Four Fish: The Future of the Last Wild Food. By Paul Greenberg. Penguin Press; 304 pages; $25.95. Allen Lane; £14.99. Buy from Amazon.com, Amazon.co.uk

FOUR wild fish species dominate the world's seafood markets, but that might not last much longer. As Paul Greenberg observes in a sharp and occasionally lyrical book, we are at a significant moment: farmed fish now make up around half of all the fish consumed by humans.

Salmon, seabass, cod and tuna all have a different relationship with man. Salmon need rivers which are quickly blocked and fouled by human development, and the teeming runs of old are now rare. But their large eggs make for easy breeding in captivity and as a result the fish farms in Chile alone now produce almost as much salmon as all the salmon rivers in the world combined.

That thought revolts Mr Greenberg, a food writer and novelist. A wild salmon's DNA has been moulded to fit the challenges of the river it must conquer in order to spawn. Escapees from fish farms risk polluting these wild genes just as the parasitic lice that plague fish farms spread to salmon that swim free. But when Mr Greenberg returns to a New York river whose native salmon had been wiped out long ago, he is surprised to find it thriving with a new stock. These may be captive-bred and genetic mongrels, but at least they are salmon.

The Mediterranean Sea offers a glimpse into one possible future for the world's oceans. European seabass were once commonly caught in its shallow, nearshore waters. Now they are born inside nets instead, with ten times as many farmed as fished. How the incredible complexity of getting a fish to breed in captivity was first overcome in the seabass makes riveting reading here, aided by the voyage of a real-life Odysseus who journeys home to Greece with a sailboat laden with live fish.

The famous story of the demise of Newfoundland's cod is a parable of all that is wrong with industrial fishing. Cod hate cages. They don't like being handled, are very sensitive to changes in their environment and are very hard to breed. That is the problem with fish-farming. Some fish are simply not suited to it.

Migratory tuna are also unlikely candidates for farming. Yet the high prices commanded by bluefin tuna and the demise of wild stocks have led many people to attempt to raise them in captivity in order to supply the same demand. Trying to farm a fish just because there is an established market for it is a waste of time and money, argues Mr Greenberg. Farm animals were domesticated because they were suitable to begin with, and only got more so over time. Aquaculture will only work, environmentally and economically, with the right sort of fish.

So far the search has turned up two good freshwater options—tilapia and the Vietnamese Pangasius or river cobbler. A marine version may still be out there. The prize is a form of protein that is far cheaper and more efficient to produce than meat. Mr Greenberg is not taken with the idea of a genetically engineered super-salmon that grows at twice the rate that other farmed salmon do. Yet, as he points out, if we don't stop heedlessly devouring the wild fish from our oceans, one day that might be all the salmon man has left. The same goes for other fish.