Why Red Tides Are an Increasing Threat to Human Health, Marine Animals, and Coastal Communities

Why Are “Red Tides” an Increasing Threat to Human Health and Life Along the Coast

Scenes of disoriented birds crashing into houses, windows, and streetlights seemed like cinematic fiction for decades, but reality has shown that such scenes can have a very specific biological cause behind them. The subject that popular culture linked to Hitchcock’s film The Birds is once again at the center of attention for scientists, doctors, and public health institutions today: harmful algal blooms, often known to the public as “red tides,” are increasingly being viewed through the lens of climate change, food safety, and risks to human health.

Although the name suggests that it is about the sea turning red, experts warn that this is only a colloquial expression. Many harmful blooms do not color the water red at all, yet they still produce toxins or cause serious disturbances in marine ecosystems. The U.S. National Oceanic and Atmospheric Administration states that harmful algal blooms occur when algae multiply uncontrollably and in the process cause toxic or other harmful effects on people, fish, shellfish, marine mammals, and birds. The problem, therefore, is not only in the appearance of the sea, but in the consequences such events can have for the food chain, the coastal economy, and public health.

The greatest attention on the American West Coast in recent years has been drawn by domoic acid, a toxin produced by certain species of microscopic algae, especially those from the genus Pseudo-nitzschia. This toxin can accumulate in shellfish and fish and then end up in the organisms that eat them, including sea lions, birds, and humans. Public health institutions warn that exposure to domoic acid can cause amnesic shellfish poisoning, a disease that in more severe cases can lead to disorientation, seizures, heart rhythm disturbances, coma, and even death. Particularly worrying is the fact that the consequences can also include permanent short-term memory loss.

From Film Inspiration to a Scientifically Confirmed Cause

The link between Hitchcock’s film and algal blooms was long the subject of debate, but today there is considerably more support for the claim that the real event on the California coast truly was connected to toxins from the sea. Film historians and media records have for years cited an incident from 1961 in Capitola, in Santa Cruz County, when seabirds began behaving unusually: they flew into houses, cars, and public lighting, and some vomited pieces of fish. According to several American sources, Hitchcock followed the local newspapers, and that very event helped shape the atmosphere of his later film.

Additional weight was given to this story by scientific research published in the journal Nature Geoscience, in which plankton preserved from the 1961 event was analyzed. Researchers identified the presence of diatoms associated with the production of domoic acid, which strongly supports the conclusion that toxins from an algal bloom contributed to the “bird madness” that later entered popular culture. In other words, what sounded for decades like almost an urban legend now has a serious scientific basis.

But the real events were different from the film dramatization. The birds were not deliberately attacking people, but were neurologically impaired after coming into contact with the toxin through the food chain. This is precisely the key difference that toxicologists and epidemiologists emphasize today: harmful algal blooms are not a curiosity of nature, but a phenomenon that can change animal behavior, endanger food safety, and raise serious questions about long-term health effects on humans.

Climate Change Intensifies Risks, but Not Equally for Every Algal Species

To the question of whether climate change increases the frequency and danger of harmful algal blooms, the scientific answer today is mostly yes, but with an important qualification: not all algae species react equally to changes in temperature, salinity, and water circulation. The U.S. Environmental Protection Agency states that warmer water, changes in freshwater and marine systems, and sea level rise can contribute to more intense and more frequent blooms in an increasing number of water bodies. In its reviews, NOAA also warns that marine heatwaves, altered nutrient inflow regimes, and other climate anomalies can create conditions favorable for dangerous blooms.

This does not mean that every warming event automatically translates into the same type of bloom or the same toxin concentration. The latest research on coastal waters shows that warming and changes in salinity can increase the frequency of certain harmful blooms in some seasons, while reducing them in others. That is exactly why experts warn that it is dangerous to reduce the problem to a single formula. The ecological “niche” of each algal species is different, so local conditions, from ocean currents to the inflow of nutrients from land, must be analyzed separately.

Despite this complexity, the broader picture is clear: warmer oceans and more frequent marine heatwaves are increasing pressure on coastal ecosystems. At the beginning of April 2026, American media reported unusually high sea temperatures off Southern California, which once again raised fears of a longer-lasting marine heatwave and new disruptions in the food chain. Such conditions are particularly worrying for researchers monitoring domoic acid because previous severe episodes on the West Coast were often linked precisely to unusually warm seas and changes in the upwelling of nutrient-rich water from deeper layers.

