Imagine being told that the neighborhood where you were born, grew up, and raised your own children is slowly being poisoned. Not all at once. Gradually, invisibly, until one day there simply isn't enough air to breathe. That's what's happening to fish in the Baltic Sea right now, and it's playing out across their entire lives from the moment they're laid as eggs to the day they die as adults.

Stretches of ocean starved of oxygen, known as dead zones, are spreading. They're growing larger, lasting longer, and creeping into waters they never reached a generation ago. Where they spread, fish struggle to survive, reproduce, and thrive. Scientists have spent years trying to understand how serious the damage really is, not just for individual fish, but for whole populations, and for the fisheries that millions of people depend on.

A new study has come closer than anything before it to answering that question, and what it found should matter to anyone who eats fish, cares about the sea, or thinks about where our food comes from.

What Is a Dead Zone, and Why Does It Matter?

A dead zone is a stretch of ocean where oxygen levels fall so low that most marine life can't survive. The technical term is hypoxia, from the Greek for "under" and "oxygen." When water turns hypoxic, fish face a stark choice: flee or stay and endure.

Some flee, abandoning the habitats they depend on. Others stay, burning extra energy to cope, growing more slowly, and dying younger. Either way, the consequences spread outward, affecting not just individual fish but entire populations, and eventually the fishing industries and coastal communities built around them.

Dead zones aren't new, but human activity is making them dramatically worse. Excess nutrients from farms and sewage systems pour into coastal waters, fueling enormous algae blooms. When that algae dies and rots, it sucks oxygen out of the water. The Baltic Sea has been on the receiving end of this process for decades, loaded with nitrogen and phosphorus from more than a dozen countries around its shores. It's now one of the most heavily polluted bodies of water in the world.

The Baltic Sea: A Sea Under Siege

The Baltic Sea sits between Scandinavia and the European mainland, and it's in serious trouble. For decades, runoff from farms, cities, and factories has pushed the Baltic toward a chronic oxygen crisis. Large portions of its deeper basin waters now regularly run hypoxic, and the problem is getting worse, not better.

What makes this especially alarming is that those deep basins are exactly where some of the region's most valuable fish go to lay their eggs. Atlantic cod spawn there. So does European plaice. In shallower coastal waters, Atlantic herring glue their eggs to algae and gravel beds — habitats that are increasingly hit by short, sharp pulses of oxygen depletion.

These aren't obscure species. Cod, plaice, and herring are the backbone of Baltic fisheries, representing centuries of fishing culture, millions of tonnes of annual catch, and livelihoods across a dozen nations. When their nurseries and spawning grounds start choking, the consequences reach far beyond the water's edge.

A New Way to Measure the Damage

Scientists from France, Denmark, and the United States have built a mathematical model that tracks the impact of oxygen depletion across a fish's entire life, from the egg it begins as to the breeding adult it becomes.

The model simulates what happens to fish populations when the places they rely on most — their spawning grounds and the shallow coastal nurseries where their young grow up — become degraded or, conversely, restored. It zeroes in on three moments in a fish's life when hypoxia can strike:

1. The egg stage: When water loses too much oxygen, eggs can die before they hatch, or adult fish are forced to spawn in less suitable spots, reducing how many eggs ultimately survive.

2. The juvenile stage: Young fish that settle in coastal nursery grounds can't easily swim away from bad conditions the way adults can. They're largely stuck, and if the water turns hypoxic during the summer growth season, they suffer for it.

3. The adult spawner stage: When mature, breeding fish are exposed to hypoxic conditions and can't escape, their death rates climb. That means fewer adults survive to reproduce the following year, and the year after that.

The team tested their model against three species with very different biological profiles: Atlantic cod, European plaice, and Atlantic herring. The species age differently, mature at different rates, and live for very different lengths of time — and those differences shaped how each responded to habitat stress in ways that weren't always obvious.

Which Stage of Life Hurts the Most?

This is where the research gets genuinely counterintuitive.

Most people would assume that damaging the egg stage is the worst thing you can do to a fish population. Eggs are the beginning of everything, after all. But that's not what the model found.

Damaging the egg stage is actually the least harmful of the three scenarios. Fish populations have a remarkable natural shock absorber called density dependence: when fewer eggs hatch and fewer larvae survive, there's less competition among the young fish that do make it. More food, more space, better odds. The population partially compensates for the loss on its own.

Damaging juvenile habitat is worse. When the nursery grounds where young fish grow up deteriorate, the maximum number of juveniles those areas can support falls. That's harder for the population to recover from, because it shrinks the foundation everything else is built on.

Damaging adult spawners is by far the most devastating. When breeding fish die in higher numbers because they can't escape hypoxic spawning grounds, that mortality compounds across years of a fish's reproductive life. For a species like European plaice, which lives up to 28 years, the damage stacks up every single season. The model showed that plaice populations are extremely sensitive to spawner mortality for exactly this reason — there are so many reproductive years that can be cut short.

