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The future of our seas has never been more precarious. Ninety years of industrial-scale overfishing has brought us to the brink of an ecological catastrophe and deprived millions of their livelihoods. As scientific guidelines are ignored and catches become ever bigger, Alex Renton tells why the international community has failed to act

A tuna transport floating tank being towed from the fishing grounds off Libya to tuna ranches off Sicily, Italy. Photograph: AFP/Gavin Newman

Anyone who cares about where their fish come from - and this should mean anyone who wants to go on eating them - should take two tools when they visit the fishmonger. One is the handy guidance provided by the Marine Conservation Society, Fish to Avoid and Fish to Eat (the latter is still the longer); the other is a ruler. My ruler is the type handed out to commercial fishermen by the international advisory body, Incofish, and has pictures of key species with marks indicating when they can be considered mature