Low-fat, easy to prepare, and delicious, seafood’s popularity is booming. Atkins dieters love the protein, nutritionists tout the benefits of omega-3 fatty acids, and sushi has become a common snack. By 2020, predicts the U.S. Department of Agriculture, we’ll be eating 25 percent more fish than we do today.

That is, if we can find it. While demand is rising (the developing world is expected to increase its consumption by 60 percent in the next 20 years), we’re now extracting as many or more fish than the sea has to give. The world catch increased by 400 percent in the years after 1950, but since 1989 it has leveled off at about 80 million metric tons of useable fish a year.

We may soon be getting less, as species after species is fished out. Many corporations have made huge investments in giant factory trawlers, so when one species crashes, they turn to another. Slimehead, say. Formerly considered a "trash" fish, slimehead was rechristened "orange roughy" in the 1980s and featured in all the best restaurants. Within a few years, it too was dangerously overfished.

In an influential study last year in the journal Nature, biologists Ransom Myers and Boris Worm showed that it only takes 10 to 15 years to reduce a fish population by 80 percent. We’ve already cleaned out 90 percent of the large predatory fish—the giant marlin, swordfish, and sharks—from the seas, they say, and those that remain are only one-fifth to one-half as large as they used to be. (See "Lay of the Land," September/October 2003.) The National Marine Fisheries Service is less pessimistic, boasting to Congress of stocks of fish removed from the "overfished" list, but Myers and Worm counter that claims of recovery are often derived from faulty baselines—the already depleted numbers of a decade ago, for example. "We are in massive denial and continue to bicker over the last shrinking number of survivors, employing satellites and sensors to catch the last fish left," Myers told the BBC.

At the minimum, Myers recommends cutting the take of sensitive species by half. Others are thinking along similar lines. Last year, the Pew Oceans Commission called for fundamental fishery reform, including a national system of fully protected marine reserves, and the creation of a federal oceans agency. And the Marine Conservation Biology Institute is pressing for controls on destructive fishing practices like bottom trawling, in which heavily weighted nets are dragged along the ocean floor, crushing fragile habitat.

The fish-eating public can also do its part. Just as you wouldn’t fricassee Martha, the last passenger pigeon, you can spurn disappearing Patagonian toothfish (which got a marketing makeover as "Chilean sea bass"). You don’t have to abstain from fish altogether (although some have taken that step); you just need to shop strategically. West Coast oysters, mussels, and clams, for example, are raised in enclosures but don’t pose the pollution problems of farmed salmon; in fact, their dependence on clean water is a powerful argument against allowing polluted runoff. Similarly, California and Alaska have thriving, well-regulated wild-salmon fisheries that depend on healthy rivers, streams, and forests. Sardines, once overfished, have made a big comeback (and are surprisingly tasty when fresh).

Luckily, you don’t need to keep track of the status of every fishery. Check out the "All Fish List" in the Seafood Watch section of the Monterey Bay Aquarium’s Web site (www.mbayaq.org), or the even more comprehensive www.seafoodinfocenter.org. Then ask your fishmonger or waiter the appropriate questions: Is the fish wild or farmed? Was it caught by hook and line, or trawler? If they don’t know, choose something else—because there are still other fish in the sea.