I contemplated the simple sandwich on the plate in front of me: a beautiful slab of glistening rainbow trout, crisp lettuce, and a freshly baked French roll. The trout skin was lightly seared and seasoned. The pinkish meat was firm and toothsome. I genuflected briefly, then two-fisted the thing and took a big bite. A slightly smoky, sweet flavor gave my taste buds a sensation long denied. I chased it with a slug of Fort Point ale. Soon, both fish sandwich and beer were gone. I am a vegan, but I was untroubled. Eating the trout seemed like the right thing to do.

The journey to that sandwich began a few months earlier with a question from a friend who wants to eat sustainably: What fish can I eat? My response was the same one I have given for years: You should eat no fish at all.

I haven’t always felt this way. I grew up on the East Coast, spent a lot of time on the Atlantic Ocean, and ate more than my share of salmon, tuna, crabs, scallops, and whatever other seafood was on offer. But a few years ago, as I began to write extensively about the relationship between humans and animals, especially the lives of marine mammals in captivity, my thinking changed. What we eat affects the health of the planet as much, if not more, than what kind of car we drive or where we set the thermostat. The more I learned, the more I came to believe that the single most powerful choice an individual can make is to stop eating animal protein. So, in 2010, I became a vegetarian. After about a year, realizing I could manage without cream in my coffee and eggs for breakfast, I took the next step and went vegan.

I didn’t evangelize about it. I made my choice; others could make theirs. But I noticed that when I was asked about my reasons, there always seemed to be special interest in the question of fish, which even the vegetarian-inclined still want to eat. Setting aside my vegan concerns about fish welfare—laugh if you like, but then go watch a beautiful, fighting-mad bluefin tuna being gaffed on YouTube—anyone who has been paying attention knows a dispiriting truth: wild fish are being decimated by the world’s increasingly teched-out, 4.7-million-vessel fishing fleet. The United Nations’ Food and Agriculture Organization estimates that 90 percent of marine fish stocks are either fully exploited or overexploited. Meanwhile, fish farming, with its reputation for overcrowding and antibiotic-laced, fecal-polluting practices, doesn’t sound like a very appealing solution. And there appears to be no shortage of crooks and liars, from fraudulent distributors to fact-twisting chefs and fishmongers, at just about every link in the distribution chain. According to a recent Associated Press investigation, you can’t even be sure your supermarket isn’t stocking seafood caught by fish pirates in Indonesia, who kidnap and enslave impoverished Southeast Asians to work on their boats. The slaves work untenably long hours for little to no pay, are locked up at night, and are often beaten if they don’t perform as told. Where’s the argument for eating fish in all that?

But the questions kept coming, and I knew my personal position didn’t provide realistic or helpful advice. Seafood is an indispensable source of protein and omega-3 fatty acids—good for the heart and brain—on a planet whose population will need a lot of protein as it swells toward a projected 9.3 billion people by 2050. My friends and family and most of the world will continue to eat fish, and despite all the seafood guides and journalism on the subject, people are more confused than ever—about whether to eat wild or farmed, about which fish are healthier, about the implications of fish consumption for the oceans.

“Just tell me what fish I can eat,” my mother pleaded. So I set out to produce a better answer, and what I learned surprised me. Not only might fish offer the best, and least ecologically damaging, solution to global food insecurity in a flesh-eating world, but some seafood is now produced so efficiently that even a vegan might be tempted to rethink his absolutist vows.

Wondering how to put it all into practice? We asked the experts and distilled their advice down to six rules of eating healthier, more sustainable seafood

I. Consider the Source

The math is simple. Global demand for fish is at about 158 million metric tons annually (and growing), which is about twice the already worrisome 80 million metric tons we take from the oceans. Against that unrelenting pressure, it seems reckless to keep scarfing down wild fish.

But Kenny Belov, a burly, high-energy 38-year-old who co-owns TwoXSea, a fish distributor on Fisherman’s Wharf in San Francisco, is quick to disabuse me of the idea that wild fish should be completely off the menu. Belov and his partner, Bill Foss, a cofounder of Netscape, caught my attention by lobbing grenades at their own industry in a probing San Francisco magazine story about seafood sustainability (or lack thereof) a few years ago, and they have been outspoken advocates for rethinking our approach to eating fish ever since. Most days, Belov shows up at the TwoXSea warehouse at 3 A.M. to supervise the shipping of 1,500 to 2,000 pounds of seafood to top restaurants and stores in the Bay Area and a few other cities—all of it sustainably caught. He is as obsessive and conservative as they come in his views about whether any fish population is healthy enough to be fished and whether the catch method damages other populations or the ocean ecosystem.

