How do South Dakota researchers count river otters when they can't even find them?

Sioux Falls Argus Leader

The early morning sun warmed the crisp autumn air, and upland game biologist Alex Solem and intern Jessica Kading kayaked down the Big Sioux River as they already had so many times this September. The slow current floated the small research team several kilometers south from Dell Rapids, the next transect of the river on their list to search.

Their objective? To find evidence of river otters.

Solem and his team of researchers have started the first survey in a five-year process to collect more detailed data about the number of river otters in eastern South Dakota as well as where they are dispersed along various bodies of water. This year's survey consists of 23 sections of the Big Sioux River, each between two and six kilometers long. From September 1 until the end of October, researchers travel both by kayak and on foot searching for indications that otters live nearby.

"River otters are secretive animals that live in hard-to-reach places," said Solem. "Compared to other wildlife in South Dakota, such as white-tailed deer, they are hard to observe or count. This makes it harder to get a reliable estimate on how many are out there."

River otter tracks are pressed into the mud on Tuesday, September 28, 2021, on the Big Sioux River just south of Dell Rapids. Otter tracks often come in groups of two with connected toe and foot pads.

The chance of seeing any live otters while out on a survey is slim; Solem said he only saw one the entire month of September. Instead, the research is focused on collecting DNA samples from latrines, a term for communal defecation sites. The team is fairly confident they've found an active river otter latrine when they can match it with nearby tracks in the mud and flattened vegetation from otters sliding from dry land into the water. Since raccoons and beavers also inhabit similar areas, it can be difficult to tell them apart from their droppings alone.

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After collecting fecal samples via both swab and forceps, those samples are sent to a laboratory partner at the University of Idaho for analysis. Because this first year of the survey is a so-called "pilot year," Solem said he is still determining what data they can realistically expect from using non-invasive genetic sampling, or NGS. One form of analysis can confirm the samples are from river otters and not another animal, while a more in-depth amplification of the DNA could potentially indicate how many individual otters used a particular latrine.

"Using NGS, we hope to determine the number of individuals detected each year within these stretches, which can help us develop trend data to monitor river otters," said Solem in an email explaining the program. "After several years, we will determine if it’s feasible to expand this type of monitoring effort to other rivers and streams in eastern South Dakota."

Otter population has increased since the 1990s

Alex Solem, upland game biologist with South Dakota Game, Fish and Parks, and intern Jessica Kading prepare to take fecal samples from a potential river otter latrine on Tuesday, September 28, 2021, on the Big Sioux River just south of Dell Rapids.

River otters were listed as a threatened species in South Dakota by 1978. Solem said unregulated harvest, loss of habitat and pollution caused a steep decline in the wild population. The Flandreau Santee Sioux Tribe reintroduced 35 otters into the Big Sioux River in the late 1990s, and their numbers have been steadily increasing statewide for over 20 years.

In 2020, the Game, Fish and Parks Department voted to delist river otters as threatened species and the first-ever otter trapping season took place, causing some contention.

Before this new method of research, the main way to track otter population growth was to spot one in the wild and report it to the state Game, Fish and Parks Department.

Solem said these reports could be documented sightings as well as vehicle kills and accidental catches while trapping other species. Necropsies performed on incidentally killed otters provided a glimpse into the sex and age demographics of the population.

"The opportunistic data on distribution, expansion, trends in abundance, and evidence of reproduction met the criteria to remove otters from the state threatened species list," said Solem.

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Solem said the department collected data showing evidence of otter reproduction throughout the eastern part of the state, enough to indicate the population was stable.

"However, there is a desire to develop a more robust monitoring plan," said Solem.

South Dakota Game, Fish and Parks intern Jessica Kading kayaks toward a river otter slide on the river bank on Tuesday, September 28, 2021, on the Big Sioux River just south of Dell Rapids.

The samples from surveys this year may provide some useful data, but more importantly, it's a year to work out the kinks and find the most effective research methods.

"Obviously there's some headaches and trials and tribulations that come up, but everybody that we're working with is great at working and communicating to find a way to properly monitor otters," he said.

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Solem said the Game, Fish and Parks Department will continue to collect data from reported otter sightings along with the new survey method.