Tandoori Salmon. (Deb Lindsey/For the Washington Post)

Come dinnertime, wild salmon is an excellent choice. Many of the Pacific fisheries are well managed, and the fish itself is healthful and delicious. The problem is that there isn’t very much of it. Worldwide, our annual wild salmon harvest comes to about 2 billion pounds, which sounds like a lot until you divide it by 7 billion earthlings and come up with one serving per person per year.

What’s a salmon eater to eat?

Go back as little as 10 years, and the answer was definitely not farmed salmon. “It was the thing you weren’t supposed to buy,” says Jason Clay of the World Wildlife Fund, which established the Aquaculture Stewardship Council (ASC) to create sustainability standards for shellfish and fin fish aquaculture worldwide. When the industry was new, salmon farms were accused of polluting the oceans, spreading sea lice, fostering disease, allowing escapees and depleting the stocks of forage fish, up to seven pounds of which went into each pound of farmed Atlantic salmon. All of those accusations were true in some locales, and some were true in all.

But the salmon farmers did a funny thing. They listened. The survival of the industry depended on farmers cleaning up their act, and so that’s what they started to do.

By 2004, the WWF, working with the industry, had started to develop detailed standards. “The industry had begun to make improvements,” says Clay. Nearly a decade later, in June of this year, those ASC standards — more than 100 pages of them — were released. Farms that meet the standards will receive ASC certification, and many already have begun the process.

Meanwhile, the Monterey Bay Aquarium’s Seafood Watch program has noted industry improvements. It awarded its very first “buy” recommendation to an open-pen salmon farm in Chile named Verlasso, a joint project of salmon producer AquaChile and DuPont, the latter of which developed a genetically engineered yeast that produces a substitute for fish oil — an important part of the salmon’s diet — that’s biochemically identical to the real thing.

Although, for now, all farmed salmon other than Verlasso’s is rated “avoid,” Seafood Watch aquaculture research manager Peter Bridson acknowledges that salmon farming has come a long way. The program is in the process of evaluating farms in other areas and will come out with revised ratings at the end of the year. “Our understanding of the science has changed, and production practices have changed,” says Bridson. “Some of the older concerns are less of a concern.”

Areas the industry has focused on include:

POLLUTION — Feed, feces and other byproducts of high fish concentrations have become better controlled. “There’s an intuitive sense of feedlots. The gut reaction is that they’re a horrific source of pollution, but it seems now, from longer-term data sets, that the impacts are restricted to a small area around the pens,” says Bridson.

“That’s not to say that all the concerns have gone away,” he adds, but he notes that we know more about finding sites where farms work well and accurately predicting their impact. Our understanding of the carrying capacity of a region — the total number of farmed fish an area can support — is better, and the farms now let some areas go fallow to allow them to recover before fish are put there again.

ESCAPEES — There are a lot fewer of them, and concern about Atlantic salmon in non-native waters, particularly, has decreased. “It’s really quite clear that Altantic salmon are bad at colonizing outside their natural range,” says Bridson.

There are still problems within their natural range. But, according to Martin Krkosek, a biologist at the University of Toronto who studies the impact of aquaculture on wild populations, “the rate of escapees has declined dramatically.” There is evidence of farmed-salmon genes in wild Atlantic salmon, and the new ASC standards acknowledge the importance of keeping farmed salmon securely penned. To earn certification, farms must limit escapees to 300 or fewer per production cycle (about three years).

FEED CONVERSION — That industry average of as much as seven pounds of forage fish to grow one pound of farmed salmon has come down to 2.5 or 3 pounds, and the best ratios approach 1:1.

One reason for the improvement is simple: Cameras detect when the feed starts falling through the pen, indicating that the salmon have finished eating, and the feed is stopped. “That one innovation saved 40 percent of feed,” says the WWF’s Jason Clay.

The content of the feed has changed as well. Forage fish provide two essential products: fish meal, for protein, and fish oil, for omega-3 fatty acids. Twenty-five years ago, fish meal made up 50 percent of feed. Now, it’s 15 percent or even less, as other kinds of protein are being substituted. Plant sources of omega-3s are replacing some fish oil, but they don’t provide the long-chain omega-3 fats that are linked to health benefits. The industry is looking into alternatives such as algae to further cut reliance on forage fish.

