Whenever Paul Greenberg, a lifelong angler and writer about seafood, mentioned that he was working on a "fish-in-danger book," the question most often asked was which fish people should eat. Farmed salmon or wild? Is swordfish OK now? Is tuna sushi a bad habit? You'll find answers -- though perhaps not the ones you're expecting or hoping for -- in Four Fish, his excellent, wide-ranging exploration of humankind's relationship with fish -- the flesh that even many vegetarians consume.

You'll also find deeper questions about our need "to reevaluate whether fish are at their root expendable seafood or wildlife desperately in need of our compassion." In this passionate study, part investigative journalism, part travelogue, part personal memoir about fishing for wide-mouth bass as a boy in Connecticut, Greenberg asks, "Must we eliminate all wildness from the sea and replace it with some kind of controlled system, or can wildness be understood and managed well enough to keep humanity and the marine world in balance?"

Four Fish builds on Mark Kurlansky's pioneering 1998 microhistory Cod, but it's more than simply Cod-plus-three. Greenberg notes that, just as human consumption of mammals and birds has narrowed to four sources each -- cows, pigs, sheep and goats in the meat department; and chickens, turkeys, ducks and geese for poultry -- "four archetypes of fish flesh" have come to dominate our fish counters: salmon, bass, cod and tuna.

He examines each in turn, showing how humans have threatened them by overfishing, destroying their spawning grounds and upsetting their food chains. Much of this sorry situation is not, alas, news, but where Greenberg's book earns its stripes is in pulling together in one superbly researched and engagingly written volume an examination not just of the problem but of some of the (occasionally misguided) attempts at solutions. Salmon farming, for example, which Greenberg asserts "is in dire need of reform," has created its own well-publicized issues of pollution and contamination of wild stock.

Four Fish is not all doom and gloom. Greenberg notes that "unlike the terrestrial environment, the essential habitat of much of the world's marine life remains reclaimable." Careful management of existing wild stock and production of viable farmed alternatives through sensible polyculture are vital steps.

In evaluating the efficacy of fish farming, a crucial factor is the feed conversion ratio -- the number of pounds of feed it takes to produce one pound of edible flesh. Neither cod nor sea bass nor tuna -- which require as much as 20 pounds of feed for every pound of flesh -- have proven to be good candidates for farming. But Greenberg visits operations producing feasible alternatives, traveling to Vietnam to look into tra, a superproductive freshwater fish with a tolerance for high stocking densities, and to Hawaii to explore kahala, which he says tastes like white albacore. Rechristened Kona Kampachi for marketing purposes, these fish have a feed-conversion ratio of 1.7 to 1 and even approach "the holy grail of aquaculture," requiring less than a pound of feed per pound of flesh, "a potential net gain of marine protein."

So, what to eat? Not an easy question. Tilapia. Barramundi. Kona Kampachi. Sustainably harvested salmon caught by the Yup'ik Eskimo Kwik'Pak Fisheries in Emmonak, Alaska, the only Fair Trade-certified fishing company in the world. What to read is easier: Four Fish.

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