MANKIND'S transition from hunting and gathering to farming began about 12,000 years ago, when people decided to stay put and cultivate the plants they liked. Although people have since domesticated a vast array of the world's animals and plants, in one area at least we have never really shaken off our hunter-gatherer roots. Most of our fish is still caught in the wild, much as it was in our ancestors' time—albeit with a few more fancy bits of gadgetry to tip the balance in favour of the hunter, rather than the gathered.

The thrill of the chase, though, may increasingly be a thing of the past. Fish farming has been the world's fastest growing food-production sector, with output rising 8.8% a year since 1970, according to the Food and Agriculture Organisation. By comparison, livestock production increased at a rate of 2.8% a year. Today, some 45% of all fish consumed by humans—around 48m tonnes—is raised on farms. That is still only half of what is caught in the ocean (much of which goes to feed livestock), but at this rate, in eight years farming will produce as many fish as are caught at sea today.

AP

Part of the solution?

Levels of wild-fish catches have been stable since the mid-1980s, and the vast majority of the world's capture-fisheries are fully exploited—or indeed over-exploited: we cannot, therefore, catch more wild fish than we do today. But the demand for fish is booming, thanks to growing numbers of people, and their increasing affluence.

By 2030 a whopping 37m extra tonnes of fish will be needed to maintain current levels of fish consumption per person. The missing fish that needs to be found to sustain levels of consumption has been dubbed the “fish gap”, and it will have to be filled by fish farming.

But fish farming has its problems, too. It does not, for instance, always increase the total amount of fish available. Carnivorous farmed fish must be fed wild fish; for every pound of salmon eaten, several pounds of wild fish must be caught. Currently, much of this fishmeal can be obtained by using industrially caught fish to feed fish rather than animals, but what happens after that? Fish farming already uses up most of the world's fish oil and a hefty chunk of its fish meal.

Many aquaculturists are now eyeing krill, a small crustacean found in the cold waters of Antarctica. It is an excellent source of nutrition for farmed fish. Unfortunately, krill is central to the Antarctic marine food-web, and it is also an excellent source of nutrition for all of the species in the Southern Ocean.

Another problem with salmon farming has been revealed in a recent paper that shows the damage salmon farms can do to nearby wild fish populations. Farmed salmon, kept in unnaturally high densities, are a breeding ground for parasitic sea lice. When young wild salmon on their way to the ocean swim past pens of farmed salmon at the mouths of rivers, they get infested with these lice. The scientists say that lice infestations could drive some salmon populations they studied in British Columbia to extinction in four years.

Such problems seem likely to worsen as fish farming grows. But there are solutions. One is to farm more vegetarian fish such as tilapia and catfish. Another is to move salmon pens to better locations. Earlier this year, John Fredriksen, the main shareholder in Marine Harvest, the world's largest salmon farming company, said that salmon farms ought to be moved away from wild salmon runs.

A better, although more expensive, solution would be to make fish farms self-contained. OceanBoy Farms, a company in Florida, produces organic inland shrimp using cleverly designed ponds that avoid another environmental side-effect of fish-farming—the dumping of fish faeces and uneaten food onto the bottoms of sensitive marine environments, such as Scottish lochs. Some have linked this filth to the growth of toxic algal blooms.

Compared to terrestrial agriculture, fish farming is young, and it has a lot of growing up to do. Like farming, it causes environmental problems but offers great benefits. The world will have to find solutions to the first in order properly to enjoy the second.