Why do River Otters Die and What Can We Do to Help?

River otters are a fairly common sight along the coastlines of the Salish Sea. Cute and curious, we have a pretty good idea of what they eat, how they behave, and what role they place in the near shore ecosystem. Unfortunately, we don’t know a whole lot about why they die, which makes it hard to fix any of our actions that might contribute to their demise. 

When studying mortality in marine mammals, scientists often depend on data gathered from stranded animals, but river otter carcasses are difficult to find. To put it plainly, river otters tend to move out of sight when sick. For example, we know sea otters were impacted by the Exxon Valdes oil spill, but locating oiled carcasses proved very difficult. 

Thanks to a recent study led by SeaDoc Society and published in the Journal of Wildlife Diseases, we now have a better picture of their most common mortality threats.

After a decade of studying river otter mortality, the team, led by SeaDoc Science Director and wildlife veterinarian Joe Gaydos, determined that human-caused death was a major cause of river otter mortality.

For the study, Gaydos and his collaborators, Martha Delaney of the University of Illinois Zoological Pathology Program and Stephen Raverty of Animal Health Center in British Columbia, investigated the death of 30 marine-foraging river otters found in the Salish Sea. For nearly half of the animals—47% of them to be exact—the cause of death was trauma, which occurred in every age class examined.

While a few of the trauma cases were due to gunshot, most of them were caused by vehicles. Other causes of river otter death included infectious diseases, metabolic derangements and cancer.

“Unlike true marine mammals, river otters need to drink fresh water,” says Joe Gaydos. “We think a lot of these otters are being hit by cars when they cross roads as they move from the ocean where they feed, to upland areas where they drink from ponds and make dens.”

Now that we know we are inadvertently killing them with cars, the next step is to identify sections of road where they are frequently hit and put in wildlife crossing underpasses or post signs to make drivers more aware that otters are in the area.

River otters are not that long-lived; the oldest female in this study was 12 years old, the oldest male, 13. They do, however, play an important role in the Salish Sea. For example, they transfer marine-derived nitrogen to the terrestrial ecosystem.

We don’t want to be inadvertently killing them with our cars. This study should help us reduce river otter mortality so we can continue to enjoy seeing them as they go about their lives and work to keep our shorelines healthy.