Atlantic cod? “It’s overfished and mostly caught by bottom trawling, which is like clear-cutting the seafloor,” Belov scoffs. “I wish we would leave Atlantic cod alone. They need more time to recover after what we did for so many generations.”

Ahi tuna? “Almost all of it is caught on pelagic longlines, which are 40-plus miles of floating line dangling a baited hook every three feet. Longlines catch everything else in the habitat.” That’s called bycatch, a somewhat bloodless term for a fishing method that indiscriminately hooks as many as 150,000 sea turtles annually, along with tens of thousands of seabirds, whales, sharks, dolphins, and porpoises.

The Alaskan pollock so often used for fish fingers? “Caught by fishing vessels that are 100 to 200 feet long,” Belov says. “Their huge nets pull in lots of other species, like squid and salmon.”

So how is it that Belov has a warehouse full of sustainable wild fish? Because he scoured the West Coast fleet for fishermen who were tapping into healthy stocks the right way. Once he found them, he paid them a premium for their catch.

Belov walks me around TwoXSea’s facilities. On the day I visit, he’s got Coho salmon—beautiful, powerful, silvery fish—that were caught by the High Hope, out of Sitka, Alaska, using a method called trolling, in which a few lines are dropped behind a boat and pulled in one by one, reducing the risk of bycatch. He’s also got black cod, targeted with baited lines set by the Eagle III in Coos Bay, Oregon, and night smelt from Eureka, California. It was harvested by a fisherman named Dude Gifford, who dips a net stretched across an A-frame of poles into the surf. “We don’t sell anything that doesn’t come directly from a fisherman,” Belov says.

The real problem, he believes, is not that sustainable fish stocks aren’t out there. It’s that a lot of unsustainable fish is passed off as OK. Belov and Foss are also partners in a Sausalito restaurant called Fish, which Foss opened in 2004, promising customers that everything on its menu could be eaten with a clear conscience. They launched TwoXSea five years later because they got fed up with all the dishonesty they encountered trying to supply fish for Fish. “There is so much seafood fraud going on when it comes to labeling species, its origin, and the captain and vessel,” Belov says. He tells me about the time he went looking for scallops that hadn’t been caught by dredging, a process that tears up the seabed. He met with two distributors from New York City and explained that he would need traceability, vessel names, and documentation to confirm the catch method. Their glib responses made it clear that the information would be meaningless. “We have a long list of boat names,” they told Belov. “Just pick any one you want.”

Belov is not being paranoid. A 2013 study by Oceana, a nonprofit that campaigns to protect and restore the world’s oceans, concluded that 33 percent of fish in the United States is fraudulently labeled to increase profits. (There is now a presidential task force trying to address the problem.) To emphasize the point, Belov walks me outside onto the pier. He gestures toward two swordfish-longlining vessels that are tied up alongside another fish distributor’s warehouse. “See those boats?” he says. “Because they unloaded their swordfish here, it can be labeled PRODUCT OF CALIFORNIA, which means it will be sold to diners as local or San Francisco swordfish, even though it was caught 1,500 miles away in the middle of the Pacific.” He says restaurants will probably describe the swordfish as “line caught,” which sounds positively artisanal.

But both Belov and Foss believe that things are getting better and that the success of TwoXSea is in large part due to a younger generation of chefs who are making decisions based on an ethical rather than a financial stance. “People are much more aware of what is going on with dishonesty in seafood and all the fraud,” Belov says. “But I still think we have a tremendous way to go,” adding that when it comes to seafood sustainability, personal choices matter.

By the time Belov is done with me, I have a few new beliefs. One is that you can eat some wild seafood without trashing the oceans—wild-caught Alaskan salmon, for example, is a well-managed fishery. Another is that, in a perfect world, we would all know the name of the fisherman reeling in our fish, but that’s not the reality for most of us. There is so much complexity in catch methods, fishery management, and the supply chain that even a conscientious seafood lover might as well throw a dart at the menu. Luckily, there’s an app for that.

