Harmful algal blooms are causing biotoxins that restaurants and consumers can't get rid of to accumulate in shellfish. Climate change is making it worse.
In 2013, around 70 Londoners developed symptoms of nausea, vomiting, and pain after eating shellfish in several restaurants. But what looked like an ordinary case of food poisoning originated 800 miles to the north, in the Shetland Islands, an archipelago in Scotland.
Several weeks before the poisoning incident, there had been an unprecedented increase in phytoplankton within the shellfish harvesting areas in Shetland. Events such as these are called algal blooms. While algal blooms can block sunlight and reduce the amount of dissolved oxygen in water, which can be detrimental to the life of fish and other aquatic life, a few types of algae also release toxins, leading to harmful algal blooms, as was the case with the London poisoning incident.
More than half of the seafood available for human consumption comes from aquaculture, where aquatic plants and animals are specifically reared for consumption. Over the past few decades, aquaculture sites have faced huge economic losses and even closures due to toxic algal outbreaks. But what happens when the effects of algal toxins go unnoticed in certain reared species? And what happens if harmful algal blooms become more frequent, more widespread, more destructive?
Unlike fish, shellfish don't always show signs of poisoning. Even after the toxins have built up in the flesh of the shellfish, they look and taste the same. These toxins cannot be removed by cooking or freezing, but they could make humans sick and can even lead to death.
Marine scientist Fatima Gianella, who studies the effects of blooms on aquaculture in Scotland, showed through her research that biotoxins that cause diarrhea, rather than labor or capital, are the most significant factor influencing shellfish production. Just one type of algae, Dinophysis, amounts to 15% of total losses.
And that's not the whole picture. Harmful algal blooms are not only detrimental to human life but can also kill fish and other aquatic life that humans require for food.
“These are specifically economic losses. I think [a harmful algal bloom] can have greater repercussions depending on the size of the farm,” she said. “We have more information on what is monitored more often, like shellfish. Other vertebrates can be impacted, like marine mammals and birds. Pretty much any animal in the sea can experience these effects.”
Oyster farmer Francisco Avelelas is used to such losses. At his farm in the Aveiro region of Portugal, he grows oysters in open cages, exposed to naturally occurring variations in currents, zooplankton, and phytoplankton.
European and Portuguese law requires Avelelas to test the water for toxins when planning a harvest. If the region’s water quality isn’t up to standard, he must send the organisms to a purification station before selling them to his customers.
Avelelas said the water analyses usually show high values of toxins. “Sometimes we have toxins in the water. Other times, we have toxins in the organisms and that leads to catch bans, so [shellfish] can’t be sold,” he said.
In cases with bad water quality, the purification stations keep the organisms in a recirculating water system for a day to maximize their natural filtering activity. In simpler terms, the oysters would be encouraged to spill their guts. After this immersion period, the mussels are sold.
But sometimes, lab tests performed at purification stations can show unexpected and unacceptably high levels of toxins. When this happens, farmers are forced to kill the entire batch of shellfish they have sent to the purification stations.
“It doesn’t make sense because the shellfish could go back to the farm until the toxin levels come down and the tests come back negative for those toxins,” Avelelas said. “Nothing justifies the destruction of so much product.”
Avelelas and his colleagues are working with the Portuguese Institute for the Sea and Atmosphere to change how this process is regulated.
“Were trying to speed up the process and advocate for faster analytical techniques so that we don't have to wait up to two days for our results and thus avoid this situation with the purification stations, which is destructive to the whole supply chain,” he said.
In recent years, climate change has proved to be a major contributor to algal growth worldwide. According to the U.S. Environmental Protection Agency, factors like warmer temperatures, sea level rise, and saltier oceans — all by-products of climate change — contribute to harmful algal blooms.
Unprecedented outbreaks in Oregon and Washington that have been attributed to climate change, both in terms of toxic levels being detected in shellfish and the number of people becoming ill, have led to the states closing their coasts to shellfish harvesting.
The impact isn’t limited to the Pacific Northwest either: the tainted shellfish was sold as far as Hawaii and New York, according to a U.S. Food and Drug Administration advisory issued in June — just like those unsuspecting London diners over a decade ago, there was nothing restaurants, retailers, and consumers could have done to protect themselves. Farmers and regulators form the last line of defense.
While farmers like Avelelas can’t completely prevent algal blooms or the effects of climate change, they are aware of certain patterns that promote them, such as warmer months and increased fertilizer or sewage discharge in estuaries, especially during tourist seasons.
Although Portugal has made significant strides in treating urban wastewater after being referred to the European Court of Justice and fined over noncompliance, Avelelas is still particularly concerned about his region’s wastewater infrastructure: certain places retain combined sewers, where rainwater and wastewater are discharged through the same system. Sustained heavy rain could lead to combined sewer overflows where untreated sewage would enter waterways and encourage algal blooms.
“We try not to schedule capture expeditions two weeks after heavy rains,” Avelelas said.
Farmers have also been trying to control factors such as oxygen and nutrient levels to prevent stock loss during a bloom event. Gianella explained that some steps include oxygenating the water column, putting tarpaulins that act like curtains around the sea cages, or taking the fish out.
But despite intensive monitoring efforts, algal blooms have been creating havoc at unprecedented rates. Avelelas has been dealing with dangerous levels of toxins near Aveiro that have prevented him from catching shellfish for weeks at a time.
“In some specific areas around the Aveiro estuary, hundreds of families depend on catching mussels and oysters,” he said. “It's a serious loss.”