Why the Problem Is Not Only Ecological, but Also a Public Health Issue

Harmful algal blooms are often perceived by the public as a nature protection problem, but for doctors and epidemiologists they are also a very concrete health issue. With domoic acid, the problem is that the toxin can end up in food. The California Department of Public Health reminds that the state has been dealing systematically with this risk since the early 1990s, when domoic acid was first detected in Monterey Bay. Today, California runs a year-round shellfish and phytoplankton monitoring program precisely so that dangerous toxin concentrations can be detected in time and recommendations or harvest bans issued.

Such monitoring also explains why the number of human poisonings in California is relatively low compared with the potential danger. The risk has not disappeared, but the public health system actively mitigates it. Regular shellfish testing, phytoplankton monitoring, and the publication of health warnings are key elements of prevention. The problem arises where such monitoring is not equally available, especially in communities that rely more heavily on gathering food from the sea on their own.

Along with domoic acid, experts also warn about saxitoxin, another powerful marine neurotoxin associated with certain harmful blooms. This toxin causes paralytic shellfish poisoning. State health authorities in the United States state that symptoms can appear very quickly after consuming contaminated shellfish, ranging from tingling and numbness to breathing difficulties, muscle weakness, and paralysis. In the most severe cases, paralysis of the respiratory muscles occurs, which is why urgent medical assistance is necessary. It is particularly important to stress that cooking does not remove these toxins, so safety depends above all on monitoring and timely warnings, not on food preparation.

Consequences for the Sea, Fisheries, and Coastal Communities

Not all algal blooms are toxic, but even those that do not directly produce toxins can have devastating consequences. When a massive mass of algae dies and begins to decompose, it consumes oxygen from the water. The result can be oxygen-poor zones in which fish and other marine organisms struggle to survive. This affects not only nature but also local economies that depend on fisheries, aquaculture, tourism, and food safety.

On the American West Coast, the consequences have been visible for years through the rescue of marine mammals as well. In 2024, NOAA also warned of toxic blooms affecting California sea lions and dolphins, explaining that the upwelling of nutrient-rich water had favored the development of a bloom producing domoic acid. When the toxin enters the food chain through small oily fish such as anchovies and sardines, the consequences are quickly seen in predators. Animals may show disorientation, seizures, abnormal behavior, and permanent neurological damage.

Such images further heighten public awareness, but at the same time they warn of a broader problem. If the same toxin can also reach humans through shellfish or other marine organisms, then the issue of harmful blooms is no longer just a topic for marine biologists. It becomes a question of public policy, food system security, and the adaptation of coastal communities to climate change.

Why Some Communities Are More Exposed Than Others

The risk is not distributed evenly. Communities that buy most seafood products through strictly controlled commercial channels have a higher level of protection than those that rely on traditional or recreational harvesting and gathering. That is precisely why expert circles often highlight the example of Alaska, where Indigenous communities are among the most exposed when it comes to shellfish poisoning linked to saxitoxin.

Data and experience from Alaska show that this is an issue that is not only about health, but also social, cultural, and food-related. For many communities, shellfish are not merely an addition to the menu, but part of a traditional way of life and an important pillar of food security. If harmful blooms spread or become less predictable, the consequences are far more serious than the occasional closure of a beach to recreational users.

That is exactly why the Sitka Tribe of Alaska developed its own laboratory and field monitoring system, and through networks such as SEATOR, multiple locations in Southeast Alaska have been monitored for years. According to available official data, the goal is to enable communities to send collected shellfish samples for testing and to regularly receive information about the safety of specific areas. Such a model shows how important it is to connect science, public health, and the needs of the local population. Where the state cannot or does not manage to test everything that people collect themselves for food, community-based and locally led monitoring becomes crucial.

UCSF, UNESCO, and the Attempt to Create a Global Response

In this context, the work of researchers from the University of California, San Francisco is particularly interesting. In March 2026, UCSF announced that toxicologist and epidemiologist Matthew Gribble had received a Pew-Hoover Fellowship in marine and biomedical science and that, together with Professor of Medicine Sheri Weiser, he was launching the UNESCO Chair in Oceans, Clean Water and Health. According to UCSF, this is the first UNESCO Chair program in the history of the university.

