Ever felt short of breath after a workout? That winded feeling – like you’re not getting enough oxygen – is called hypoxia.
The same thing can happen underwater. Fish, shellfish, and other marine animals depend on oxygen in the water to survive. When oxygen levels drop, it can mean life or death.
Every summer, parts of Long Island Sound become hypoxic. This condition can drive fish away, harm or kill stationary bottom-dwellers, and disrupt the ecosystem. It’s often referred to as an invisible sign of water pollution because you can’t see it. During extreme hypoxic or anoxic events, there may be large fish kills, which are obvious. But more often, people won’t see the impacts of hypoxia in our waters.
That doesn’t mean we won’t feel the effects, however. Hypoxic events can affect where fish go, how they grow, and their ability to reproduce. These changes can impact fishing success and, ultimately, what ends up on local tables.
Additionally, prolonged exposure to hypoxia can lead to mass mortality in shellfish populations. Shellfish like mussels, oysters, and clams are tolerant of brief periods of low oxygen, but are especially vulnerable to prolonged periods because they can’t escape low-oxygen areas. In 2021, commercial shellfishing in Connecticut brought in over $20 million. When shellfish populations decline, it hurts local harvesters and can drive up prices for consumers.
But if we can’t see it, how do we know it’s there, and more importantly, where it’s going to be? That’s where our new hypoxia StoryMap and public education toolkit come in. These communication tools can help people across the region better understand what’s happening under the surface. Developed with scientists at EPA and partners across the Long Island Sound Partnership, the toolkit brings together the latest data, visuals, and explainer content – including a hypoxia forecasting tool, an animated video, and more.
These resources make it easier than ever to understand what causes hypoxia, why it matters, and how science is helping us respond.
Still, knowing when and where hypoxia will occur has remained a challenge. That’s why scientists scoured decades of monitoring data to develop the forecasting model to predict the extent and severity of hypoxia in the Sound. For 2025, it forecasts that up to 31 square miles of bottom waters could be affected, peaking around August 15.
Even if the forecast isn’t exact, it’s a major leap forward. “This is the first year we’re using the model to forecast hypoxia in Long Island Sound, so it’s a big step forward – even if the forecast isn’t perfect,” said Cayla Sullivan, EPA Habitat and Reporting Coordinator for the Long Island Sound Partnership, and the coordinator in developing the forecast. “The data we gather this season will help us refine the model and improve its accuracy in the future. Each year, we’re getting closer to understanding, and predicting, how hypoxia develops in the Sound.”
And why does that matter? Because better forecasting enables better decisions for resource managers, fishermen, shellfish growers, and anyone who relies on a healthy, productive Sound. It means knowing when and where marine life might be at risk. Making forecasts also challenges scientists to improve their models.
Together, the resources in the toolkit give us new tools to protect life in the Sound and adapt to a changing environment.
We may not be able to see hypoxia, but with tools like these, we can see it coming, and do something about it.
The toolkit is available in the Reduce Nutrients section of the LIS Partnership website.
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