Reports of over-fished seafood populations — such as Atlantic swordfish or Chilean sea bass — have increased over the last decade, and discerning diners may be troubled about harming the food chain. A new book by noted marine biologist Carole C. Baldwin and researcher Julie H. Mounts, One Fish, Two Fish, Crawfish, Bluefish: The Smithsonian Sustainable Seafood Cookbook (Smithsonian Institution Press, 2003), with dazzling illustrations by Charlotte Knox, seeks to set the record straight about what choices consumers can make and the questions they can ask seafood retailers or chefs. The book also presents recipes from 150 US chefs, including Al Forno’s Johanne Killeen and George Germon, which utilize the suggested seafood species.

During a fall lecture/tasting at Al Forno, Baldwin presented slides to back up her points of why certain shellfish or finfish should be avoided and others sought out. She pointed out that problems of sustainability arise not just from over-fishing. They also stem from environmentally unsound dredging practices, which destroy habitat, especially for young fish; from wasteful trawling or long-lining, which can bring in more by-catch (shellfish or small fish) than the targeted species; and from inefficient fish-farming where small fish are fed to make larger ones, depleting the supply of the small ones. Baldwin hopes to encourage people to think more about sustainable or well-managed seafood in the same way that they’ve come to understand recycling — as a conservation of resources — and she believes that if consumers diversify the fish they eat, endangered supplies can be saved.

She began by talking about salmon. Wild Atlantic salmon has all but disappeared, but Pacific salmon is managed so well that it received a stamp of approval from the Marine Stewardship Council. She maintained that almost all marketable Atlantic salmon is being farmed, but that wild fish are being killed to feed them. Farmed salmon have been found to have 10 times the amount of PCBs as wild ones. So, if you can’t get Pacific salmon (pink, sockeye, Soho, king and chum), its close relatives, Arctic char and rainbow trout (the US is second worldwide in farmed trout production), are suitable substitutes.

Snapper and grouper have been severely over-fished, partly because so many juveniles are destroyed in the by-catch. Catfish and tilapia, however, are plentiful — tilapia entered the US top-10 seafood list in 2001 — and they are raised on environmentally sound farms. Instead of the over-fished Chilean sea bass, you should look for sablefish, also called black cod, and American butterfish, both an excellent source of omega-three, with a flavor similar to the so-called sea bass (actually a Patagonian toothfish).

Orange roughy has also had a rough life in recent decades. It can live 150 years, but doesn’t reach breeding maturity until it’s 25 or 30, so it has been quickly fished out. Mahi mahi (dolphinfish) on the other hand, can reach 20 pounds in one year, reproduces at an early age, and lives only four or five years. Mahi mahi and its close relative cobia are good sustainable seafood options. Since bluefin tuna was almost eliminated during the first wave of sushi enthusiasm in the ’70s, Baldwin recommends albacore, yellowfin, skipjack, or blackfin as the tuna of choice. And since Atlantic halibut is almost extinct, she’s emphasizing Pacific, California, or Greenland halibut or sand dabs.

How’s an inquiring seafood consumer to find out some of these facts? Just keep inquiring, suggests Baldwin. Eventually, retailers and chefs, in an effort to keep customers satisfied, will ask the same questions of wholesalers or fishermen themselves.

Farms are now the source for most oysters, mussels, and clams in the US, and even in Rhode Island, farming oysters and mussels have become viable businesses. Since these bivalves filter the water in which they live, the pollution problems that have plagued some other seafood farming operations have been minimal. It’s also a good idea to ask for "farmed," because the wild populations have been so depleted and since the dredging methods used to pull them from the wild end up destroying so much underwater habitat.

The same is mostly true for scallops and for similar reasons: dredging practices, severe depletion and the commonplace addition of additives to shrimp and shucked scallops to extend their shelf life. Shoppers and diners should choose handpicked diver sea scallops, or farmed bay scallops, such as Taylor, which are shipped in the shell.

Imported shrimp, even if they are farmed, also have a chemical additive to beware of. US-farmed and -trawled shrimp, however, are well-regulated. It can be tricky to determine from the labels where frozen shrimp came from, but it’s worth looking carefully. Texas-farmed, Louisiana-caught, and Northern pink shrimp (Maine shrimp) are the ones to grab.

In crabs, look for three species: blue, stone and Dungeness, a West Coast species. Both hard and soft-shell stages of blue crab are popular. Stone crabs regenerate their claws, if only one is removed at a time, so they are naturally recyclable. Snow crab legs and king crab legs come from very over-fished populations. It’s been discovered that only a small percentage of lobsters in a specific area go into traps — they’re not all that stupid. Thus, their population is not as threatened as previously thought.

There’s much more information in Baldwin and Mounts’s book, and the recipes are truly enticing. As Baldwin repeated at the end of her lecture, there would be a beneficial impact on the world’s seafood resources if people just diversified their eating habits.