Offshore from Vancouver Island, British Columbia, a team hauls up a line laden with meter-long fronds of sugar kelp (Saccharina latissima), a floppy, brown seaweed with crinkled edges. The harvest from Cascadia Seaweed, a kelp farming company founded in 2019, has over the years made its way into a seaweed-enriched “sea spice” condiment mix and a host of trial snack products including protein puffs and tortilla chips. Now the company is focused on processing its seaweed into animal feed and soil additives—anything that will help kelp make the world a better place.

Seaweed farming has a long history in Asia, and now it is spreading around the globe. Over the past 30 years, according to the Food and Agriculture Organization of the United Nations (FAO), world production of seaweed has boomed more than sixfold to over 35 million tonnes, with emerging markets in the Americas (particularly the Pacific Northwest, Alaska, and Maine), Europe, and Africa. The most recent numbers, tallied from 2019, show that North America produces some 23,000 tonnes of wet seaweed (referred to as wet tonnes by the industry) per year. Cascadia claims to be the largest cultivator in Canada, with a production of 50 wet tonnes in 2022, and aims to become the largest in North America with a projected harvest of 200 wet tonnes this spring and plans for expansion. Farms, small and large, are popping up fast: while hard numbers are difficult to come by, Alaska, Maine, France, and Norway have all reportedly more than doubled their seaweed production since 2018, with each region now cranking out hundreds of tonnes per year.

Many see this expansion as a good-news story. The Safe Seaweed Coalition—an umbrella research and industry group founded to promote safe expansion of the industry—says humanity could, and should, be harvesting 15 times more seaweed by 2050. Coalition member Vincent Doumeizel works as the food program director at the Lloyd’s Register Foundation and is a senior adviser on oceans to a United Nations program working toward corporate sustainability. Doumeizel talks about a “seaweed revolution” he feels is necessary to feed the growing global population—a transformation of food systems as dramatic as the ancient shift to land-based agriculture. “In the ocean, we are still hunter-gatherers,” he bemoans.

A “seaweed manifesto,” edited by Doumeizel and published in 2020, is an urgent call for that to change. The Safe Seaweed Coalition estimates that seaweed—high in much-needed protein and nutrients—could add 10 percent to the world’s present supply of food using just 0.03 percent of the ocean surface. One recent study concluded that substituting 10 percent of human food with seaweed by 2050 could also spare 110 million hectares of land for purposes other than agriculture—like staying wild. That’s about two percent of all the land used for agriculture today, or an area the size of Bolivia.

Advocates like Doumeizel cast seaweed as the savior of many social and environmental woes: the industry requires no fresh water, pesticides, or fertilizers, they note; it doesn’t take up any land and can overlap with other uses of the ocean like offshore wind farms; its low-tech operation can help to alleviate poverty and provide jobs for women in the developing world; and some companies, including Cascadia, are collaborating with Indigenous communities. Seaweed creates a rich habitat for sea life, soaks up CO₂, counteracts acidification, and absorbs run-off nutrients. Seaweed farming logically should do more of the same—although firm evidence quantifying how farms, specifically, help with such things, including sequestering carbon long term to tackle climate change, is still thin on the ground.

Many large environmental organizations, including the World Wildlife Fund, Oceans 2050 (spearheaded by Alexandra Cousteau, granddaughter of Jacques-Yves Cousteau), and the Nature Conservancy, have come out in support of the idea of expanding seaweed farming to help the oceans, the planet, and people. The Intergovernmental Panel on Climate Change’s 2019 special report on the oceans concluded that coastal ecosystems like mangroves can reduce the risks and impacts of climate change, and for that reason, seaweed aquaculture “warrants further research attention.”

But the industrial boom also makes many wary. Past rapid expansions in aquaculture have not been rosy: fish farms have spread disease; shrimp aquaculture has plowed down mangroves to make room for farms. Will industrial-scale seaweed farms, some wonder, also bring hazards like imported diseases or unwanted ecosystem changes, or see big business grabbing leases to large swaths of the ocean? One study of seaweed farming’s expansion in Europe, for example, concluded that while small-scale projects seem low risk, more work is needed to be sure that the risks of large-scale cultivation are balanced against the benefits.

Safe Seaweed Coalition member and biosecurity expert Elizabeth Cottier-Cook, who works at the Scottish Association for Marine Science, notes that while some regulations exist regarding which seaweed species can be grown where, in many places “it’s a free-for-all; you can do what you want, really.” That makes some observers nervous. What if many of the concerns of land-based farming just get transferred to the ocean?

