The point of the book comes down to the push and pull of our desire to eat wild fish, and the promise and fear of consuming the farmed variety. As Greenberg follows his four species, and our pursuit of them, farther and farther out into the ocean, he posits the sense of privilege we should feel in consuming wild fish, along with the necessity of aquaculture.

Along the way, Greenberg raises real-life ethical questions of the sort to haunt a diner’s dreams, the kind of questions that will not be easily answered by looking at the Monterey Bay Aquarium’s seafood-watch card. In truth, he shows, there is rarely such a thing as a good wild fish for any of us to eat, at least not if all of us eat it.

Combining on-the-ground and on-the-ocean reporting from the Yukon to Greece, from the waters of Long Island Sound to the Mekong Delta, along with accounts of some stirring fishing trips, Greenberg makes a powerful argument: We must, moving forward, manage our oceans so that the fish we eat can exist both in aquacultural settings and within the ecosystems of wild oceans.

Wild fish were once everywhere, of course, in such numbers as to astound. (And still, Greenberg reports, the current global catch of wild fish measures 170 billion pounds a year, “the equivalent in weight to the entire human population of China.”) Wild fish seemed to be, as Greenberg puts it, “a crop, harvested from the sea, that magically grew itself back every year. A crop that never required planting.”

Once, Greenberg writes, as many as 100 million Atlantic salmon larvae hatched every year in the upper reaches of the Connecticut River and eventually made their way south to Long Island Sound, and north from there to Greenland before returning to the Berkshire foothills to spawn. Dams, overfishing and more dams still have taken their grim toll on their descendants. Today, every piece of Atlantic salmon you’ll find at your local supermarket or fishmonger, smoked into lox, wrapped around mock crabmeat, or lying flat and orange against crushed ice, is farmed. As Greenberg explains clearly and well, the process by which that farming is undertaken threatens the future of what wild salmon remain here and in the Pacific. The amount of wild fish needed to feed farmed salmon, the threat of farmed salmon escaping and crossbreeding with wild salmon stocks, the rise of pollution from the farms themselves — when it comes to the business of domesticating salmon, Greenberg writes, “we should have chosen something else.”