For decades, shellfish farmers in cooler coastal waters, including northern Europe, the Pacific Northwest, and parts of Canada, worked with a fairly predictable threat calendar. Harmful algal blooms showed up in certain seasons, in certain places, and producers and regulators had learned to plan around them. That predictability is breaking down. Shellfish biotoxin risks are moving into new waters, and the shellfish safety monitoring systems built to catch them have not fully caught up.
A 2026 study in Environmental Monitoring and Assessment makes this plain: as ocean temperatures rise, the algae that produce marine biotoxins are spreading into waters where they were rarely or never seen before. The maps we use to assess shellfish harvest safety need updating.
The Science: What Is Actually Moving
The most well-documented shift involves the algae behind paralytic shellfish poisoning (PSP). Species like Alexandrium are losing ground in warmer tropical and subtropical waters, but expanding steadily toward the poles. Coastal regions in higher latitudes that have rarely had to worry about PSP toxins are now sitting in the path of that expansion.
The same pattern is emerging with domoic acid, the toxin responsible for amnesic shellfish poisoning (ASP). The diatoms that produce it are also moving poleward, with projections showing meaningful spread around the British Isles, Scandinavia, and sub-Arctic coastlines.
The most surprising development is tetrodotoxin (TTX). This is a powerful nerve toxin that most people associate with pufferfish in tropical waters. It is now showing up in mussels and oysters along the European Atlantic coast, including in the UK and Portugal. Laboratories across multiple countries have confirmed these detections independently. How shellfish are accumulating TTX is still being studied, but the findings are consistent and can no longer be treated as isolated incidents.
Why This Matters for Shellfish Safety Right Now
New marine biotoxins showing up in new places would be concerning enough on its own. The bigger problem is that our shellfish safety systems were not built for a moving target.
The rules governing shellfish biotoxin testing in the EU, the US, and most other major markets were written around the toxins that were known risks at the time. Tetrodotoxin, for example, has no legal limit in EU shellfish because it was never considered a European problem. There is no approved rapid screening method for it. A farmer harvesting oysters in an area where TTX has now been detected has no simple field tool to check whether their product is safe. That is not their fault. It is a structural gap between where the science currently stands and where the rules and shellfish testing tools have gotten to.
On top of that, harmful algal blooms are no longer as seasonal as they used to be. Warmer water for longer periods means bloom events that once ended in late summer are now running deeper into autumn. Producers are being asked to stay vigilant over a longer window, sometimes without any increase in monitoring program resources to match.
What Shellfish Producers and Testing Labs Should Do
Geographic risk profiles need regular review. A site that has never triggered a shellfish harvest closure is not automatically low risk forever. If that site sits in the projected expansion zone for PSP or ASP-producing species, it deserves a closer look. NOAA’s HAB Forecasting System and EFSA’s harmful algal bloom monitoring data are both free to access and useful for tracking long-term trends in bloom activity near your area.
Fast, on-site shellfish toxin testing is more valuable now than it has ever been. Sending samples to a central lab and waiting days for results was already a stretch when bloom patterns were predictable. With bloom windows getting longer and appearing in new locations, being able to run a rapid shellfish test at the point of harvest and get a result within minutes gives producers options that centralized lab testing cannot match.
If you operate in a region that is seeing new bloom activity or new toxin detections, get ahead of it with your regulatory contacts now. The rules around emerging toxins like TTX will tighten eventually. Producers who already have a shellfish biotoxin monitoring routine and documentation in place will be in a much stronger position when that happens.
The Baseline Has Shifted
Shellfish safety has always demanded attention and care. What is changing is the environment that attention has to cover. Bloom species are moving into new waters. Marine biotoxins are turning up where they were not expected. And the official shellfish monitoring programs meant to protect consumers and producers are, in some cases, still working from an older picture of the risk landscape.
Recognizing that the baseline has shifted is the starting point. Building shellfish biotoxin monitoring programs that reflect where the risks actually are today is what comes next.
Sources: Environmental Monitoring and Assessment (2026); PMC9598431 — Projecting Future Climate Change Impacts on PST-Producing Dinoflagellates; PMC9863508 — Impacts of Climate Change on ASP-Producing Diatom Biogeography; Foods 2025, 15(12):2103 — Potential Increase in Known and Emerging Biotoxins Due to Climate Change; EFSA emerging biotoxin risk assessments.