For several years, biotech companies have been promising “clean” meat, “cell-based” meat, “cultured” meat – whatever you want to call it – as a way to enjoy the taste of chicken, pork and beef without the brutality of animal slaughter or the environmental damage of big agriculture. But what about fish? What about something as prized as buttery bluefin tuna, a delicacy that has become the forbidden fruit of the sea because of the many threats that have landed the fish on threatened and endangered species lists?

Where are the Silicon Valley start-ups promising to free us from the guilt of gobbling down a finger of otoro sushi, the rich bluefin belly meat, without contributing to the decline of the fish or the decline of our own health via mercury that accumulates in the flesh of this apex predator?

Well, there is at least one scientific pilgrim: Brian Wyrwas is the co-founder and chief science officer for Finless Foods, a San Francisco Bay area biotech dedicated to growing bluefin tuna in a lab. He can tell you all about the difficulties of his task, starting with the bone-weary process of securing bluefin tuna samples, the pristine source material for much of the science that follows in this field known as cellular agriculture.

Bluefin tuna is processed at ABS Seafood in San Francisco.

Unlike scientists who grow chicken or cow cells in a lab, Wyrwas can’t exactly biopsy a living animal for tissue, given that bluefin tuna travel the world’s oceans at speeds approaching 40 mph. Nor can he grab a sample from one of the precious few bluefin tuna farms, which would view him as competition. Nor can he walk into a fish processing plant and request a sample. Bluefin tuna die on ship, many miles from shore, their cells slowly decomposing even when frozen or on ice.

No, to get an uncontaminated sample, Wyrwas has to head out to sea. Wyrwas, 26, and his Finless co-founder, Mike Selden, 27, don’t like to talk specifics when it comes to sourcing bluefin tuna samples. In the competitive, tight-lipped market of cellular agriculture, no company likes to volunteer information that it earned the hard way: Through scientific trial and error or, in Wyrwas’ case, through countless hours sitting on boats, fighting the elements and his sterile equipment to secure a quality sample that could, hopefully, provide healthy stem cells.

LEARNING TO GROW THE CELLS

Even once he succeeded, Wyrwas and the Finless team had to learn how to culture, or grow, bluefin tuna cells without the actual animal. Without the fish’s natural habitat. And without the fish’s standard diet of squid, mackerel, herring and more. The scientists had few blueprints to follow.

“The cell culture would often die because we were sort of shooting in the dark in the beginning,” says Selden, sitting in a conference room at Finless’s offices in Emeryville, Calif. “We didn’t know how to culture bluefin tuna cells because basically nobody knows how to culture bluefin tuna cells.”

Jennifer Tang inspects tissue cultures of bluefin tuna samples at the Finless Foods lab.

But if they can figure out the science from beginning to end and, perhaps more important, figure out how to scale up the process into a viable commercial venture, the folks at Finless Foods hint at an almost utopian reversal of fortunes for humans, fish and the environment.

Consumers could enjoy bluefin tuna above current recommended levels – one serving per month, says the Environmental Defense Fund; avoid altogether, counters the Monterey Bay Aquarium Seafood Watch – without fear of ingesting mercury, plastic or other contaminants. Just as important, the three bluefin species could begin to recover from decades of overfishing, which has decimated wild populations mostly to cater to the Japanese market, by far the largest consumer of bluefin tuna. (The Pacific bluefin tuna population, for example, has dropped by more than 97 percent from its historical high.)

What’s more, marine ecosystems could begin to restore the harmony that’s disturbed when a top-level predator is removed in such large numbers. To cite just one example, scientists predict that jellyfish populations could explode without an apex predator, affecting both tourism and fishing operations. Plus, without the need for commercial fishing boats to chase after tuna, the oceans could see a drop in the pollution from these vessels, whether discarded plastics or dumped fishing gear.

So, has Finless Foods figured it out? Yes, in part.

The challenge ahead: to produce the fish in large quantities – and in a form that sushi lovers would recognize.

LAB-GROWN MEAT HAS A WAY TO GO

In 2013, when Dutch researcher Mark Post debuted what would become the world’s most famous lab-grown meat – a five-ounce beef patty mixed with bread crumbs – the response from tasters was tepid. Which was not surprising. The beef was grown without any fat. Regardless, the tasting was designed more as a public-relations stunt to drum up interest in an emerging field that promised to give diners their meat with fewer of the harmful side effects – such as greenhouse gases, animal waste, reckless use of freshwater resources and animal suffering – of big ag.

Mike Selden checks on bluefin tuna cultures in a storage device at the Finless Foods lab.

But that staged burger tasting – especially the resulting photo – created a false impression about cellular agriculture, says Ben Wurgaft, a writer and historian who researched the industry for five years.

Post and his colleagues “left the media with the impression that you grew a burger in something that looked like a petri dish,” says Wurgaft, author of “Meat Planet: Artificial Flesh and the Futures of Food,” set for release next year. “It’s like imagining that rice grew in a bento box.”

In reality, the scientists grew thin sheets of bovine muscle cells – thousands of sheets, each no wider than a strand of hair – which they essentially fused together with a kind of meat glue. The process is “obviously not scalable,” Wurgaft says.

In a whitewashed room that smells like bleach, Jennifer Tung, a senior cell biologist for Finless Foods, actually does rely on something that looks like a petri dish. It’s called a cell-culture flask, and Tung uses a lot of them to keep bluefin tuna stem cells alive. It’s a standard part of the R&D process. Each flask contains a thin layer of grapefruit-colored liquid – it’s the food, or “media” as its known in the trade – that allows the cells to grow. The only way to see the cells is under a microscope.

