There’s no question fish is a smart menu choice. It’s a staple of many healthy eating plans, such as the Mediterranean diet. Most types are low in fat and calories. And the varieties of fish that do contain generous amounts of fat, such as salmon and mackerel, are brimming beneficial omega-3 fats linked to a lower risk of heart disease, stroke and other health problems.

But there’s a catch: Thanks to pollutants and overfishing, not all fish varieties are good choices for you or the environment.

It’s well known that many rivers, streams and ponds are choked with toxins such as dioxins, PCBs and heavy metals. For example, sport fishermen in the Mississippi Delta have been warned not to eat more than two servings per month of the fish they catch in the river or nearby streams and lakes because of industrial pollutants. These fish include buffalo fish, gar, carp and large catfish.

Even the mighty ocean is tainted with methylmercury, a nerve poison.

As pollution increases, so does the contamination of certain fish species. A recent study found that mercury in Hawaiian yellowfin tuna, a sashimi staple also known as ahi, have been increasing by 3.8 percent or more a year, largely because of pollution from coal-fired power plants and artisanal gold mining.

Related5 Surprising Sushi Do’s and Don’ts

Meanwhile, increasing demand has led to an overfishing crisis. According to the World Wildlife Foundation, “more than eighty-five percent of the world’s fisheries have been pushed to or beyond their biological limits.” Some species, such as Atlantic bluefin tuna (sorry, Charlie!), are on the brink of extinction.

Smart choices for the seafood lover in you

Many experts recommend eating two fish-based meals a week, but figuring out which ones aren’t is sometimes tricky.

Choose low-mercury species. According to Sonya Angelone, RDN, a spokesperson for the Academy of Nutrition and Dietetics, choices that tend to have less of this toxin are:

  • Salmon
  • Cod
  • Canned light tuna
  • Trout
  • Mahimahi
  • Whitefish,
  • Pollack
  • Anchovies
  • Herring
  • Sardines
  • Shrimp

Go wild when it comes to salmon. Farmed salmon, according to one study, can contain up to seven times more PCBs than wild salmon, though levels can vary from farm to farm. “There are some ethical fish farms that take good care of their fish and limit contaminant exposure,” says Angelone.

Since it’s tough for the average consumer to tell which ones they are, err on the side of safety by choosing wild-caught varieties, including sockeye, chum, Coho and Chinook (sometimes called King). Most are harvested in Alaska, though some come from Washington, Oregon and California or are imported from Canada or Chile.

Favor smaller varieties. According to the Natural Resources Defense Council, small varieties of seafood tend to contain less mercury. They’re also more plentiful, so they’re in less danger of being overfished. Some good choices include:

  • Squid
  • Oysters
  • Mackerel
  • Sardines
  • Mussels

Related: Bait and Switch: Why the Fish You’re Eating May Not Be the Fish You Think You’re Eating

Be leery of large fish. Some large species of fish, particularly swordfish, shark, king mackerel and tilefish, often have high levels of mercury because they’ve been in the water for long periods and therefore absorb more of the toxin, according to the Food and Drug Administration. According to Angelone these include:

  • Albacore (“white”) tuna
  • Red snapper
  • Sea bass
  • Largemouth bass
  • Marlin
  • Pike
  • Orange roughy
  • Bluefish
  • Lobster
  • If you’re pregnant or nursing, avoid these altogether. Don’t serve them to young children either.

Learn how to safely eat local. The Environmental Protection Agency has a useful interactive website that details fish advisories. By entering your state and the body of water that you’re interested in, you can find out if there are any varieties of fish or seafood you should steer clear of. The site will also direct you to specific state fish advisories.

Stick to sustainable fish. Here’s a handy way to keep from eating overfished varieties: Download Monterey Bay Aquarium’s Seafood Watch app (free and available for both iPhone and Android). The app allows you to search for seafood and sushi by name and will tell you if a certain type of fish is domestic or imported, farmed or wild, overfished or sustainable and more. You can also download printable state-by-state guides on the website.

Related: Food Safety Fails: 10 Common Mistakes That Could Make You Sick

Denise Foley is a veteran health writer and a former contributing executive editor at Prevention magazine.