Guest blog post by Dr. Oldemar de Oliveira Carvalho Junior
For four decades, we’ve been quietly watching otters along the southern coast of Brazil. Not just because they’re charismatic and playful, but because they are among nature’s most honest diagnosticians. Just like a doctor checks your pulse, blood, and behavior to assess your health, scientists can use otters to monitor the condition of estuaries, those vital transition zones where rivers meet the sea.
Published in Estuarine Management and Technologies, our new study, ‘Otters as Bioindicators of Estuarine Health,’ synthesizes 40 years of field data from from Projeto Lontra in Brazil’s Peri Lagoon, along with global ecological research. The findings reinforce the role of semi-aquatic mammals as “living sensors” of ecosystem integrity.
This study is part of the topical collection: Otters as Bioindicators of Estuarine Health: Innovations in Monitoring and Management from Tropical Coastal Brazil
Why otters and not another animal?
Otters sit at the top of the food chain in estuaries. They need clean water, healthy fish populations, connected habitats, and safe denning sites. If any of these elements falter, otters are among the first to respond, with fewer sightings, changes in their diet, or markers of disease.
While a water sample offers only a fleeting snapshot, otters aggregate environmental signals across time and space.
In fact, their very presence, or absence, tells a story. In regions where Atlantic Forest fragmentation has severed the connection between land and water, otter sightings have plummeted by 30% in just a decade.
This isn’t just a loss of biodiversity; it’s a warning for everyone downstream. Fewer otters mean degraded waterways, disrupted food webs, and weakened natural buffers against floods and erosion.
A “living record” of pollution
The diet of otters consists of 70-80% fish, as well as crabs and other aquatic animals. As they consume prey, they also accumulate pollutants, like heavy metals, microplastics, and industrial chemicals, that have built up in those animals. Their bodies effectively become archives of contamination.
Think of an otter as a biological hard drive: by analyzing its scat (feces), we can reconstruct what pollutants are moving through the estuary, even if they’re invisible to the naked eye.
For instance, in Brazil, we’ve found microplastic fibers in otter droppings from protected lagoons, proof that no coastal system is truly isolated from human impact.
The tropical knowledge gap
Globally, over 70% of otter research focuses on temperate species like the Eurasian or sea otter. Meanwhile, the Neotropical otter (Lontra longicaudis) receives less than 4% of scientific attention, despite inhabiting the world’s most biodiverse and threatened estuaries.
This imbalance in the research poses a great conservation risk. Tropical estuaries face unique pressures: rapid urbanization, mangrove deforestation, and climate-driven sea-level rise. Without focused research, we risk managing these ecosystems with tools designed for entirely different environments.
Our work aims to close this divide, establishing the Neotropical otter as a key indicator to drive evidence-based conservation across the Global South.
Land-based diseases in aquatic sentinels
One of our most striking findings showed that 66% of otters tested positive for Toxoplasma gondii – a parasite shed only by domestic cats. How does a land-based pathogen end up in an aquatic mammal? The answer is through runoff.
The rain washes cat feces from the streets, gardens, and informal settlements into rivers and lagoons. This reveals a sobering truth: even within protected areas, otters are exposed to threats originating far inland. To protect these sentinels of ecosystem health, we must implement integrated management strategies that address sewage overflow, urban runoff, and domestic animal impact.
What comes next?
We’re finalizing the Otter Health Index (OHI), a low-cost toolkit for municipalities, NGOs, and community groups to monitor estuaries using simple metrics: spraint frequency, habitat connectivity, contaminant screening, and local knowledge. It’s designed for places with limited labs but rich ecological insight.
After 40 years of observing otters, we’ve learned this: protecting them 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.
Otters aren’t just inhabitants of our estuaries; they are their guardians. If we listen to what they’re telling us, we’ll know exactly what these ecosystems need to survive and thrive.
Original sources:
Carvalho Junior, O., Bez Birolo, A., Tosatti, M., Haddout, S., Ljubenkov, I. and Jayachandran, P.R. eds., (n.d.). Otters as Bioindicators of Estuarine Health: Innovations in Monitoring and Management from Tropical Coastal Brazil. PEstuarine Management and Technologies. doi:https://doi.org/10.3897/emt.coll.317
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.
