Knowledge can be such a burden. Knowing about fish – I’m a fisherman – is great. I can tell the difference between a sea trout and a salmon, a pike and a dinosaur. But knowing about how fish come to us makes shopping hell. I stopped eating Atlantic salmon four years ago because I knew our wild stocks were disappearing. I knew because I had stood in the very rivers to which they should be returning. (Before you ask, I don’t fish for salmon. I just like standing in rivers.) Then I stopped eating North Sea cod, which meant fishfinger sandwiches were a thing of the past. Friends would get edgy when I came to dinner. “It’s not cod,” they would say, “it’s Antarctic sea bass.” I would eat it – it’s delicious (pirate fishermen call it “white gold”), and I’d rather not embarrass my host – but in my heart I knew that “Antarctic sea bass” was really Patagonian toothfish: one of the most endangered fish in the oceans.

Then even I started to stick my head in the sand. Press releases would arrive on my desk that I would hide, but not before noticing headlines such as “Tuna: Endangered”, “Swordfish: no!” Buying fish was becoming a minefield: was there anything I could pick up without thinking, “Is this the piscatorial equivalent of eating panda?” It would take me so long to choose fish that the frozen prawns would start to defrost in my basket – and I knew in my heart that I shouldn’t really be buying prawns anyway, not without knowing how they had been caught. I had also brainwashed my friends, and would get frantic mid-shopping phone calls: “Is haddock OK?” (Yes, if it’s line-caught and from Iceland.) “Lobster?” (Australian rock lobster is fine – but expensive).

Last month the Marine Conservation Society published a book called The Good Fish Guide. Bernadette Clarke of the MCS spent a year collating information so as to find out which fish are OK to eat, and which aren’t. There’s a “20 species most at risk” list, too (which you can also see on the website: www.mcsuk.org). This is fantastically useful: at last, the consumer has easy access to information that was previously available only to the industry. It imparts basic facts, such as which of our fish stocks are “sustainable” and the impact that fishing has on the rest of the environment.

But there is quite a lot to take in, even if you are a fish maniac like me. It’s not about one species being good and another being bad. It depends on where the fish is caught, and how. “I’m even more depressed now,” said one friend to whom I gave the book, “I got so confused in Marks that I ended up buying chicken.”

In other cases, supermarkets tell us if products are organic, where they come from, who they are approved by. So why can’t the supermarkets, where most of us buy our fish, help us with labelling – for example, with some nice big “sustainability” stickers? You may have seen a few labels – probably the best-known is the blue tick of the Marine Stewardship Council, which means you can “eat fish with a clear conscience” (but don’t beat yourselves up about it too much. Its chief executive doesn’t always manage to buy fish from sustainable sources either, although he does try). Most likely you have seen it on a relatively new fish called hoki, which is being marketed to take some of the pressure off cod. Bird’s Eye is currently working on a hoki fishfinger.

The problem is that there isn’t one nationally recognised symbol, and not enough fisheries have yet been certified. And, off the record, the supermarkets are nervous about putting “sustainable” on one species because they know it will make it even more obvious which ones aren’t.

The thing to do is learn about the fish you like eating. If cod is your thing, you can take some solace from the fact that four-fifths of the cod sold is not from the North Sea, which is the most endangered kind. Ideally, you should choose cod from Icelandic waters (all of Waitrose’s is). Remember the cod wars of the 1970s? Canny Iceland hung on to its fishing waters, unlike the UK, which sold off 60%. Iceland has a 200-mile exclusion zone; no one else is allowed to fish in its waters, and it is generally accepted that it is a good fishery. Because our fishing waters are mostly shared, there’s more of an every-man-for-himself attitude. Without naming names (the Spanish and Italians), not all EU nations are as responsible as we are about sticking to quotas (you can imagine the Englishman, can’t you: “No, you first – no, really…”).

So, as a rough guide, fish from Icelandic waters is generally OK. Wild Atlantic salmon is a no-no, but Alaskan is fine because it’s sustainable – it’s also from the Pacific ocean. Do keep up. If you buy farmed, try to make sure that it’s organic, as this answers a lot of the problems associated with farmed fish: what they eat, how many are kept to a cage, etc.

According to The Good Fish Guide, of the top 20 species to avoid, 12 really have no alternatives and should therefore be avoided completely. These are: ling, Patagonian toothfish (beware, this fish is called many other things, such as Chilean sea bass), dogfish, grouper, marlin, monkfish, orange roughy, shark, skates and rays, snapper, sturgeon (and caviar) and swordfish. These are the names to focus on and avoid. And of those, how many do you really eat, unless you’re a big show-off and go to fancy restaurants? For most people, ignoring the swordfish and skate on the menu would be the biggest sacrifice they would have to make. Tuna, the third most popular fish we eat, is generally well labelled enough now for us to avoid the really naughty ones and go for dolphin-friendly skipjack or yellowfin instead. Not on the 20 least-wanted list but worth a mention, I think, is whitebait. Avoid it (I know it’s delicious) because whitebait are juvenile herring that have never had the chance to spawn. You’re eating babies, for heaven’s sake.

Warm-water prawns are the ones most people don’t think about. These are your big tiger prawn-type things, and they are fished for either by trawling – which damages the sea bed and accounts for a third of the world’s total bycatch (the catching and killing of unwanted species) – or by farming, which is often carried out irresponsibly. Workers tend to be badly treated, mangrove forests are devastated, and the farming causes massive pollution. So look for Madagascan prawns – these are much more responsibly farmed. Waitrose was the first to use them, but you can now get them in other chains.

A word of advice about supermarkets. Where they sell prepacked fish and fish on a counter, buy from the counter – even if it involves queueing or dealing with a surly youth. The more environmentally friendly and sustainable (ergo, more expensive) fish is sold there. This is because we “buy with our eyes, not according to price” from the fresh fish counter. Sainsbury’s, for example, sells Icelandic cod on the counter but cod from less sustainable sources is prepacked. If you buy more of the former, the chances are they won’t stock so much of the latter.

As to how the fish was caught, look for “line-caught” which generally means long-line-caught (rod and line is probably the best of all, but rare). Again, there are degrees of responsibility: long-line fishing can kill seabirds (they dive for the bait, get hooked and drown), so if you want to be really good, try and find out – no mean feat – whether the lines are weighted. That means they sink faster than the seabirds can swoop.

The important thing is not to get a headache, think, sod this, and just not eat fish. If you have a decent fishmonger, you’ll get more information out of him or her than from any supermarket. But if you do buy from a supermarket, give them something to do: ask loads of questions. Police those fish counters. The alternative is unthinkable – becoming vegetarian.

This was first published in The Guardian.