In 1950, when tuna usually ended up in sandwiches and casseroles, the worldwide catch totaled an estimated 660,000 tons (600,000 metric tons). Today that annual number has grown to more than 7 million tons (6.6 million metric tons) as the fish has gone gourmet and demand has soared.
The tuna boom has led to a host of concerns about the global fishing business, the state of our oceans, and the health impact of consuming an apex predator.
Which kind of tuna is best to buy is a complicated question because it involves a number of conflicting factors. "There are health concerns and culinary needs as well as choices based on sustainability," says Valerie Craig, who manages the National Geographic Seafood Decision Guide. "How can you know about all those issues for every species?"
The first thing to know is that what we call tuna is actually several different kinds of fish. Each has been affected by the fish's boom in popularity, but some are suffering drastic declines in population.
Bluefin tuna, for instance—featured in the March issue of National Geographic magazine—have been so overfished that they can't reproduce fast enough to replace what's caught. If you care about sustainability, they should be on your do-not-eat list.
How fish are caught also affects their sustainability. Longlining can be especially devastating because it involves one line that can have 3,000 baited hooks and stretch for up to 50 miles (80.5 kilometers). The hooks dangle at a depth between 328 feet (100 meters) and 492 feet (150 meters), where the largest tuna—such as the threatened bluefin—tend to swim. The hooks also catch more than 80 kinds of nontargeted creatures, including endangered sea turtles, which often die on the line before the fishing vessel reels in the catch.
Going after the biggest fish also serves up health concerns to the people who eat them. Big tunas like bluefin feed high on the food chain, so they ingest all the mercury that their prey and their prey's prey have taken in. The U.S. government offers guidelines for pregnant women, nursing mothers, and small children, but other consumers are also concerned about how to balance the health benefits of eating fish against the adverse effects of mercury. A good general rule: The bigger and older the fish, the bigger the risk.
The best advice is to study up before you go to the supermarket or a restaurant. A good guide outlines the kinds of tuna, where they're fished, and what kinds of gear are used to catch them in different parts of the world. It may also make suggestions about the greenest and healthiest choices.
In addition to the National Geographic guide, the Monterey Bay Aquarium Seafood Watch offers printable regional pocket guides as well as a smartphone app, and the National Oceanic and Atmospheric Administration's Fishwatch website offers a list of seafood profiles that include tuna. These three sources are the best at keeping up with the latest changes in the tuna business.
To help sort through the issues, here's a basic rundown of the fish sold at supermarkets and restaurants in the U.S.:
In Cans or Pouches
Tuna labeled "light meat" is most likely skipjack, classified biologically as a cousin of true tunas.
Skipjack makes up about 70 percent of the canned or pouched tuna. It's plentiful, so sustainability isn't an issue. And it's cheap.
It's also a small, fast-maturing fish that's relatively low on the food chain, so the level of mercury in its flesh is low.
The downside to canned skipjack is that the texture is often mushy, and the taste can be aggressively fishy.
Albacore has a mild taste and produces firm chunks of meat. It's labeled "white meat," and accounts for about 30 percent of the canned-tuna market.
Many albacore are now caught by longlining, so sustainability and mercury content may be issues.
At a Restaurant
With a firm texture and mild flavor, yellowfin tuna often appears on restaurant menus. It may be called "ahi," a Hawaiian word for tuna. The term "ahi" is also used for bigeye, which may occasionally land on a menu when available. More about them below.
A number of yellowfin populations are overfished now, so only pole-caught fish are considered a good choice for sustainability. Mercury is a concern for those caught by longline.
At the Sushi Bar
The menu may not give much of a clue about the kind of tuna that's being served.
It may just say maguro—Japanese for tuna. "The restaurant may be posting a standard menu and then serving whatever tuna they're able to get that day," says Craig. In the U.S. that's often high-grade yellowfin.
Other words on the menu refer to the part of the fish the meat comes from.
A cut called "toro" was traditionally taken from the buttery soft belly of the bluefin tuna. More specifically, otoro comes from the belly close to the head, while chutoro comes from the middle or back of the belly and is less fatty than otoro. Click here to see a sushi diagram of the whole fish.
Bluefin is so rare these days that its price has soared. A single bite-size piece of otoro could now set you back $25. So if your local sushi hangout offers two pieces of toro sushi for $10, that's not bluefin. It may be bigeye, which is generally a better option in terms of sustainability. It reproduces and matures quickly, and though some of its populations are declining, they're not as devastated worldwide as some others.
But bigeye tuna has a downside: Each fish can grow to more than 400 pounds (181 kilograms), so mercury can be a concern, depending on where and how they're caught. Pole-caught bigeye tend to be younger, so the mercury level is likely to be lower.
Even if you've studied the tuna guides before you go out to eat, how do you know what's being prepared in the kitchen? Ask. The servers may know about the kind of tuna or its place of origin; if not, they can check with the chef. If the answer sounds fishy, maybe you should go for chicken instead of the chicken of the seas.