When it comes to seafood, the general consensus is that fresh is best (fresh fish are some of the world's healthiest foods), but for many people, geography and budget often necessitate the purchase of canned seafood.

It may surprise some folks, but when it comes to canned fish, shellfish, and other ocean delights, there are actually many healthy and sustainable options available, including oysters, anchovies and mackerel. However, things aren't always so cut and dry when it comes to navigating the waters of the canned fish aisle, and there are some products that are best avoided altogether, whether due to environmental concerns and sustainability, toxicity, labor ethics, or all of the above.

Fortunately for the conscientious shopper, there's plentiful research available to help you steer clear of the worst offenders. If you're opting for the canned version next time you get a seafood craving, be sure to leave these four selections off your shopping list.

Most Canned Tuna

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What to Avoid

Tuna is perhaps one of the stickiest wickets when it comes to the canned seafood conundrum. In 2015, the environmental watchdog Greenpeace released a buyer's guide for canned tuna, ranking more than a dozen major brands based on sustainability, mercury load and other key factors. They discovered that “more than 80 percent of the tuna sold in the U.S. comes from unsustainable, destructive sources.”

Many tuna companies use longlines, which contain hundreds of hooks, and can span miles of ocean. This indiscriminate method catches not only the sought-after tuna but also creatures such as sea turtles, marine birds, sharks, and other animals. Additionally, Greenpeace found unethical practices plaguing the processing facilities—with workers being underpaid, forced to work in poor conditions, or at worst, being treated as slave laborers.

No matter where your tuna comes from, it all carries a risk of mercury exposure. According to the Washington Post, canned tuna labeled “light” is safest in terms of mercury levels and can be eaten a few times a week, whereas albacore should only be a monthly indulgence.

What to Buy

If you're going to eat tuna, seek out brands where the fish has been ethically sourced and is labeled as “pole caught.” Look for the Marine Stewardship Council's seal of approval as well. A “dolphin safe” label means very little and doesn't necessarily imply that the tuna you're about to eat was caught without bycatch or processed by well-treated workers. Fortunately, as customer demand for better canned tuna amps up, so does the availability of products to fill that niche, and now canned tuna that meets these strict criteria can be found even at large retailers such as Walmart and Kroger.

Some Canned Salmon

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What to Avoid

There is no denying that salmon is a powerhouse when it comes to the healthiest fish, and while many people find it difficult to afford—or even find—the fresh, wild-caught sockeye we're told to seek out, canned salmon is used in salmon cakes, salads, seafood stews, and pastas.

According to Berkeley Wellness, almost all canned salmon is wild caught, but not all of it is, so a little label reading is in order. If a can indicates that the fish inside is Atlantic salmon, you'll want to pick a different brand. Atlantic salmon are always farmed, as they have been nearly driven to extinction in the wild. Farmed salmon, of course, is a poor choice due to antibiotic use, toxins such as PCBs, and pollution of waterways (where the salmon are kept in vast pens).

You should also try to avoid salmon that has been shipped overseas for processing. If the label tells you your salmon is a “product of Thailand” or some other country, that means the fish was caught in the U.S., shipped across the world, processed, and then shipped back to the states for sale. That's an awful long way for your food to travel, all in the name of cheaper labor.

What to Buy

First, make sure the label indicates that the salmon you're buying is either Alaskan pink salmon, sockeye or red salmon. All of those terms indicate that your salmon is the wild stuff from North American waters. The Alaskan salmon fishery is one of the most well-managed, safe, and sustainable fisheries in the world, so as long as your salmon is wild-caught, it's almost always an ethical choice on all fronts.

Some Canned Crab

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What to Avoid

Seachoice.org tells us the number one indicator that the crab you're contemplating might be a poor choice is in its name. If the crab is labeled as “swimmer crab,” “swimming crab,” “blue swimmer crab,” “jumbo lump crab,” or “backfin lump crab,” you'll need to do some further investigation. Most crab under the aforementioned names comes from Asia, where the crab industry and fishing methods are very loosely regulated—if they're regulated at all. Trawl methods and gillnets are often employed, leading to devastating bycatch of other marine creatures, and there are almost no management practices in place to ensure a stable crab population.

What to Buy

North America—particularly the West Coast—has one of the strongest, cleanest, and healthiest crab fisheries on earth. The David Suzuki Foundation urges crab-buyers to seek out trap-caught Dungeness crab from Canada, Washington, California, and Oregon. Fortunately, this delicious crustacean is available in canned versions, and if you can't find it in your local grocery store, you can easily order it online. Chances are, if the label says “Dungeness,” you can be certain you're making the right choice. (Note: Domoic acid levels in crabs have now been deemed safe along the entire West Coast.)

Many Canned Shrimp

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What to Avoid

Canned shrimp is perhaps even more fraught with ethical, health, and environmental problems than tuna, with farmed shrimp being the absolute worst offender. A recent piece by Mother Jones gives us a litany of reasons to shun anything other than wild-caught shrimp, including the abhorrent treatment of workers in the Asian shrimp trade, the “massive carbon footprint” of shrimp farms, and rampant antibiotic abuse (shrimp are treated with at least eight different drugs to accelerate their growth rates). To put it simply, farmed shrimp is a bad idea, and a lot of the shrimp you buy in cans off the supermarket shelf falls into that category.

What to Buy

Read the fine print, and if you can't figure out what country that can of shrimp came from, choose another option. When in doubt, simply don't buy it. A sure bet is shrimp harvested in the United States, either from the Gulf of Mexico, the West Coast, or Alaska. Most shrimp labeled “salad shrimp” or “tiny pink shrimp” are products of the United States, but the best thing you can do is simply call the company in question or visit their website. If they can't—or won't—tell you the source of their shrimp, move on to a company that will!

This story originally appeared on Rodale’s Organic Life in June 2016.