A couple years ago, salmon passed tuna as the most popular fish in the United States. Our doctors have told us to eat more of it; our fitness and diet regimens have put it in heavy menu rotation.

The problem is not all salmon is created equal. More than 90 percent of the fresh salmon eaten in this country comes from giant fish farms, and those farms have problems. Big ones. Here are the facts about America's favorite fish that you should know but perhaps wish you didn't.

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• Large-scale salmon farms in the United States, Canada, Scotland, Norway and Chile have attracted hideous marine insects called sea lice that attach themselves to the fish, causing skin lesions and secondary infections, killing the host or rendering meat inedible. The damage these lice have inflicted has caused salmon prices to soar in the past 18 months. To get rid of these parasites, farmers doctor their feed with a pesticide called Slice, or emamectin benzoate, which causes tremors, spinal deterioration and muscle atrophy when administered to rats and dogs.

• Large salmon farms also use high levels of antibiotics to treat bacteria that cause lesions and hemorrhaging in infected fish. Why is that bad? Overuse of antibiotics, either in farming or for human medical treatment, speeds up the development of antibiotic resistance.

• Farmed salmon are fed pellets made out of fish oil and smaller fish, ground-up chicken feathers, poultry litter (yes, that's poop), genetically modified yeast, soybeans and chicken fat.

• Wild salmon get its lovely rose color from eating krill and shrimp. Farmed salmon, because it eats those pellets, is grey. To make it more appetizing to consumers, farmers add dyes to their feed.

• Studies show that farmed salmon contains up to eight times more PCBs — cancer-causing industrial chemicals that were banned in 1979 — than wild, as well as high levels of mercury and dioxins from herbicides like Agent Orange.

• We've all heard that omega-3 fatty acids are essential nutrients for nervous system, heart and brain health. Omega-3 in fish are derived from plants like algae, leaves and grass. Because farmed salmon are fed a lot of soy, they are high in omega-6, which you don't want: Omega-3 fights inflammation while omega-6 promotes it.

• Then there are environmental concerns: pollution from fish excrement and uneaten feed; farms releasing diseases to wild fish stocks; escapees unwittingly released into the wild where there are no natural populations and then outcompeting native fish populations. Many people also have ethical concerns about farming carnivorous fish. To produce farmed fish such as salmon, you must feed them about three times their weight of wild-caught fish. That's like feeding a whole sheep to a chicken to get that poultry to harvest weight.

Sources: Seafood Watch, Washington State Department of Health, healthline.com, watershed-watch.org

Laura Reiley, Times food critic