II. Red Light, Green Light

The Monterey Bay Aquarium, two hours south of San Francisco, is housed in an old sardine cannery, and one of its feature attractions is a 335,000-gallon viewing tank that contains a forest of California kelp. Jennifer Dianto Kemmerly, 42, the director of the aquarium’s Seafood Watch program and an environmental scientist, sits down at a table in the cafeteria, her hair still damp from an early-morning dive into the kelp. There are a lot of seafood standards out there, but Seafood Watch’s are arguably the most independent, comprehensive, and rigorous, and it has taken a tough, truth-telling approach to assessing fisheries’ sustainability. “When we started red-rating fisheries in our own backyard, that was a really bold move,” Kemmerly says. “But after ten years lots of fish came back, and the message was heard by fisheries up and down the coast.”

Seafood Watch was launched in 1999, after aquarium visitors started walking off with cafeteria display cards listing a few which-fish-to-eat recommendations. Seeing an opportunity, the aquarium put together a program to produce detailed, science-based evaluations of specific fisheries to publish on its website and in its app. Fish from well-managed, abundant populations—caught in a way that caused little harm to other species or habitat—got a green Best Choice designation. Fish that were OK to buy but were harvested in a way that caused Seafood Watch some concern, garnered a yellow Proceed with Caution tag, since changed to Good Alternative. Fish from a badly managed, overfished, or destructive fishery got called out with a red Avoid label.

Today, Seafood Watch has more than 2,000 unique recommendations, on both wild and farmed seafood, updated at least every three years. Of those, 22 percent are Best Choice, 38 percent are Good Alternative, and 40 percent are Avoid. More than 1,000 North American companies use this information in their buying decisions, and more than a million users have downloaded the app.

Seafood Watch data affirms that, in well-managed U.S. fisheries, sustainable wild fish is available. The program’s scientists recently assessed 129 species, which account for three million metric tons of catch annually. Of that haul, more than half a million tons, or 19 percent, are rated Best Choice. And just 2 percent are Avoid, which leaves 79 percent in the yellow Good Alternative limbo—a rating that worries some seafood advocates because it sounds too much like a buy recommendation. I mention to Kemmerly that eating a lot of yellow-rated seafood doesn’t seem very sustainable. “It isn’t,” she says. “But if we’d set the bar that you could only buy green, I don’t think this market-based-incentive movement would be pragmatic.”

Still, the truly conscientious seafood eater should aim to buy Best Choice, taking a precautionary approach and reducing our impact on fragile and complex ocean ecosystems. Plus, the more sustainable the rating, the less likely you are to eat seafood caught by fish pirates. “If there is a known IUU”—illegal, unreported, unregulated—“issue, then that will result in a red Avoid rating,” says Kemmerly. Though she cautions, “Unless there is full traceability from boat to plate, one can’t be sure if the product comes from a vessel that engaged in IUU activity.”

Eating in the green zone takes some dedication. That is, when you can find it and afford it—wild-caught Alaskan salmon can cost upwards of $15 a pound. And even with the app, you’ll have to ask a lot of questions.

Take albacore tuna. If it was caught by trolling or with a pole, in the North Atlantic or Pacific, Seafood Watch rates it a Best Choice. But if it was caught anywhere in the world on a longline—except off Hawaii and in the U.S. Atlantic, which have strict bycatch limits—it gets a red Avoid rating. Will the person selling you the fish know how it was caught and where, and can you be sure that person’s information is accurate?

Clear labeling at supermarkets and restaurants would make life a lot easier for consumers, and that is starting to happen. Seafood Watch and the Safina Center, a New York–based ocean-conservation nonprofit run by marine ecologist Carl Safina, have partnered with Whole Foods on labeling, which has sold no red-listed wild seafood since 2012. Safeway, Target, and other supermarkets are working to implement similar changes. Meanwhile, 145 restaurants listed on the Seafood Watch site have also gone no-red.

What about when there’s no labeling at all, which is the case in most restaurants and stores? Use the app and ask questions about catch method and location. “It shows businesses that they have to stay on it,” Kemmerly says.

After she takes off for a meeting, I check out the aquarium’s cafeteria menu. The fish tostadas, at $15, are made using albacore tuna. Troll- or pole-caught in the Pacific, and not by longline, I assume, after glancing at my app. But I’d have to ask.