One vital step in culturing meat is to create an “immortalized” cell line, which theoretically can grow forever, meaning you never again need to go out to sea to capture fresh samples.

Beakers hold culture samples that may one day be developed into bluefin tuna meat at Finless Foods.

“We think our bluefin tuna line is immortalized,” Selden says. “We’re pretty sure.”

As important as that development is, however, “it is not the same as being able to make meat,” Wurgaft cautions.

In fact, growing stem cells into something that precisely mimics the fatty flesh of bluefin tuna is not considered possible yet. The technology for such a textured product is still years away from a commercial application, say Selden and others. At present, biotech firms can grow cells in devices called bioreactors, but the resulting meat is more paste than flesh. Which is why Just, the San Francisco company behind a plant-based version of mayonnaise, plans to first release cultured meat products that don’t rely on firm, fleshy textures.

Before the end of the year, Just expects to introduce a chicken product to some still-unnamed restaurants in Asia. It won’t be a cultured chicken breast or thigh, but something closer to the consistency of a nugget, with fried-chicken skin and with plant-based materials serving as binder and flavoring agents.

“If you buy Tyson chicken nuggets, some percentage of the nugget is plant-based,” says Josh Tetrick, co-founder and chief executive of Just, formerly known as Hampton Creek, a company with almost as many controversies as successes. “A chicken bite is much easier than bluefin tuna.”

Then there’s taste. The flavor of the chicken you now eat occurs naturally, in part, from the animal’s diet. Tetrick and his team at Just say they have found a way to incorporate plant-based material into the food media so that when chicken cells are cultured into paste, they end up tasting like the real thing.

In an experimental kitchen at Just’s headquarters in the Mission District, Chris Jones gets to play around with the plant-based materials and cultured meats that others in the company discover or create. A former chef de cuisine at Moto, the once-celebrated and now-closed restaurant in Chicago, Jones is vice president of product development for Just. Recently, he’s been dehydrating cultured chicken paste so that it resembles skin, presumably for those nuggets.

“I actually think it tastes cleaner, and better, than real chicken skin,” Jones says. He hands me a golden sliver of the lab-based skin. It crackles under tooth, both salty and savory. Most people would never know it was developed in a lab.

HOW TO MIMIC NATURE?

Over on the east side of the San Francisco Bay, where Finless Foods has its offices, the seven-person team has yet to determine the exact food media mix necessary to give their bluefin tuna the proper flavor, ruby-red color and healthful omega-3 fatty acids that consumers desire. But they do have one advantage over the researchers who produced that cultured hamburger five years ago: The Finless folks have figured out how to grow three kinds of tissues from bluefin stem cells: Muscle, fat and connective tissue. They even claim they can manipulate the amount of fat to mimic the lush flavor of otoro tuna.

Last year, Finless hosted a tasting of its first fish prototype, a cultured carp paste, which a local chef mixed with potato into a croquette. Selden and Wyrwas figured that, if they had produced a pound of this cultured carp, it would have cost $19,000, not including labor. A reporter for the Guardian sampled the croquettes and found them “both delicious and disappointing … I just about detect a pleasant aftertaste of the sea, though not fish as such.”

Flavor profiles are just one obstacle. Fetal bovine serum, or FBS, is an essential ingredient in the culturing process. The serum stimulates cells to divide and grow outside the animal’s body. The problem is, as the name implies, FBS is derived from fetuses removed from pregnant cows during slaughter, which, as Tetrick notes, connects “clean” cellular agriculture to a sometimes inhumane system that the start-ups are trying to disavow. Just, Tetrick adds, has developed its own plant-based serum to replace FBS.

Selden and Wyrwas with Finless say they’re working on their own alternative serum, too, which they plan to have ready in time to launch their first bluefin tuna product – a paste that could be used in sushi rolls and other dishes – by the end of 2019 or beginning of 2020.

“Barring major regulatory shake-ups,” Selden notes.

The small cellular ag community is still waiting to learn what U.S. agency, or agencies, will have oversight of the industry. Both the U.S. Department of Agriculture and the Food and Drug Administration can justifiably lay claim to the task. But Just’s Tetrick, for one, isn’t waiting around for the government; he’s looking toward Asia. If the politicians in America, Tetrick says, can’t clear a path to market for cultured meat, other countries will.

“And we will be buying our meat for the next 30 years from them,” he adds.

Consumer acceptance is another hurdle. One study, conducted several years ago when cultured meat was just entering the public consciousness, indicated that only a quarter of the participants would be willing to try the product. One factor was cost, which Finless is working to reduce. Selden and Wyrwas say they already can produce a bluefin tuna paste that compares favorably to retail prices at California sushi restaurants.

But even if consumers are hesitant, some meat producers and fish processors are already on board. Cargill and Tyson Foods, two of the largest meat producers in America, have both invested in Memphis Meats, another Bay Area cultured meat company. In an email to The Post, Uma Valeti, co-founder and chief executive of Memphis Meats, said that “we believe that Tyson can help us on our journey to scale up production and bring products to consumers.”

Henry Ichinose, owner of ABS Seafood in San Francisco, sees the potential of cell-based bluefin tuna. Standing in his warehouse on the famous Fisherman’s Wharf, oblivious to the chilly temperatures required to process fish, Ichinose says: “The oceans are already taxed. Nobody really knows how bad it is out there.” He thinks the seafood industry needs to embrace change to survive as the planet’s population continues to grow and its resources continue to shrink. But will chefs, home cooks and diners accept cell-based fish?

“I don’t see why not,” Ichinose says. “Ultimately, it’s cells dividing and growing, just like any other animal or plant.”