Before ordering tuna, you may want to ask what sort of tuna it is and where it was caught. (ISTOCKPHOTO)

Most of us have become aware of the ethical implications of eating meat. That’s a good thing: Intensive animal farming damages the land, sea and air, in addition to being cruel to the creatures that die to feed us.

But food ethics can become so intense as to be comical. The satirical television series “Portlandia,” for example, featured a couple so worried about the origins of their chicken dinner that they made the waitress hold their table while they visited the farm.

But what about seafood? There are hundreds of species of fish now available, some wild-caught, some farm-raised. The tools of the fishing trade, from poles to lines to nets to boxes, all have different effects on marine ecosystems.

The easiest thing is to plead ignorance and buy whatever you like. It doesn’t have to be complicated, though. Here’s a three-step guide to buying sustainable fish and easing any omnivorous guilt.

First, look for products carrying a reputable sustainability certification. The best-regarded certification group is the Marine Stewardship Council. This organization certifies fisheries all over the world, and its stamp of approval appears on more than 17,000 wild-caught products. (Farmed fish have an equivalent certification, issued by the Aquaculture Stewardship Council.)

Fisheries have to convince such groups that they satisfy three principles of sustainability. First, the fish population from which the fishery is drawing must be either steady or growing on a year-to-year basis. Counting fish isn’t an easy job.

“For some fish, researchers use mid-water sonar studies to measure density, then extrapolate,” says Kerry Coughlin a regional director for the Marine Stewardship Council. “In other places, biologists fly over rivers and count fish, or college students sit on the shore and count fish as they swim by. For some species, fisheries have to submit the size of their catch each year.”

The second principle is that the fishery must not adversely impact the larger marine ecosystem. The fishing method must not snag large numbers of unintended species, a result known as bycatch. The process also must not degrade coral or the ocean floor. Even if the population of the target fish is steady, researchers must also confirm that animals that prey on that species, or are preyed upon by it, won’t be damaged by fishing.

Finally, the fishery must be well managed. For example, if a particular species is sensitive to overfishing, the managers must have the capacity to adjust their take on a monthly or yearly basis.

You might not be able to find fish that carry the certification. Seventeen thousand products is a lot, but it pales in comparison to the number of seafood items on the market. If your favorite restaurant, grocery store or fishmonger doesn’t carry sustainability-certified products, you can try Step Two: Become an advocate. Tell the waiter or the manager that you care about sustainability and responsible marine management, and that you’ll have to take your business elsewhere.

If you simply must have fish and there’s no certification in sight, move on to Step Three: Check out guides to sustainable fish. These handy surveys put fish into such categories as “good choice” or “avoid.”

Unfortunately, poring over a seafood guide can be complicated.

Let’s say you’re thinking of buying a sushi tuna roll, so you check a seafood guide. The results are mind-boggling. There are seven separate listings for fresh tuna: albacore, bigeye, blackfin, bluefin, skipjack, tongol and yellowfin. The restaurant menu probably just says “tuna,” so you inquire with the server, who informs you that it’s bigeye. You click on bigeye, and now you have more questions.

If the fish is caught with pole and line by a U.S. fisherman in the Atlantic, it’s highly sustainable. If caught in the same area but using a longline, it’s somewhat sustainable. Bigeye tuna caught with a longline from outside the portion of the Atlantic bordering the United States is considered an unsustainable choice.

Here’s hoping that your server knows the difference between a pole-and-line and a longline. (Pole-and-line fishing is the traditional method of baiting a single line, which produces very little bycatch, whereas a longline is miles long and contains hundreds of hooks.)

Why are sustainable seafood guides so complicated?

“As fishery sustainability becomes more and more the norm, our requirements are becoming more granular,” says Tom Pickerell, senior science manager at the Monterey Bay Aquarium, which produces the Seafood Watch guide. “The Chilean sea bass is the poster child for this. Previously, the recommendation was a blanket red [no use], but, as various fisheries change their management, different regions or methods turn to yellow or green.”

Believe it or not, the dizzying complexity of seafood guides indicates that progress is being made. We’re living in a transitional period, when some fisheries have moved to sustainable-harvest practices. Pickerell estimates, somewhat hopefully, that nearly all U.S.-based fisheries will be earning green lights in the next decade.

Is it complicated? Certainly. Will you get some eye rolls from waiters and fishmongers? Probably. But if they don’t know where their fish came from, that’s their fault, not yours. It’s not like you’re asking them to sit tight wait while you visit the fishery to confirm its sustainability.