III. Modern Farmer

As diligent as you might be with the Seafood Watch app, there isn’t enough sustainable wild fish to feed the growing world. To fill the gap, many suppliers have turned to aquaculture, which has exploded from producing 1.6 million metric tons in 1960 to 66.6 million metric tons in 2012 and now provides about half of all the seafood we consume.

Farmed fish has confused consumers for years. Is it healthy? Bad for the environment? “Like any farming, aquaculture can be done well or it can be one of the most destructive things,” says Safina, who has fought for the oceans for more than two decades. “Particulars matter. There is sustainable aquaculture, and there are also fish farms that have wrecked coastal zones and mangroves and done bad things to poor people.”

Nearly 60 percent of fish farming takes place inland, in ponds and closed aquaculture systems, and produces finfish like tilapia, catfish, and carp, as well as shrimp. Pond farming conjures up images of overcrowded, feces-filled pools that require chemicals and antibiotics. But these days, most U.S. inland farming is done in line with good, healthy standards. U.S.–farmed catfish, salmon, and shrimp are all Seafood Watch Best Choices. Tilapia is also popular, and if it’s farmed in Canada, the U.S., or Ecuador, it too rates a Best Choice. Farmed tilapia and carp from China and other parts of Asia often get dinged to yellow for questionable chemical use and waste-management practices.

It’s the seafood raised in marine environments—especially salmon and shrimp—that has given aquaculture its controversial reputation. Waste, chemicals, antibiotics, and unused feed pollute nearby waters, farmed-fish escapees from these net pens threaten to spread disease and alien DNA to wild populations, and sensitive coastal environments become industrialized.

Marine fish farming also has a resource-use problem. It’s known as the fish-in, fish-out (or FIFO) ratio, and it’s an important measure of sustainability. Consider farmed salmon. According to Seafood Watch, it can take three pounds of smaller forage fish, like anchovies, menhaden, and sardines, to create the feed needed to produce a pound of salmon; even the most efficient farms have a ratio of 1.5:1. That’s not a particularly sustainable way to produce fish. For all these reasons, until recently Seafood Watch slapped most finfish farmed in marine environments with a red Avoid rating.

Kemmerly, however, believes that we’re on the verge of a paradigm shift, thanks to advances in aquaculture over the past decade or so. “It can be done responsibly,” she says.

To see what the future could look like, I seek out Josh Goldman, CEO of a company called Australis Aquaculture. I find him, bespectacled and busy, in a cavernous two-acre warehouse complex in Turners Falls, Massachusetts, on the Connecticut River, which serves as company headquarters. Inside, it’s warm and humid, and the air is redolent with the sharp smell of a million fish, the sweet aroma of pellet feed, and the earthy fug of damp concrete.

Goldman walks me around, past massive tanks enmeshed in a complex web of -filters and industrial piping, until we stop at a Jacuzzi-size tank teeming with beefy-looking fish. They’re called barramundi, which is Aboriginal for “large-scaled fish.” In the wild, they can be found from northern Australia up through Southeast Asia and beyond, all the way to the coastal waters of India and Sri Lanka. These fish, though, did all their growing in Goldman’s tanks, which collectively contain 2.5 million gallons of water. Over 300 days, they were transformed from tadpole-size hatchlings, weighing just one-third of a gram, into meaty fish weighing one to two pounds.

Goldman has been experimenting with aquaculture since he first got hooked on the natural sciences at Massachusetts’s Hampshire College in the early 1980s. He thinks we’re in a transition from wild fish to farmed fish that is similar to the transition 13,000 years ago from hunting meat to domesticating it. “But we have the opportunity to learn from the mistakes,” he says.

To Goldman, the key is domesticating the right fish. After years of trying to improve on farming methods for popular species like striped bass, Goldman created a matrix of qualities that would make for a better farmed fish and, in 2000, started prospecting the world to find it. He ticks off some of the reasons barramundi fit his better-fish matrix: they have the high fecundity of a marine fish (large females can produce up to 40 million eggs in one season); they travel up rivers to forage, which means they are tough and adaptable; and they eat a flexible diet that includes plants. By experimenting with feed compositions over the growth cycle, Goldman managed to drive the FIFO at Turners Falls down to an impressive 0.98:1, meaning that it takes less than a pound of wild fish to produce a pound of barramundi. Seafood Watch approved, rating Goldman’s indoor barramundi, which has a sweet, buttery flavor and is packed with omega-3’s, a Best Choice.

