When I was a child, my father used to take me for walks, often along a river or by the sea. We would pass people fishing, perhaps reeling in their lines with struggling fish hooked at the end of them. Once I saw a man take a small fish out of a bucket and impale it, still wriggling, on an empty hook to use as bait.

Another time, when our path took us by a tranquil stream, I saw a man sitting and watching his line, seemingly at peace with the world, while next to him, fish he had already caught were flapping helplessly and gasping in the air. My father told me that he could not understand how anyone could enjoy an afternoon spent taking fish out of the water and letting them die slowly.

These childhood memories flooded back when I read Worse Things Happen at Sea: the Welfare of Wild-caught Fish, a breakthrough report released last month on fishcount.org.uk. In most of the world, it is accepted that if animals are to be killed for food, they should be killed without suffering. Regulations for slaughter generally require that animals be rendered instantly unconscious before they are killed, or death should be brought about instantaneously, or, in the case of ritual slaughter, as close to instantaneously as the religious doctrine allows.

Not for fish. There is no humane slaughter requirement for wild fish caught and killed at sea, nor, in most places, for farmed fish. Fish caught in nets by trawlers are dumped on board the ship and allowed to suffocate. Impaling live bait on hooks is a common commercial practice: long-line fishing, for example, uses hundreds or even thousands of hooks on a single line that may be 50-100km long. When fish take the bait, they are likely to remain caught for many hours before the line is hauled in.

Likewise, commercial fishing frequently depends on gill nets – walls of fine netting in which fish become snared, often by the gills. They may suffocate in the net, because, with their gills constricted, they cannot breathe. If not, they may remain trapped for many hours before the nets are pulled in.

The most startling revelation in the report, however, is the staggering number of fish on which humans inflict these deaths. By using the reported tonnages of the various species of fish caught, and dividing by the estimated average weight for each species, Alison Mood, the report's author, has put together what may well be the first-ever systematic estimate of the size of the annual global capture of wild fish. It is, she calculates, in the order of one trillion, although it could be as high as 2.7tn.

To put this in perspective, the United Nations Food and Agriculture Organisation estimates that 60 billion animals are killed each year for human consumption – the equivalent of about nine animals for each human being on the planet. If we take Mood's lower estimate of one trillion, the comparable figure for fish is 150. This does not include billions of fish caught illegally nor unwanted fish accidentally caught and discarded, nor does it count fish impaled on hooks as bait.

Many of these fish are consumed indirectly – ground up and fed to factory-farmed chicken or fish. A typical salmon farm churns through 3-4kg of wild fish for every kilogram of salmon that it produces.

Let's assume that all this fishing is sustainable, though of course it is not. It would then be reassuring to believe that killing on such a vast scale does not matter, because fish do not feel pain. But the nervous systems of fish are sufficiently similar to those of birds and mammals to suggest that they do. When fish experience something that would cause other animals physical pain, they behave in ways suggestive of pain, and the change in behaviour may last several hours. (It is a myth that fish have short memories.) Fish learn to avoid unpleasant experiences, like electric shocks. And painkillers reduce the symptoms of pain that they would otherwise show.

Victoria Braithwaite, a professor of fisheries and biology at Pennsylvania State University, has probably spent more time investigating this issue than any other scientist. Her recent book Do Fish Feel Pain? shows that fish are not only capable of feeling pain, but also are a lot smarter than most people believe. Last year, a scientific panel to the European Union concluded that the preponderance of the evidence indicates that fish do feel pain.

Why are fish the forgotten victims on our plate? Is it because they are cold-blooded and covered in scales? Is it because they cannot give voice to their pain? Whatever the explanation, the evidence is now accumulating that commercial fishing inflicts an unimaginable amount of pain and suffering. We need to learn how to capture and kill wild fish humanely – or, if that is not possible, to find less cruel and more sustainable alternatives to eating them.

• Copyright: Project Syndicate, 2010.

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