The significance of such an initiative goes beyond academic prestige. UNESCO’s Chairs and Networks Programme is designed to connect universities and research centers in addressing global challenges, from education to natural sciences and public policy. In the case of oceans and health, the idea is to establish a hub that will coordinate research, education, and cooperation among institutions and communities. This is particularly important for topics such as harmful algal blooms, where a local incident can very quickly prove to be part of a broader international pattern linked to climate, food trade, and public health surveillance.

Gribble’s profile at UCSF further confirms that his work is not limited to the laboratory level. There he is listed as co-founder and director of the UCSF Center for Oceans & Human Health and as the lead academic partner of a tribally led center for oceans and human health. This combination of academic research and work with communities is particularly important because local communities are the first to feel the consequences when fishing grounds are closed, animals are poisoned, or shellfish consumption bans are imposed.

What Most Concerns Scientists Today

Although much more is known about harmful algal blooms than a few decades ago, serious gaps in knowledge still remain. One of the key open questions concerns long-term exposure to smaller doses of toxins. Severe acute poisonings are easier to recognize and connect to a cause, but it is much more difficult to assess what consequences repeated exposure to lower levels of domoic acid or other biotoxins over a longer period may have.

This is important both for communities that regularly eat seafood and for regulators who decide on safety thresholds. If it turns out that even lower but repeated doses can leave neurological or other health consequences, then monitoring standards as well as public health recommendations would also have to be adjusted. That is exactly why there is growing insistence on long-term research that connects toxicology, epidemiology, oceanography, and clinical medicine.

The second big question is predictability. Scientists today have better models, better satellite monitoring, and stronger laboratory analytics than before, but harmful blooms still remain a complex phenomenon influenced by numerous factors. Changes in sea temperature, nutrient availability, winds, ocean currents, rainfall events, and local ecology can together determine whether a bloom will appear, how long it will last, and whether it will be toxic. That is why early warning systems and local field measurements remain irreplaceable.

A Problem That Can No Longer Be Viewed as a Passing Anomaly

If the story of “red tides” is reopening today, the reason is not only fascination with unusual scenes from the sea nor nostalgia for Hitchcock’s classic. The reason is that harmful algal blooms are showing ever more clearly how climate change, human health, food safety, and the condition of marine ecosystems overlap with one another. From California to Alaska, from rescuing sea lions to testing shellfish that communities gather themselves for food, it is the same pattern: a natural process that existed before is now taking on new weight because conditions are more unstable and the consequences are broader.

In that sense, the story that once helped inspire one of the most famous thrillers of the 20th century now feels less like a cinematic oddity and more like a warning. When toxins from the sea break through the food chain and reach birds, marine mammals, fisheries, and ultimately humans, it becomes clear that this is a topic that can no longer be pushed to the margins of interest. According to the available scientific and public health data, the response will not come from one laboratory or one country, but from the sustained combination of monitoring, medicine, local knowledge, and international cooperation.

Sources:

• UCSF – interview and overview of Matthew Gribble’s work on harmful algal blooms, domoic acid, and public health risks (link)
• UCSF – announcement on the Pew-Hoover Fellowship and the launch of the UNESCO Chair in Oceans, Clean Water and Health (link)
• UCSF Center for Oceans & Human Health – description of the center’s goals and the connection between oceans and human health (link)
• UNESCO – official overview of the UNESCO Chairs programme and international university cooperation (link)
• NOAA National Ocean Service – explanation of what harmful algal blooms are and how they affect people and ecosystems (link)
• NOAA NCCOS – overview of the state of science, forecasting, and monitoring of harmful algal blooms (link)
• California Department of Public Health – official marine biotoxin and shellfish monitoring program (link)
• California Department of Public Health – overview of domoic acid risks and amnesic shellfish poisoning (link)
• Washington State Department of Health – symptoms and health risks of amnesic shellfish poisoning (link)
• Oregon Health Authority – official overview of paralytic shellfish poisoning and the effects of saxitoxin (link)
• Nature Geoscience – scientific paper linking the 1961 Capitola event with algal toxins and inspiration for the film The Birds (link)
• HISTORY – overview of the historical event in Capitola and its relationship to Hitchcock’s film (link)
• Sitka Tribe of Alaska – official description of the work of the Environmental Research Lab and monitoring of risks for the community’s marine food supply (link)
• U.S. Climate Resilience Toolkit – presentation of the cooperation of Alaska Native communities in assessing harmful algal blooms (link)
• EPA – overview of the effects of climate change on harmful algal blooms in aquatic systems (link)