Goldman’s next challenge was to take barramundi out of the tanks and grow it to scale in a marine net-pen farm. “People were looking at aquaculture in coastal zones as an environmentally harmful activity,” he says. “I wanted to right that wrong.”

He went prospecting again and found the location he needed in Van Phong Bay, on Vietnam’s southeast coast. Australis Aquaculture Vietnam started production in 2010. Today it turns out some 2,000 tons of barramundi a year—more than three times the output at Turners Falls, at roughly half the cost—and has permits to scale up to 10,000 tons annually. Its frozen fillets are shipped to more than 4,000 stores across North America and cost a reasonable $9 a pound at my local Whole Foods. Careful net-pen siting and low fish densities reduce pollution and the threat of disease. Antibiotics are used sparingly and only as needed, and escapees are rare and not much of a concern, since local barramundi were used as the brood stock in Australis’s hatchery. Last year, after careful inspection, Seafood Watch gave Australis Aquaculture Vietnam’s barramundi the first green Best Choice rating ever granted to a marine net-pen fish.

“I don’t think there is any question that barramundi can be a real player in global supply,” says Goldman, who already has his eye on another fish that looks farm friendly, though he won’t say what it is yet.

Meanwhile, for the consumer, it’s now much easier to find farmed Best Choice options. Of the 176 farmed recommendations on Seafood Watch, 52 percent are Best Choice, 40 percent Avoid. From inland farms, there is rainbow trout, Arctic char, and salmon. Then there’s marine net-pen fish like Goldman’s barramundi and New Zealand’s newly green-rated Chinook salmon. If that’s hard to find, Best Choice farmed tilapia and catfish, while not high in -omega-3’s, are still a healthy and affordable protein.

IV. Vegan Fish

After visiting Turners Falls, I start imagining a world increasingly fed by innovative aquaculture. It’s a hopeful vision, except for one glitch—the FIFO problem. Most of aquaculture relies on forage fish to provide fish meal for protein and fish oil for omega-3 fats, which they get from eating microalgae and phytoplankton in the ocean. But global forage-fish harvests have maxed out at 20 million to 25 million metric tons, a volume that some experts worry is too high. The industry has been doing a better job scavenging from fish-processing waste, but there are still a limited number of forage fish that can be taken from the sea, which is a serious impediment to sustainable aquaculture growth.

This problem inspired Bill Foss, Belov’s partner at Fish and TwoXSea, to ask fish farmers a question that could change everything: Why do you need to have fish in your feed?

“It’s been a well-known fact that the amount of fish needed to feed a fish is a pretty asinine way to produce a fish,” says Foss, sitting in a coffee shop in Petaluma, California. Foss, who is 50 and has little patience for the shortsightedness of the human race, tells me that about five years into supplying fish for his restaurant, Belov found out that their farmed Best Choice tilapia and trout—both of which can feed on plants—weren’t vegetarian. Neither wanted to serve fish that consumed overstressed wild forage-fish stocks. Besides, the cost of fish meal and fish oil has more than tripled over a decade.

To formulate a novel fish-free feed, Foss turned to a freethinking scientist: Rick Barrows, at the USDA’s Agricultural Research Service in Bozeman, Montana. Barrows is a slim, deliberate man, with wire-rim glasses and a wide mustache. His official title is research physiologist. That’s a sterile description for someone who has been on a decades-long quest to find the holy grail of aquaculture: replacing fish meal and fish oil in the feeds that sustain the industry. His research mainly takes place inside a lab tucked into the foothills outside Bozeman, where 320 holding tanks are arrayed in neat clusters, most containing small populations of rainbow trout. When Barrows shows me in, two assistants are netting, weighing, and grading the growth of some trout—“the white rats of aquaculture,” Barrows jokes—that are eating a pistachio-based feed made from deformed nuts rejected for human consumption.

Barrows has looked for an alternative to fish-based meal in everything from corn and soy to pistachios and peas. He’s even experimenting with black soldier fly larvae. “It was fairly easy to come up with a variety of new protein sources,” he says.

