Despite having spent the last couple of decades hiking around the Sierra Nevada to conduct my research I'd never seen a river otter nor heard much about their presence in the mountains.That all changed a few years ago when a colleague observed a family of five otters swimming across a lake in northwestern Yosemite National Park. The following winter he observed otter tracks along a stream while doing snow surveys in the Park. And then this past summer I found evidence of otters at a lake in northern Yosemite. My curiosity was definitely piqued and I had to learn more.
Otters are typically described as feeding mainly on fish, and trout are a favorite prey item. So were river otters found historically in the Sierra despite the absence of fish in most water bodies? Or did they expand their range into the Sierra following the introduction of trout into many lakes and streams starting in the mid-1800s? And if they were found in the mountains prior to fish introductions what did they eat?
Several of us are still trying to answer these questions but the otter sign I found this past summer and some additional information sources we've turned up have provided some intriguing details. First, otters have been reported in the High Sierra for many decades, suggesting that their presence predated trout introductions. Second, some notes recorded by a Yosemite wilderness ranger in the 1970s suggested that otters were eating Sierra Nevada yellow-legged frogs (Rana sierrae). He reported seeing otters in a lake in the Vogelsang area and also found numerous pairs of discarded frog legs around the lake shore. Given how abundant R. sierrae were historically it certainly seems possible that otters could have subsisted on them. And finally, the otter sign that I found this summer was several otter scats atop a lakeshore boulder. I collected the scats and dissected them back in my lab. What I found amazed me. The scats contained dozens of bones of a size and structure that likely makes them those of Rana sierrae adults and juveniles. But the dominant remains in the scats were from aquatic invertebrates, including giant water bugs (Lethocerus americanus), backswimmers (Notonecta sp.), and dragonfly and damselfly larvae (Aeshna and Enallagma, respectively)! Given the relatively large size of frogs their inclusion in otter diets isn't particularly surprising. But invertebrates?? Notonecta are barely an inch long and yet otters were eating them by the dozens.
To return to the questions I posed at the beginning of this story, I'm guessing that otters utilized Sierran lakes prior to the introduction of trout and subsisted on a diet of amphibians, aquatic reptiles, and invertebrates. With the introduction of trout, otters had another food item on their menu. But did this new food item allow otters to increase in numbers? If so, could their elevated numbers be negatively impacting the few frog populations remaining today? I wish I knew. I'd sure love to be able to radio track some of these guys around Yosemite and figure out what habitats they use and get a more complete picture of their diets.
If any of you readers have observations of river otters in the Sierra Nevada, I'd love to hear about them.
Back to The Mountain Yellow-legged Frog Site.