For 40 years, scientists have been monitoring the Neotropical otter (Lontra longicaudis) along the southern coast of Brazil. Recent study published in Estuarine Management and Technologies reveals that these charismatic mammals are far more than just inhabitants of the coast, they are "living sensors" providing information of ecosystem decay.

The study synthesizes 40 years of field data from Projeto Lontra in Peri Lagoon along with global ecological research. As otters sit at the top of the food chain, with a diet consisting primarily of 70-80% fish, as well as crabs and other aquatic animals, they aggregate environmental signals across time and space. When the water is polluted or the food web is broken, the otters are the first to show the symptoms.

“In fact, their very presence, or absence, tells a story.” says Dr. Oldemar de Oliveira Carvalho Junior, one of the authors of the study. In regions where Atlantic Forest fragmentation has severed the connection between land and water, otter sightings have plummeted by 30% in just a decade.

One of the most striking findings of this study showed that 66% of otters tested positive for Toxoplasma gondii - a parasite shed only by domestic cats. So, how does a land-based pathogen end up in an aquatic mammal? The answer is through runoff, as rain washes cat feces from streets, gardens, and informal settlements into rivers and lagoons.

This finding reveals a sobering truth: even within protected areas, otters are exposed to threats originating far inland. It underscores that effective estuary management must be watershed-wide, and include responsible pet ownership, sewage control, and green infrastructure.

"Think of an otter as a biological hard drive," says Carvalho Junior, "By analyzing their scat, we can reconstruct exactly what pollutants- from heavy metals to microplastics - are moving through the estuary, even when those pollutants are invisible to the naked eye." For instance, the researchers have found microplastic fibers in otter droppings from protected lagoons, further proof that no coastal system is truly isolated from human impact.

The research highlights a critical Tropical Knowledge Gap. While 70% of global otter research focuses on temperate species like the Eurasian or sea otter. Meanwhile, the Neotropical otter receives less than 4% of scientific attention, despite inhabiting the world’s most biodiverse and threatened estuaries. "Without focused research, we risk managing these ecosystems with tools designed for entirely different environments.", explains Carvalho Junior.

In response to these findings, the team is finalizing the Otter Health Index (OHI). This low-cost toolkit is designed for municipalities and NGOs to monitor estuarine health using simple metrics like habitat connectivity and contaminant screening allowing for better monitoring in areas with limited labs but rich in ecological insight.

"Protecting otters isn’t just about saving a species. It’s about safeguarding the intricate web of processes that sustain clean water, fisheries, flood resilience, and human well-being. If we listen to what the otters are telling us, we’ll know exactly what these ecosystems need to survive."

concludes Carvalho Junior

Original source:

Birolo, A.B., Tosatti, M., Junior and Assiya Haddout (2026). Otters as bioindicators of estuarine health: Scientific gaps, field-based insights, and a framework for future research. Estuarine Management and Technologies., 3, pp.65–78. doi:https://doi.org/10.3897/emt.3.185117.

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