“We don’t have an adequate regulatory framework in Maine to protect us from very large-scale experiments by very well-funded actors,” says Severine von Tscharner Fleming, a wild seaweed harvester and organic farming advocate with the North American group Seaweed Commons. Meanwhile, she says, there’s a lot of money and rhetoric from companies that large-scale projects will absorb carbon and produce copious food without any negative impacts. “It sounds a lot like the sunny, optimistic projections from conventional agriculture that those of us in organic ag have spent our lives trying to confront,” she says. “That over-rosy outlook is uncomfortable.”

Seaweed beds are a lush and important part of coastal ecosystems worldwide, with more than 10,000 types of green, brown, and red algae now cataloged. Despite the name weed, they are algae, not plants, without true roots or leaves: they typically grow from a holdfast on rocks in shallow waters and float up toward the surface. Some grow quickly; giant kelp, a common brown seaweed, can grow up to 30 centimeters or more a day in favorable conditions, rapidly blooming into dense underwater forests. Some are perennial, surviving years. Others are annual, wiped away by each winter’s storms.

The notion of harvesting seaweed for food has an extremely long history. Researchers have proposed that high-nutrient seaweeds may have super-charged the evolution of the hominin brain millions of years ago. Traces of seaweed meals were found in Chile in one of the earliest-known habitations in the Americas, dating back 14,000 years.

Seaweeds can be eaten raw or dried, boiled in soups, or wrapped around sushi. They are fed to abalone, used in fertilizers, and added to cattle feed: extracts from some red algae have been found to dramatically reduce methane in cow burps, helping to reduce the climate footprint of ranching. Seaweed extracts, agar and carrageenan, are used as gelling agents or stabilizers in everything from ice cream to toothpaste. Seaweeds are a potentially useful ingredient for everything from bioplastics to biofuels to concrete.

According to FAO, back in 1969, the world used some 2.2 million tonnes of seaweed, and only half of that was grown on purpose. The other half came from wild harvest: people would simply go out and chop off fronds. Wild harvest has stayed steady at around 1.1 million tonnes per year, but cultivation has boomed to 34.7 million tonnes in 2019. Today, seaweed makes up nearly 30 percent of the wet weight of all seafood produced by aquaculture globally. About half of cultivated seaweed is red algae and about half is brown; of the brown, most is kelp. In some places, the scope of seaweed farming is epic. In Indonesia, seaweed farms account for 40 percent of the national fisheries output and employ about one million people.

With Asia still accounting for 97 percent of seaweed production as of 2019, the scope for growth elsewhere is huge. For now, the Americas contribute just 1.4 percent to global seaweed production; Europe just 0.8 percent. This is the market hole that companies like Cascadia Seaweed are hoping to fill.

Cascadia’s farming process is simple. Sugar kelp trimmings are taken from local wild plants in the autumn, and their spores are spread onto twine wrapped around pipes in a nursery composed of a couple of tanks in adapted shipping containers. After about a month, when the babies are still smaller than a pinky fingernail, they are transported out to ocean farms, where the twine is rewrapped around long lines anchored a few meters below the surface. Here, they grow until early spring; by March or April, several-meter-long fronds are ready for harvest. The company leaves about 10 percent of their seaweed out in the water as a habitat service and to provide source material for the next year’s crop.

One of the possible arguments for expanding kelp farming is that it would benefit the ecosystem by, say, restoring lost biodiversity or creating habitat for fish. Support for this idea is more nuanced than it might seem. Cottier-Cook says she just helped complete a survey of more than 20 experts and nearly 300 research papers to find hard evidence of wider ecosystem-scale benefits of seaweed farming, and “there’s not much out there,” she says. “There are lots of claims being made at the moment but not a lot to back them up.” The European Union, she says, is now trying to plug those gaps with research funding.

A 2016 survey of kelp forests concluded they’re declining globally at around 1.8 percent per year—but it’s patchy, with some spots improving and others facing extreme declines (one of the most dramatic examples is in Northern California, where warming waters overlapped with a disease that struck predatory sea stars, letting urchins run rampant and devour 95 percent of kelp forests).

The spots facing declines aren’t necessarily places where farms will fare well. Much of California, for example, with its relatively open and wild coast, is too rough for easy farming. In British Columbia, the wild kelp beds are relatively healthy, though they vary from spot to spot: “I’m happy to report that kelp in BC is doing actually quite well,” says Anne Salomon, a Simon Fraser University marine ecologist who studies kelp. This implies that farms aren’t exactly needed to “repair” the ecosystem in British Columbia, though there might be bare patches that would benefit from kelp.

Ecosystem impacts can be complicated. A dense kelp forest, say, in a spot where one did not previously exist, can affect the light and nutrients in the surrounding water—not necessarily for the better. Kelp could, in theory, shade out other organisms below, for example. Intentionally growing huge quantities of seaweed in a massive global scale-up could have far-reaching negative impacts on ocean chemistry and the food chain. And growing large quantities of a single crop could raise the specter of monoculture problems—on land, monocultures pursued for high profit margins have notoriously proven problematic for biodiversity and resistance against extreme weather or disease.

