What river otter poop reveals about life in Chesapeake Bay

River otters have roamed the shores and inlets of Chesapeake Bay for a long time. But there’s still a lot we don’t know about how they interact with their environment. What do they eat? Where do they hang out? And oddly enough – where do they go to the bathroom?

To answer these questions, researchers in Maryland spent nearly a year observing river otters and collecting data on their habits.

The team, based at the Smithsonian Environmental Research Center (SERC), analyzed scat samples and habitat use to better understand the otters’ diet, behavior, and exposure to parasites.

Study senior author Dr. Katrina Lohan is a parasite ecologist and head of the Coastal Disease Ecology Laboratory at SERC.

“River otters in the Chesapeake Bay eat a wide range of animals, including those that live in the water and on land. Parasites, too, are crucial parts of their diet,” said Dr. Lohan. “We also found that river otters use manmade structures for latrines or choose areas with specific characteristics.”

Otter poop on the dock

It all started with an email. A colleague spotted some odd, watery scat on a dock and sent a message to Dr. Lohan. Inside the scat was a bright red worm.

Cameras later confirmed that the dock had become a favorite toilet spot for a local river otter. This was the beginning of a full-scale study.

Graduate researcher Calli Wise took the lead on collecting samples. Over 11 months, she gathered scat from 18 active otter latrines. On average, each latrine held about 28 scat piles.

“Scats usually smelled strongly of fish and were full of scales or crustacean shells,” Wise said. “We wore gloves and used sterile tools and tubes to collect scat samples, to avoid contamination with other DNA.”

Otter toilets are social hotspots

The latrines aren’t just toilets. Otters use them to mark territory with urine and feces, but they’re also social spaces.

Otters gather there to eat, play, and rest. The latrines often show signs of repeated use: flattened grass, worn vegetation, and nearby cover.

Most were located near the shoreline. The researchers found that otters liked areas with woody debris and easy access to the water.

Some latrines stood out. Five were found directly on man-made features like docks, boardwalks, or stairs leading to the water.

What the otter poop reveals

The team used two main methods to analyze the samples: metabarcoding, which identifies DNA fragments, and traditional microscopy. Both approaches helped piece together what the otters had been eating – and what else they might be spreading.

“We collected most scats in the summer, when river otter diet is likely diverse,” Wise said. “We found evidence of invasive species like common carp and white river crayfish in river otters’ diet.”

The otters also ate American blue crabs, other fish, and crustaceans. Occasionally, they even consumed ducks or amphibians.

But diet was just one layer. The scat also showed which parasites the otters were exposed to – either through their prey or directly. Most parasites infected teleost fish, the primary type of fish eaten by otters. A few may have infected the otters themselves.

“It is possible that river otters, like other top predators, wouldn’t be able to find enough food to eat without parasites,” noted Dr. Lohan.

Why parasites might be a good thing

The researchers couldn’t identify every parasite. Gaps in DNA databases made some sequences impossible to match. They also couldn’t link poop to specific otters, so individual diets remain a mystery.

Still, the findings suggest otters might be playing an even bigger role in the ecosystem than we thought.

“Since so many of the parasites are actually infecting otters’ prey, it could mean that river otters are culling sick individuals from the populations they are preying upon,” explained Dr. Lohan.

Removing infected animals could reduce the spread of disease and even affect the gene pool of prey species. Sick animals don’t get to reproduce if they’re eaten. There’s also a potential connection to human health.

“Some of the parasites that infect river otters could potentially also infect humans, who also are mammals,” said Dr. Lohan. “Thus, we could use river otters as ‘disease sentinels,’ and study them to learn about what public health threats occur in certain areas.”

This study opens new questions about how river otters influence, and are influenced by, their environment. Their poop tells stories about what they eat, where they go, what parasites they host, and what role they might play in managing disease in coastal ecosystems.

And in Chesapeake Bay, it turns out those stories often start at a latrine – sometimes even on a dock.

The full study was published in the journal Frontiers in Mammal Science.

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