It's 6.30am at Newlyn market in Cornwall. Men in yellow wellies off-load plastic boxes of fish and ice from the boats moored at the quay outside. The fish merchants huddle round, bidding with subtly raised eyebrows and chins. It's a quieter morning than usual, but the catch is still a mermaid's dream: box after box of pollack, hake, haddock, sea bass, red mullet, cod, spotted ray, star ray, lemon sole, dover sole, brill, monkfish, turbot, john dory, megrim and glistening mackerel, firm with freshness and plucked from the sea just an hour or two before. It's an awe-inspiring sight.

The reason I'm here, with early morning bags under my eyes and meltwater under my feet, is all down to the Tate. Sean Davies, executive head chef for the gallery's restaurants, regularly takes his kitchen brigade and front-of-house staff on jaunts to meet their suppliers. Today, they're visiting the renowned Cornish fish merchant Matthew Stevens, and I've managed to blag a ride. It seems somehow fitting that fish from Newlyn will end up on the gallery's menus: you can see the mackerel fishermen hand-lining in the bay from the windows of the Tate's Cornish outpost on the coast in St Ives.

Ironically, all the talk of dwindling fish stocks has made me eat more of the stuff, not less - particularly mackerel, which is still so abundant at certain times of year that it practically jumps into the boats.

I've also been won over by the rebranding of the Cornish sardine (a fish formerly known as the pilchard). Simply grilled, or filleted and fried in a devilish coat of mustard, cayenne and flour, there are few more summery lunches. It's mad that we preferred to bury them in dodgy tomato sauce for so long. Sardines are one of those fish, like red mullet, that we tend to think of as southern European, but they're just as much part of the West Country catch. Eating them is not only a pleasure - it makes a lot of sense.

For lists of sustainably caught fish, visit fishonline.org, a website set up by the Marine Conservation Society

Don’t want to cook? Check out the sustainable tinned fish from Fish4Ever, including tuna, mackerel and sardines in organic olive oil. Just add salad. From £1.75, from Asda, Morrisons, Waitrose and delis.

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