Last year, Barrows made more than 150 feeds for 22 fish species, and his research has proved that at least eight popular fish-farm species, including trout, salmon, and sea bass, can grow just as fast, or faster, on fish-meal-free feed. “If we can do it with those eight, we can do it with any fish,” he says.

Replacing fish oil, the key to providing the two omega-3’s—DHA and EPA—that are associated with good brain and heart health, was more difficult, Barrows says. We head out to look at the feed mill, where Barrows and his team homebrew their experimental feeds. They start with a mash of whatever ingredients they’re using and then run it through a massive twin-screw extruder, a machine used to make everything from dog food to Froot Loops. Today it’s spitting out small orange pellets, which drop into a large plastic garbage can. I suggest to Barrows that people should skip the fish and just consume the pellets directly. He grins. “Sure, you could eat it instead of cereal. Foss gave it a try.”

Foss and Barrows eventually solved the omega-3 problem by adding an algae-based DHA supplement, often used in baby formula, to the vegan feed, which also contains pea protein and flax oil (and love, Belov jokes). Since 2010, they have tried it out on successive generations of rainbow trout at a farm Foss and Belov purchased in the Sierra Nevada near Susanville, California, run by David McFarland. The results have been interesting. The trout show a good DHA profile and also seem to be converting at least some of the DHA into EPA. “So DHA is all we need—the trout does the rest. Kinda cool, huh?” says Foss. “Plus, the algal DHA has none of the mercury or PCBs that come from forage fish, so we’re ahead in the health game.”

Unfortunately, the DHA supplement is expensive, which means that the vegan feed is pricey—$1.50 a pound rather than the 80 cents a pound for standard forage-fish-based feeds. That adds about 15 percent to the cost of the trout. If the costs of fish meal and fish oil continue to climb, the price differential will shrink or disappear. More demand and scaled-up production of the vegan feed would also bring the cost down. “The biggest thing holding us back is that someone like Whole Foods hasn’t said, ‘We want a million pounds of what you’ve got,’ ” Foss grumbles.

Australis’s Goldman, among others, is also experimenting with alternate plant-oil sources that might produce omega-3’s in fish. Until more farmed fish fed a vegan diet are widely available, try to add in FIFO-light options, like tilapia and catfish, to your menu. There’s also another solution.

V. The Seafood Chain

It’s tempting to think that as long as something is a green Best Choice, you can eat as much as you want. But Barton Seaver, a former chef who is now the director of the Healthy and Sustainable Food Program at Harvard’s T. H. Chan School of Public Health, would like seafood lovers to be conscious of more than a rating. Seaver, 36 and lanky, is a thoughtful presence who carries the slightly haunted air of a man who is wearied by all he knows. To Seaver, smaller portions and variety are key elements of sustainability. “What’s important is eating less seafood more often,” he says, noting that we get more nutrients than we need when we chow down on a large slab of fish.

Seaver spent his summers as a child fishing and crabbing on the Chesapeake Bay’s Patuxent River. In 2007, when he was 27, he opened Hook, a popular seafood restaurant in Washington, D.C. He got interested in sustainability after he called up a seafood supplier to place his first order. “Send me bluefish, crab, oysters, rockfish,” he said, eager to feature all the Chesapeake bounty he had loved as a boy. “Kid, what are you talking about?” the supplier responded. “We ate all those.”

“I realized that natural selection in our world is firmly holding a fork,” Seaver says.

I met with Seaver in Portland, Maine, in January, to see his friend Gary Moretti’s Casco Bay mussel farm. Moretti, a 63-year-old with the cheerful spirit of a man who loves being on the water, co-owns Bangs Island Mussels with his son, Matt. As he gets ready to back his converted lobster boat away from the wharf, a seal pops its head up. “Hey, Loretta, get out of the way,” he calls. “Don’t worry, I’ll give you a fish later.”

Within minutes we’re chugging toward Clapboard Island, where Bangs Island keeps four mussel rafts—40-by-40-foot steel frames dangling 400 fuzzy ropes to a depth of 30 to 40 feet, for mussels to adhere to. In the relative warmth of the boat’s wheelhouse, Moretti explains that siting a mussel farm is all about thinking like a mussel. You want plenty of phytoplankton, minimal sediment, and nice current-driven water flow that is uncontaminated by golf-course or industrial runoff. “But this is Casco Bay. Here you can pretty much grow mussels anywhere,” he says of the beautiful seascape around us.