Verlasso, the first producer to win approval from Seafood Watch, has reduced its fish-in, fish-out ratio to 1.3:1 by using the fish oil substitute made from yeast. Asked whether forage fish could be eliminated from salmon feed, Verlasso founder Scott Nichols was dubious. “They still need some fish meal as part of their feed,” he says, “because salmon are picky.”

Another concern about feed is added astaxanthin and canthaxanthin, carotenoids that give salmon its characteristic pink color. In the wild, fish get it naturally. On the farm, it has to be added to the feed (that’s what “color added” on the label means). Canthaxanthin, in large doses, can cause retinal damage in humans, and the FDA limits the allowable amount in salmon feed accordingly.

CONTAMINANTS — In 2004, a controversial study found higher levels of polychlorinated biphenyls (PCBs) and dioxins in farmed salmon than in wild salmon. That scared consumers, although the methodology of the study was criticized by health authorities, who continued to recommend eating farmed salmon. More recent research weighing the contaminant risk against health benefits from omega-3s concluded that every serving of salmon, wild or farmed, is a net positive.

Contaminant content varies too much to make generalizations; some farmed salmon has higher levels of contaminants than some wild, and vice versa. Increased reliance on plant oils in feed has naturally lowered contaminant content and, because farmed salmon generally has more omega-3s, it is sometimes a more healthful choice than wild.

PARASITES AND DISEASE — This is probably the most serious problem, particularly in areas where farmed salmon and wild salmon populations coexist. “Sea lice and viruses continue to be issues,” says Bridson, and the problem varies by region. “In Chile, there’s not much evidence [of impact]; there are no native populations. . . . There are several recent studies that show that there is still impact from sea lice in the Atlantic.”

Both Bridson and biologist Krkosek point to the difficulty in assessing the impact on wild salmon mortality of parasites and disease from farmed salmon. Krkosek explains that naturally, only 1 percent to 5 percent of wild salmon survive to spawn, so it’s difficult to assess an incremental change. Estimates of impact vary and are controversial.

Although “it’s fair to say that there’s pretty broad agreement that fish farms can raise parasite levels in wild fish,” Krkosek says, the fish farms are getting better at combating parasites. By using parasiticides just before the wild salmon come through the area, they decrease the chance of transferring parasites to wild populations. The chemicals, though, might have other effects. In high doses, they can harm crustaceans, but long-term, low-dose exposure is less well understood. A bigger problem is that the sea lice, which can be fatal to salmon, are beginning to show resistance.

The new ASC standards call sea lice a “pressing challenge.” They require monitoring of both wild and farmed fish populations, public release of data, and levels of lice “near zero during sensitive periods for wild fish.” There are also requirements to cull all fish in any pens harboring infectious salmon anemia, a lethal version of which has affected farmed salmon in many areas and could threaten wild populations.

ASC standards cover every aspect of salmon farming: water quality, feed composition, escapees, antibiotics, biosecurity, energy consumption, greenhouse gas emissions, worker wages and many others, including transparency. All of the major producers worldwide — who, among them, produce 70 percent of the nearly 5 billion pounds of farmed salmon harvested annually — have signed on to them and have committed to compliance by 2020.

Salmon farms will never have zero impact on the environment. Despite industry improvements, Krkosek said, he still avoids farmed salmon because be believes that wild is a more sustainable choice. And he points out that other fish, such as tilapia, can be farmed more efficiently than salmon. But the WWF’s Jason Clay also comes at the problem from an environmental perspective, and he has concluded that, in the face of a growing population that needs to be fed — and likes salmon — the answer isn’t to oppose salmon aquaculture. It’s to make it sustainable.

Everyone, on both sides, agrees there is work to be done before that goal is met. But everyone, on both sides, acknowledges that progress is being made.

Come dinnertime, that’s good news.

Haspel writes and blogs (at starvingofftheland.com) about food and health from Cape Cod, where she also fishes. She’ll join today’s Free Range chat at noon: live.washingtonpost.com.