We now need to eat two portions of farmed salmon to equal the amount of omega-3 intake that we would have gotten just five years ago, says a study from Stirling University in Scotland. The change appears to be due to a reduction in the amount of ground-up anchovies added to their feed.

Farm-raised and wild caught salmon contain the same amount of cholesterol, but wild salmon have half the fat of farmed in a typical half-filet serving.

Salmon farming is only about four decades old, but it is the fastest-growing food production system in the world according to WWF. Globally, about 3.5 million tons are caught or raised each year, and salmon accounts for 17 percent of the global seafood trade. About 70 percent of the world's salmon production is farmed.

Salmon is among the most popular seafoods in the U.S., where we eat 2.3 pounds per person each year. We prize salmon for its omega-3 fatty acids. National Oceanic and Atmospheric Administration (NOAA) says that consumption of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are key omega-3s found in seafood, may help to prevent high blood pressure, heart disease, certain types of cancer, clinical depression, anxiety and macular degeneration. Of the salmon consumed in the U.S., half is farm-raised.

Wild catch vs. farm-raised seafoodMarine Harvest

NOAA also states that farmed seafood is safe and healthy to eat, but many have questions about the practice. Crowded conditions in the pens used for raising salmon provide an ideal breeding ground for sea lice, which are now invading wild Alaskan salmon populations. Sea lice can be lethal to juvenile pink and chum salmon. In farms in some parts of the world, a pesticide is used to combat sea lice that is toxic to marine life and banned by both the European Union and U.S. Food and Drug Administration.

The greatest concern, though, centers around interbreeding of farmed and wild salmon. In September, a study by Canada's Department of Fisheries and Oceans found that more than 750,000 salmon have escaped from fish farms in Newfoundland since aquaculture began, and that these fish are breeding with wild salmon and producing offspring. A separate study in Norway found that half the wild salmon tested had genetic material from farmed fish. It's unclear which traits might impose themselves on wild salmon, but farm-raised fish are bred to grow big and to grow fast.

Farm-raised and wild caught salmon contain the same amount of cholesterol, but wild salmon have half the fat of farmed in a typical half-filet serving. Farmed fish also deliver three times the saturated fat as wild. But to feed a growing global population and provide the omega-3s they need, wild fisheries may not be up to the job.

On the West Coast of North America, salmon are in trouble. The number of endangered or threatened salmon runs on the Columbia River system has jumped from four to 13. In British Columbia, the sockeye salmon run this year was the lowest ever seen. Alaska's pink salmon catch is the worst it has been in 40 years.

Farmed salmon can still be ecologically friendly. According to WWF, it takes 10 to 12 pounds of feed to produce one pound of beef, but less than two pounds to yield a pound of salmon. Recognizing the need for fish farming, WWF worked to create global standards for salmon aquaculture designed to address the worst impacts. The Aquaculture Stewardship Council (ASC) now manages the standards and provides a certification program that retailers and consumers can use to ensure they are buying responsibly-farmed salmon.

The standards require farms to minimize diseases and the occurrence of sea lice while limiting the use of medicines to a set of strict conditions. Farms are also required to monitor and control water quality and prevent fish escapes as much as possible. The ASC also limits use of wild fish as feed, which is now seen to be responsible for reducing omega-3 levels in farmed salmon.

"We, and many others, are working very hard at developing new sustainable alternatives to fish oil and fish meal as sources of these long-chain omega-3s," wrote Dr. Douglas Tocher, one of the authors of the study, in an email to EcoWatch. "These include microalgal sources and genetically-modified oilseed crops."

The U.S. imports 91 percent of the seafood it consumes. Currently, oysters, clams and mussels account for tho-thirds of farmed seafood produced in the U.S., but NOAA opened up the hurricane-prone Gulf of Mexico to fish farms in January. That's the first time federal waters have been available for fish farming. So far, no commercial proposals have been received.

The World Bank estimates that almost two-thirds of the fish we eat in 2030 will be farm-raised. "Aquaculture will be an essential part of the solution to global food security," said Jim Anderson, bank advisor on fisheries, aquaculture and oceans for the World Bank Group. "We expect the aquaculture industry to improve its practices in line with expectations from the market for sustainable and responsibly produced seafood."

Aquaculture may also be the only answer to overfishing of the seas. Almost one third of global fish stocks are overfished, according to the United Nations. WWF says that stocks of all current food species of fish could collapse by 2048. But we'll need to feed 9 billion people by then.

"The solutions are very much in the pipeline," wrote Dr. Tocher. "Farmed salmon still deliver more omega-3 than wild salmon. And there is also absolutely no harm In eating two portions of farmed salmon."