River otters shape Chesapeake Bay food webs by culling sick fish

A pile of otter droppings at a Maryland research dock turned into an unexpected window into how these predators keep an entire ecosystem in balance.

Scientists at the Smithsonian Environmental Research Center spent 11 months collecting samples from 18 active otter latrines around Chesapeake Bay. The work sounds unglamorous — and it is — but what they found in the scat tells a story about predators, parasites, and the hidden mechanics of a healthy bay.

What otters actually eat

River otters in Chesapeake Bay aren't picky. They consume fish, crustaceans, invasive species like common carp, and — surprisingly — parasites. Lots of them. When researcher Calli Wise examined the samples, most scats reeked of fish and were packed with scales and shell fragments. But the real discovery came when the team used genetic analysis to identify which parasites were present in the otters' prey.

Many of those parasites infect the fish that make up the otters' primary diet. This matters because it suggests otters aren't just eating randomly — they're likely consuming disproportionately more sick or infected fish than healthy ones. "River otters, like other top predators, wouldn't be able to find enough food to eat without parasites," notes Dr. Katrina Lohan, the study's senior author and a parasite ecologist at the research center.

When otters eat infected fish, they remove those sick individuals from the population before they can breed. Over time, this culling effect shapes the genetics of fish populations in the bay, potentially making them more resistant to disease. It's a form of natural selection happening in real time, driven by a predator most people never see.

Why this matters beyond the bay

The research also hints at something useful for human health. Some of the parasites the team identified could potentially infect people. River otters, in effect, act as early-warning systems — disease sentinels that signal when pathogens are present in the ecosystem. By monitoring what parasites otters carry, researchers can track emerging public health threats in the region before they spread to human populations.

This kind of work is still relatively rare. We know surprisingly little about how river otters actually live in places like Chesapeake Bay, despite their long presence there. But by starting with something as humble as otter poop, scientists are building a clearer picture of how these predators hold together the invisible architecture of a healthy estuary.