We pull alongside a mussel raft, and Moretti and Seaver hop onto the ice-slicked girders. They tug up some lines to show me the thick clusters of blue-black bivalves growing under the raft. It occurs to me that I am looking at the ideal farmed protein. It requires no feed beyond the nutrients in the water, so it has a perfect FIFO—no fish in for lots of shellfish out. It filter-feeds, improving water quality instead of polluting it. There are a multitude of coastal zones around the globe where mussels can grow in abundance. And while they don’t pack the omega-3 wallop that salmon does, they do deliver a shot—three servings a week gets you to the recommended minimum. Another bonus: being low on the food chain, mussels have little mercury, more than 30 times less than larger predator species like swordfish and tuna.

“The benefit of mussels is you can’t be greedy and wolf them down,” says Seaver. “There is an elegance and mindfulness to eating them.”

If ever there was an animal protein that a vegan could adopt, the mussel is it, I decide. Because of their rudimentary nervous system, they likely feel no pain and would give me some DHA and EPA omega-3’s, which are mostly absent in the vegan diet. (Flaxseed, popular with vegans, provides a different omega-3.) Farmed mussels are a Seafood Watch Best Choice, but I start to think of them as a Super Green Choice.

I realize that an interesting thing happens when you approach seafood with sustainability and health in mind: you end up eating a diverse diet that pushes you lower down the food chain and away from the rut of salmon, shrimp, and tuna, the most commonly eaten seafood in the U.S. The healthiest, most sustainable seafood that’s also high in omega-3’s and low in mercury? Wild Pacific sardines, a Best Choice. Other green-listed options that have decent omega-3’s and low mercury: U.S.–farmed striped bass, U.S.–farmed rainbow trout, farmed Arctic char, Australis’s barramundi, and wild or farmed mussels. Wild or farmed oysters, farmed scallops, farmed tilapia and catfish—use your Seafood Watch app to find the Best Choice for these—are also highly sustainable and provide good protein, some omega-3’s, and little mercury. For an occasional treat and a massive shot of omega-3’s, have some Best Choice wild Alaskan salmon every once in a while. That’s a pretty green way to get all the protein and omega-3’s you need without going too heavy on the FIFO scale.

VI. Navigating the Marketplace

When I get home, I make a run to Costco to shop at the seafood counter with fresh eyes. It isn’t easy. I see a few Marine Stewardship Council CERTIFIED SUSTAINABLE SEAFOOD stickers. MSC is a nonprofit that certifies fisheries that meet its sustainability and traceability standards. Though the standard has critics, Seafood Watch recommends most MSC-certified fisheries and says that they are equivalent to at least a yellow Good Alternative. Still, most of Costco’s seafood is unlabeled. So it’s me and the Seafood Watch app.

I work my way down the cooler. Farmed salmon from Chile—red, with an unappealing label that says “color added.” (Some farmed salmon are fed the carotenoid astaxanthin to give their flesh the orange color they’d normally get from eating shrimp and krill in the wild.) MSC-certified wild Atlantic cod—yellow. Ahi tuna from the Marshall Islands in the western Pacific—there’s no information on how it was caught, and the Costco employee stocking the cooler doesn’t know, so I worry it was a longline and would rate red. I find some tilapia farmed in Honduras, but Seafood Watch is still in the process of rating it. In the frozen section, it’s more of the same. Most everything seems to fall into that yellow, not-egregious-but-not-really-OK category. I call Belov for guidance. “It’s so complicated, and there are too many standards,” he says sympathetically. “We have a long way to go, but all we can do is keep pushing and asking questions.”

Later, when I check in with Bill Mardon, Costco’s assistant general merchandising manager in fresh seafood and poultry, he explains that Costco doesn’t sell 12 of the most overfished species and is working toward having more of its seafood supply meet MSC or Aquaculture Stewardship Council standards. (ASC was created by the World Wildlife Fund and the Sustainable Trade Initiative; its certified catfish, shellfish, and shrimp equate to at least a Seafood Watch Good Alternative.) “One hundred percent of our tilapia is ASC certified, and this year we are getting going on salmon and shrimp,” Mardon says. “Call me in two, three, or four years, and I hope we will be at 80, 90, or 100 percent.”

