highly restricted in their diet and are heavily dependent on cutthroat trout. Our findings suggest that continued declines in the cutthroat trout population could negatively impact otters, potentially disrupting their role in linking the aquatic and terrestrial systems of Yellowstone Lake