You probably don’t know that most of the fish you eat is grown on farms, not caught in the wild. That’s right, your fillet of tilapia likely didn’t come from the ocean but from a farm, just like your steak or pork chop. This sounds great for the environment, but it may not be, recent studies show. Some scientists say that farmed fish may actually harm wild fish populations, and those who consume them.

If you have eaten salmon, it most likely was farmed in the U.S., the United Kingdom, Norway, or Canada. A 2004 study at Indiana University showed that farmed salmon have higher levels of toxins than wild salmon. The salmon at the fish farms live in pens where thousands are crammed into a space the size of a swimming pool. In addition to being crowded, the pens are full of dirt, fecal matter and uneaten food.

The waste from fish farms ends up polluting the ocean; National Geographic reports that scientists discovered a direct link between the explosion of sea lice in farmed fish populations and the decline of Scottish sea trout after a study where wild sea trout shared coastal waters with penned salmon. But there is an even greater problem: the farm population greatly outnumbers the wild salmon by as much as 85 to one, and ultimately it is impossible to keep the fish completely separate. Storms and wild animals damage the pens and let thousands of fat, farmed salmon into the sea; up to 40% of all salmon in the Atlantic come from farms.

So what, you ask? Well, the farmed salmon have been given antibiotics, which, according to Greenpeace, lead to the presence of antibiotic-resistant bacteria in the sediment under the pens.

“These bacteria could pose a risk to human consumers as well as to the wider ecosystem in which the nets are placed,” says ecologist Dany Garant of Oxford University. “The nets are typically located in the fast-flowing waters of estuary heads, so that the toxic feces, uneaten food pellets, parasitic lice, dead fish, escaped non-native fish as well as chemical and antibiotic residues, are distributed over the whole estuarine ecosystem.” When farmed fish escape, they spread diseases to wild fish. When they interbreed with wild salmon the result is sickly, weak fish less fit to survive, which “ultimately threatens the long-term genetic integrity of native populations.”

Farmed fish are not as good for the environment as they sound. Do your own research and decide for yourself, but eating wild fish is actually better for you and the health of the oceans than choosing a frozen fillet of farm-raised fish.