Fishing Lines: Why we should believe in cod

Ten unusual things that have been found in cod stomachs: a baby's dummy; a set of false teeth; a wrapped sandwich; a turnip; a baby hare; a miniature whisky bottle, contents intact; a packet of cigarettes (several were left in the packet, leading us to conclude that cod are probably non-smokers); a pair of nail scissors; a slightly tattered Barbara Cartland paperback; and a set of car keys.

From this, we can learn two things. First, cod are not fussy eaters. Second, your plate's wodge of batter has probably had a more interesting life than you.

The imminence of a cod-fishing trip off Bradwell in Essex, and rereading Mark Kurlansky's book Cod: A Biography of the Fish That Changed the World, have made me acutely interested in a fish that most know only from its association with chips. Cod should be extinct, given the fishing pressure they have sustained. Off Newfoundland, once one of the largest cod populations, stocks have declined more than 99 per cent since the 1960s.

So how come cod haven't joined the giant moa, the tarpan, Stellar's sea cow and the quagga? Well, they are pretty cute at survival. A 20lb female can lay more than six million eggs. In 12 days, those eggs become tiny fish. When the sea temperature nears 0C, cod rely on a form of anti-freeze in their blood.

They're pretty smart, too. Cod can recognise their own kind. They have acute hearing, and talk to each other by a series of grunting noises, rather like teenagers. They have even learnt to conserve energy by allowing themselves to be carried on the tide, a sort of aquatic train service.

Norway lobsters, or scampi, are one of their favourite foods, and they can dig up buried shrimps and crabs, often working as a team to do so. Their sense of smell is so acute they can detect some amino acids at concentrations of less than one part to a million.

So the seas should be bursting with cod. But of those six million eggs, just two will survive to become parents. And overfishing is wreaking havoc. Trawlers with nets so large they could accommodate 16 Boeing 747s and more are taking all the bigger fish.

Older fish that have spawned previously are more successful at it than younger fish spawning for the first time. As David Conover of New York's Stony Brook University told the American Association for the Advancement of Science: "By selectively harvesting the largest fish, we end up changing the whole biology - not only growth rates, but egg size, fecundity, feeding behaviour.

"The scary part is that when we stopped size-selective harvest, the biology didn't change back. It was permanent." Small wonder that the Marine Conservation Society recently tried to stop supermarkets selling cod captured from unsustainable sources.

So maybe anglers shouldn't be fishing for them at all. But here's another cod mystery. How come a fish that gulps down plastic cups thrown off a cross-Channel ferry (oh, didn't I mention that one?) seems singularly uninterested in the yummy chunks of squid and lugworm I offer them?

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