Ethical isn't easy. All of us who have cupboards stuffed full of “bags for life” know that. But one must try to shop decently - and I try especially hard with fish. Because I like them: catching them and eating them. For a start, I would rather my omega-3 came with scales and fins than in a gluten-free capsule.

There will be no commercially viable stocks of wild fish left by 2050, one recent report claimed. And by 2010 half the world's fish will be farmed. Despite all the evidence of disappearing species, European governments are still subsidising the industry to hoover up the fish, with all the thought for the future of a slash-and-burn farmer in the Amazon jungle. I'm especially cross at the way they catch the wild fish that I love - anchovies, mackerel, sardines - to process them into feed for the Day-Glo orange travesties that are farmed salmon. How daft is that?

So, if you want your grandchildren to eat more than jellyfish and crab sticks, you have to make a fuss. I do pester the fishmonger, and even the chip shop owners, with questions about where their fish comes from. I bore my friends in restaurants. (“If that sea bass is not handline-caught, you can jolly well have the fish pie.”) I've taken a ruler around the fish market in Barcelona (they eat babies over there; they do!); I peer for the blue tick “sustainability” logo of the Marine Stewardship Council.

But it isn't easy. Working out what is ethical in the fish market is harder than deciphering the codes on the back of sweet packets. Part of the problem is that there are no packets. My friend Melissa says she always asks the fishmonger how the catch was landed: “And I know they always lie! But it isn't really feasible to go in there with a big book of what's sustainable and what's not and start an argument.”

I've had fishmongers try to sell me “fresh organic tropical tiger prawns” and “sustainably caught North Sea cod” - both of them impossiblities. Recently I found myself in an Edinburgh restaurant that promised its ingredients were local and “sourced in a way that respects both habitat and history”. Charmed, I asked where their “fresh scallops” were from. “How do you mean?” replied the waiter. Well, were they dived or trawled, I asked. After consultation, she announced with a confident smile: “They're from the west coast and they were definitely not diver-caught.” So they were trawled? “Absolutely, yes.” Trawling for scallops is not respectful at all, of course: it's essentially the dragging of a huge rake across the seabed. It kills everything in its path and leaves a desert.

So how do you become a better fish buyer? The Marine Conservation Society and other ocean-watching groups have just published an encyclopaedic guide, The Good Catch Manual. Covering 50 species, from anchovy to zander, it is aimed mainly at professionals, but you can download it free from its website (www.goodcatch.org.uk). It's fascinating, both on the characteristics of fish and on the ins and outs of what you can eat, guilt-free, and when. But it weighs as much as a fish supper for four, and thus is not easily deployable on the high street. For people such as Melissa, who want to be tooled up for a row at the fish counter, there is a wallet-sized weapon in the MCS's Good Fish Guide, which you can order or download at www.fishonline.org.

Tinned tuna is now much easier to buy with a clear conscience. Last month Greenpeace usefully issued a top ten of the most popular brands, ranked according to their fishing practices and labelling systems. Sainsbury's came out best and John West worst. I'm impressed by the Fish4Ever brand, which you can buy in independent shops or from its website, and which is so friendly to the tuna that it's almost as if it has apologised to the fish on your behalf. Being good is easier if there's someone to organise it for you.

alex.renton@thetimes.co.uk

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