Herring tell a different story. They live shorter lives and mature later relative to their lifespan, which makes them less vulnerable when adult fish die off, but more vulnerable to disruptions at the egg stage, since each spawning season carries a bigger share of what keeps their numbers up.

When Two Problems Hit at Once

In the real world, hypoxia doesn't politely pick one habitat and leave the rest alone. It can hit juvenile nurseries and spawning grounds at the same time. So the researchers pushed their model further and asked: what actually happens when both critical habitats are damaged simultaneously?

The answer surprised them. When pressure falls on both juvenile and spawning habitat together, the total damage is often less than you'd get by adding the two individual impacts together. Scientists call this an antagonistic effect — the two stressors partially offset each other in ways that are hard to predict without a model like this one.

But it's not straightforward good news. In other scenarios, the combined damage is actually greater than the sum of its parts, a pattern known as a synergistic effect. Which way it goes depends on the species, which life stage is under pressure, and which aspect of population health you're measuring.

The practical takeaway is sobering. Managing fisheries under multiple, simultaneous environmental pressures is far more complicated than current policies typically assume. You can't just add up individual threats and expect the total to tell you the full story.

What It Means for Fisheries Management

These findings carry real weight for the people whose job it is to set fishing quotas and manage fish stocks.

Right now, stock assessments are built mostly around counting adult fish and tracking how hard they're being fished. Habitat quality barely enters the picture. What this research shows is that's a significant blind spot. Habitat health shapes population health, and it does so differently depending on which life stage is affected.

For western Baltic cod, this matters urgently. Scientists and managers have struggled for years to explain why the stock isn't recovering the way their models predict, even when fishing pressure drops. The new research points to a plausible reason: hypoxia in both spawning and juvenile habitats may be steadily draining the population in ways that fishing restrictions alone can't address. Until the habitat problem is tackled, the stock problem may simply not go away.

For herring, the situation is arguably worse than even this model captures. Herring eggs aren't drifting freely in open water — they're fixed to gravel and algae on the seafloor, which means they can't escape when a pulse of water stripped of its oxygen rolls through. That makes herring eggs sitting targets for hypoxic events in a way the current model doesn't fully account for.

For plaice, things look somewhat more encouraging. Their spawning appears to occur in deeper channel waters that funnel oxygenated seawater into the Baltic, giving them a degree of natural protection. That may partly explain why plaice stocks have actually grown in recent years while cod stocks collapsed in the same water.

Can Anything Be Done?

There's real hope woven into this research, and it's worth sitting with for a moment.

The same model that maps how populations shrink when habitats degrade also shows how they bounce back when habitats improve. Reduce hypoxia in spawning and nursery grounds, and fish populations respond. Sometimes the gains are substantial.

The difficulty is that reducing hypoxia in the Baltic is slow, complicated, and politically demanding. Forty years of work to cut nutrient runoff haven't reversed the dead zone problem. The watershed draining into the Baltic stretches across 14 countries, and the farming, industry, and urban infrastructure feeding nutrients into the system are deeply entrenched. Climate change is also working against recovery — warmer water holds less oxygen and disrupts the natural circulation patterns that periodically ventilate the Baltic's deeper basins.

These efforts at a local level do help. Restoring coastal seagrass beds, building artificial reefs, and expanding shellfish farming all contribute to cleaner water with more oxygen near shore. But these can't substitute for the bigger fix. Reversing the dead zone problem ultimately requires sustained, coordinated cuts to nutrient pollution at a continental scale — the kind of cooperation that's easy to call for and very hard to deliver.

A Tool Built for What's Coming

The most lasting contribution of this research may not be what it revealed about any specific species. It's the tool itself.

The model was built from the start to travel beyond the Baltic. It can be adapted to almost any marine fish species, anywhere in the world, where habitat degradation is threatening populations. It can grow to include spatial complexity, environmental variability, and stressors other than oxygen depletion. It gives scientists, managers, and policymakers something they've rarely had: a rigorous, transferable way to compare what it actually costs, in fish, to let a habitat degrade — and what can realistically be gained by restoring it.

Fish aren't just numbers in an assessment spreadsheet. They're creatures shaped by where they're born, where they grow up, and where they return to breed. When those places become hostile, entire populations pay the price, across years and generations, and the fishing communities built around healthy seas pay it alongside them.

The oxygen is running out in parts of the Baltic. Fish are paying the price. And this research makes it harder than ever to pretend the problem is simple, or that adjusting fishing rules alone will be enough to turn things around.

Publication details: Year of online publication: 2024 Journal: ICES Journal of Marine Science Publisher: Oxford University Press on behalf of the International Council for the Exploration of the Sea DOI: https://doi.org/10.1093/icesjms/fsae178

Credit & Disclaimer: This article is based on the original research paper "Modelling the impact of hypoxia on critical essential fish habitats throughout the life cycle of exploited marine species," published in the ICES Journal of Marine Science (2024) after rigorous scientific review. Readers are encouraged to consult the full research article for complete methodology, data, and scientific detail. The article is openly accessible at: https://doi.org/10.1093/icesjms/fsae178