One of the most intuitive concerns about seaweed farming revolves around the risks presented by foreign imports. In Hawai‘i, for example, an Asian red seaweed was brought in for aquaculture in the 1970s; it has since spread several kilometers from where it was introduced, with escapees smothering some coral reefs. Likewise in the Philippines, says Doumeizel, the use of foreign farm spores (called seed by the industry) drove down local seaweeds. “They did everything we would not do today,” says Doumeizel. “They took a seaweed that was not native and grew it as fast as they could.”

Cascadia Seaweed’s chairman, Bill Collins, says there aren’t yet any explicit rules in British Columbia about where exactly seed needs to come from; out of an abundance of caution, Cascadia is currently limiting itself to parent algae found within 50 kilometers of its farm. More research is needed to see if that’s necessary, Collins says.

University of Wisconsin–Milwaukee biologist Filipe Alberto, who studies seaweed genetics, agrees. “There are still a lot of unknowns. We know much more about any other crop,” he says. His work is helping to scope out the genetic diversity of seaweeds in different regions, including Puget Sound, Washington, and to see if bringing in other genetic variants would be bad, disrupting local adaptations, or good, enriching the local genetics. Another option he’s exploring is to naturally select sterile algae for propagation so they cannot interfere with local wild seaweeds. “We have a funded project to show we can do this for giant kelp as a proof of principle,” Alberto says.

Disease is also a concern. According to a United Nations University and Scottish Association for Marine Science policy brief, in the Philippines, a bacterial infection called ice-ice has whitened and stunted seaweed crops dramatically: farmers there saw a 15 percent loss in one species from 2011 to 2013, representing 268,000 tonnes of seaweed. It isn’t clear where the infection came from, nor why it has taken hold, but the incident has fueled concerns that farms might cause new diseases to sprout or spread.

On the plus side, seaweeds can soak up nutrients. One recent study of two kelp farms in Alaska, for example, showed that algae could usefully pull excess nitrogen from the water. In theory, this could help to prevent harmful algal blooms. But some seaweed farms are thought to have had the opposite effect, too, by spurring the growth of nuisance algae. In China, for example, when the area of red seaweed farms doubled from 2007 to 2008, the Yellow Sea also saw the start of green-tide invasions of harmful Ulva prolifera seaweed, which grows on the bamboo rafts and rope nets used on the seaweed farms.

While scientists are still pinning down if and how kelp farming might be a climate change champion, the case is further complicated by the fact that climate change itself could throw a wrench into the growing industry. Kelp is relatively sensitive to temperature: in Australia, a marine heatwave in 2010–2011 devastated kelp forests. In 2012, giant kelp forests in that country’s southeast became the first endangered marine community listed under the Australian government’s biodiversity conservation act. In British Columbia, encrusting animals called bryozoans have been whitening kelps in warm-water years—an effect researchers started noticing in 2015. Can the industry survive if warming waters bring ever-more and ever-longer disease outbreaks? “The kelp aquaculture industry does very much need to be, and are, concerned about bryozoans,” says Salomon.

Overall, most biologists and industry specialists alike agree that seaweed farming can be done well and presents a far lower ecological risk than most other industrial or agricultural activities. But it does need to be well studied and well regulated, and it’s unclear whether that’s always happening. The Pew Charitable Trusts has highlighted the red tape and bureaucratic difficulties of aquaculture in the United States. Squabbles continue about the allowed size and scope of kelp farming operations in Maine, with the Seaweed Commons issuing a position paper last spring calling for more thoughtful regulation. In British Columbia, the regulation framework is still evolving.

Salomon’s group is interested in research questions about how different densities of kelp affect everything from nutrient levels to temperature, light, bryozoan growth, and interactions with wild kelp patches nearby. Her team aims to help map out the kinds of things that may need oversight, looking at how to balance ecological and social concerns, including Indigenous rights.

In general, says Salomon, small is beautiful when it comes to kelp farming. “I worry about the scale at which people are imagining this might need to be done for it to be profitable,” says Salomon. “I think at a small, artisanal scale, where the objectives are to support local communities, local industries, local sovereignty, it can be a good thing, locally. But as soon as it’s commercialized at an industrial scale, I think there can be ecological and important social costs.” In particular, she says, “the Indigenous communities along the coast whose waters people are interested in farming need to be part of the decision-making.”

Everyone agrees that there are problems to avoid. “We made a big mistake with monoculture on land. Let’s not make the same mistakes [in the ocean],” says Doumeizel. “I want my kids to be part of the generation that feeds the world while tackling climate change. We need to do things in a better way; but we need to do things.” Leaving things as they are isn’t an option.