A few days later I hit up my local Whole Foods, and the experience is a lot simpler. Whole Foods sells MSC Certified Sustainable wild fish and puts Seafood Watch labeling on any wild fish that MSC hasn’t certified yet. A graphic atop the counter explains the color coding and tells shoppers that if it’s red, “We don’t sell it!” “The whole point of having these high standards is that any choice you make is a responsible one,” says Carrie Brownstein, the global seafood quality-standards coordinator for Whole Foods and a former research coordinator at the Safina Center.

I see a lot of choice: croaker, halibut, cod, and hake. Most of the Seafood Watch labels are yellow—which reflects the state of wild fisheries—but I spot some green-rated Best Choice wild Spanish mackerel for $9 a pound. (Brownstein told me that what’s available varies seasonally and regionally, which affects how much green-rated wild fish you might find at any given time.) There’s also a lot of farmed seafood—tilapia, catfish, shrimp, Arctic char. Whole Foods has certified it with its own Responsibly Farmed logo, which requires aquaculturists to meet a strict standard on pollution, chemical and antibiotic use, and other criteria. Most of it, as far as I can tell, would earn a green, and I can see a Best Choice menu here that my mother could happily live on. There’s a good supply of oysters, mussels, and clams, both farmed and wild, and the farmed tilapia and catfish. She might also be tempted by Spanish mackerel. In the frozen section, I find some of Australis’s Vietnam barramundi fillets.

“Anything you buy regularly, I’d stay in the green,” says Safina, who tries to eat only seafood that he catches himself. “But if it’s something you splurge on once a summer, then yellow is probably OK.” Still, he adds: “If you really want to be conscientious about seafood, you should eat rice and beans.”

I agree. While I’m encouraged by the promise of better fisheries management and aquaculture innovation, I still don’t intend to eat fish, for the same reasons I stopped in the first place. I believe in author Wendell Berry’s observation that “how we eat determines, to a considerable extent, how the world is used,” and I want to use the world less. Fish raised in tanks, no matter how well cared for or sustainable, are inevitably the human processing of living things. Even TwoXSea’s delicious vegan rainbow trout, which spend their lives high in the Sierra Nevada in perhaps the most beautiful farming environment on the planet, tug at my conscience. To see them idling in concrete raceways instead of chasing an insect hatch is a reminder that farmed life is a faint facsimile of life in the wild. But I will maintain my exemption for mussels, which in my opinion are an ethically defensible animal protein.

Regardless, a sustainable approach to seafood has a lot to offer. Andy Sharpless, the CEO of Oceana and the coauthor of a book about fish called The Perfect Protein, says that if we stopped overfishing and gave spawning stocks a chance to rebuild, most fisheries would fully recover within ten years and allow sustainable harvests that are 20 to 40 percent higher than the current global catch. “A well-managed global ocean could provide the equivalent of a healthy seafood meal for a billion people every day forever,” he says.

Meanwhile, it’s worth asking: How many apps rate other kinds of meat according to its environmental footprint? “If we are scared away from buying farmed salmon because it is red-listed, what do we do instead? We go buy ground beef,” Seaver points out. “If you look at the environmental factors of protein by category, often those other proteins—beef, pork, chicken—have a larger impact than even the worst of the seafood products.”

So with apologies to Michael Pollan, I’d recommend this for conscientious nonvegans: Eat a lot less meat and a lot more sustainable seafood, wild when you can verify it, and lower on the food chain, but mostly farmed, particularly mussels, clams, and oysters.

On the way back from Clapboard Island with Seaver and Moretti, stamping our feet in the wheelhouse to stay warm, I fantasized out loud about a universal food-labeling system that would rate everything according to its environmental impact and health benefits. Of course, I’d like an animal-welfare rating, too, but I don’t want to get carried away. Moretti told me about his fantasy: converting used offshore drilling platforms into massive mussel farms to help feed a growing world population. “In my dream, Bill Gates or Warren Buffett calls me up and says: ‘Hey, Gary, I really think we should do that oil-rig-mussel thing. Here’s $100 million.’” We smiled at the improbabilities—and the potential.

Correspondent Tim Zimmermann (@earth_ist) was an associate producer of the documentary Blackfish. He wrote about zoos in March.