In the late 1990s, the Flandreau Santee Sioux Tribe reintroduced approximately 35 river otters into the Big Sioux River. Otters, which at one time could be found throughout the Upper Midwest, had become nearly extinct in South Dakota due to habitat loss, pollution and unregulated harvest.

Following a successful reintroduction, the otters began to repopulate the rivers of eastern South Dakota. First placed on the state’s threatened species list in 1978, river otters were taken off in 2020 when the South Dakota Game Fish and Parks determined the population had stabilized.

“Understanding [the river otter] population after being delisted from the endangered species list is crucial for maintaining sustainable populations,” said Amanda Cheeseman, assistant professor in South Dakota State University’s Department of Natural Resource Management.

Currently, little is known about South Dakota’s river otter population or its habitat needs. Cheeseman and SDSU graduate student Jessica Speiser have an ongoing project that seeks to learn more about this rebounding population.

Researchers do have a solid understanding of the otter’s prey base — a crucial component to sustainable populations. Their diet tends to consist primarily of fish, such as minnows, bullheads, perch, sunfish and pike, along with other aquatic species, like crayfish, mussels and frogs.

“This project will fill in the gaps of presently missing data on otter distribution, habitat needs in in grassland systems, as well as identify potential barriers to habitat connectivity,” Cheeseman said.

Last summer, the research team placed trail cameras in riverbeds and wetlands throughout eastern South Dakota to get a handle on the distribution of river otters.

Trail cameras are an effective tool when trying to gain a greater understanding of populations in the wild. Most rely on infrared sensors, which take pictures when movement occurs in front of the camera. Because river otters are semi-aquatic and secretive, trail cameras using the sensor method aren’t always reliable for capturing pictures of the elusive otter. Cheeseman and Speiser used a different method for the trail cameras. Rather than take a picture once movement is sensed, they set up the trail cameras to automatically take a picture every minute.

The research team also used scat — courtesy of the Great Plains Zoo in Sioux Falls — to attract the playful mammals out of the water.

Following fieldwork last summer, Speiser had acquired roughly 3 million photos — far too many for any two researchers to sort through. Instead of manually searching for the pictures, they utilized artificial intelligence to help separate the images into two groups: ones with animals present and ones without.

“This was pioneering work using AI with trail cameras,” Cheeseman said. “This research will begin to give us some idea where the river otters are and how many there might be.”

With a more manageable number of photos to sort through, Cheeseman and Speiser were able to identify where river otters can be found in South Dakota and, subsequently, what their habitat preferences are.

The most common area where river otters are found is along the banks of the Big Sioux River. Otters were also spotted in the lower James Valley River, the Little Minnesota River, the Vermillion River and along the Missouri River near Pierre. Some were even spotted in western South Dakota.

Preliminary data shows that river otters prefer riparian and/or woody habitats surrounding rivers. They tend to also utilize beaver structures, like dams or dens, for their habitat. In South Dakota, riparian and woody habitats have been degraded and fragmented by human development, making this research even more crucial to their long-term sustainability.

“River otters’ use of riparian areas can also provide information on the overall health of waters and riparian habitats,” Cheeseman said. “This project will better inform river otter conservation and management as well as provide information on riparian habitat health.”

Using the available data, Cheeseman is assessing the impacts of streambank land use, land management practices, habitat structure and water quality on river otter habitat quality.

Fieldwork will continue this summer, with more trail cameras being set up throughout the state.