WNC Professor of Biology Says Incidents Serve as Reminder That We Need to Learn More About Wildlife in the Region

River otters almost never attack people.

A scientific review of reported otter attacks worldwide from 2011 through 2021 found just 20 incidents over an entire decade — only 17 involving wild otters. Most occurred in North America, and more than half involved people participating in water-related activities.

That makes what happened at Lake Tahoe this summer all the more unusual.

Within three weeks, two swimmers on opposite sides of the lake reported remarkably aggressive encounters with North American river otters — Lontra canadensis — leaving both injured and wildlife experts considering a difficult question:

Why?

On Aug. 9, Stateline resident Bob Daly was swimming near Pope Beach on Tahoe's California side when an otter reportedly began repeatedly charging and striking him. Daly suffered multiple bite and scratch wounds and received precautionary rabies treatment.

Then, on Aug. 30, longtime open-water swimmer and Reno swim instructor Lynnette Bellin was swimming near Hidden Beach on Tahoe's Nevada shoreline when she said an otter grabbed her leg and pulled her underwater.

Bellin reported that the animal continued pursuing, biting and scratching her as she tried to escape. She eventually reached rocks and pulled herself from the water. Her injuries required 38 stitches and precautionary rabies treatment.

The incidents are alarming precisely because they are so uncommon.

 

Lynnette Bellin

Wildlife officials emphasize that river otters generally avoid people, and an aggressive encounter should not be interpreted as evidence that Tahoe's otters suddenly pose a widespread threat to swimmers.

But the attacks do offer a rare glimpse at an animal that is more complicated — and more formidable — than its popular image suggests.

For many people, "otter" conjures an irresistible picture: an animal floating belly-up, perhaps cracking a shell on its stomach.

That's actually an image more closely associated with the sea otter.

Tahoe's otters are North American river otters — native, semi-aquatic carnivores and highly efficient predators. Fish comprise a major part of their diet, supplemented by crayfish, frogs, insects and other aquatic prey.

Adults can measure several feet long and weigh roughly 10 to 30 pounds. Their webbed feet, streamlined bodies and muscular tails allow them to move through water with speed and agility. Their jaws and sharp teeth are designed for capturing prey.

Lake Tahoe and its surrounding watershed provide plenty of suitable habitat. Otters use shorelines, wetlands, tributaries and connected waterways, including the Truckee and Upper Truckee rivers. Their territories can be surprisingly large — commonly several square miles — meaning an animal seen in one location may use a much broader portion of the watershed.

Wildlife officials say the regional otter population appears to have increased over approximately the past 15 years, although there is no reliable published census establishing exactly how many otters currently inhabit the Tahoe Basin.

So, what could explain two attacks in one summer?

There isn't yet a definitive answer.

Female river otters can aggressively defend their young, and a swimmer may unknowingly come too close to an adult or juveniles. Otters may also react defensively if they feel threatened or cornered, particularly in areas important for feeding or resting.

There is another factor researchers consider in human-wildlife conflicts: overlap.

More people swimming, paddling and recreating in wildlife habitat creates more opportunities for encounters. In the worldwide review of reported attacks between 2011 and 2021, 53 percent of wild otter attacks occurred during water-related human activities. Attacks also peaked during summer months, when human recreation and otter activity around young can overlap.

Rabies is considered whenever a wild mammal behaves unusually, which explains the precautionary treatment given to the Tahoe victims. But aggression alone doesn't mean an animal is rabid. In that decade-long review, only two cases involved suspected rabid otters, and rabies has not been established as the cause of either Tahoe attack.

“Two incidents in such a short period of time are certainly unusual and worth paying attention to, but scientifically, we have to be careful about drawing conclusions from such a small number of encounters,” said Western Nevada College Professor of Biology and Chemistry Dr. Elizabeth Tattersall. “Lake Tahoe may look peaceful and familiar to us, but it is a complex, dynamic ecosystem with a variety of organisms living there. We tend to think of Tahoe as a place we visit and recreate, but ecologically, we are the visitors. Incidents like these remind us how much there still is to learn about the wildlife residing in this remarkable watershed.” 

There is recent precedent nearby.

In summer 2023, swimmers were attacked in separate incidents at Serene Lakes in Placer County, northwest of Lake Tahoe. One victim, experienced open-water swimmer Matt Leffers, suffered approximately 40 puncture wounds after being repeatedly bitten. Those incidents raised similar questions about territorial behavior and increasing overlap between people and otters.

Still, the Serene Lakes incidents were not in Lake Tahoe itself, making two serious attacks in Tahoe within weeks particularly noteworthy.

Avoiding an Otter Encounter

The best response begins before an encounter.

Never approach, follow, feed or attempt to interact with a wild otter, even one that appears curious or playful. Give otters considerable space, particularly if more than one animal is present or young may be nearby.

Swimmers should pay attention to an otter that appears unusually interested in people — repeatedly surfacing nearby, following a swimmer or closing the distance rather than moving away. In those circumstances, calmly leave the water and give the animal room.

If an otter does attack, wildlife officials recommend fighting back and getting out of the water as quickly as possible. Once safely on shore, anyone bitten or scratched by a wild mammal should promptly seek medical attention because of the risk of infection and the need to evaluate possible rabies exposure. The encounter should also be reported to the appropriate wildlife agency.

The recent incidents shouldn't fundamentally change how people view swimming or recreating at Lake Tahoe. Millions of people visit the Tahoe Basin while serious encounters with river otters remain extraordinarily uncommon.

They do, however, offer perspective about what it means to recreate in one of the West's great natural ecosystems.

Lake Tahoe isn't simply a spectacular outdoor playground. It is habitat — a vast interconnected ecosystem occupied by fish, birds, bears, coyotes and a native aquatic carnivore that can move almost unnoticed through its waters.

Scientists may eventually identify a common thread connecting this summer's two attacks. They may prove entirely unrelated. For now, the explanation remains uncertain.

That uncertainty may be one of the most interesting aspects of the story.

Even at Lake Tahoe, visited by millions of people and studied by scientists for generations, wildlife continues to remind us that observing a natural system is not the same as completely understanding it.