Ethical eating? It's a fishy business...

With stocks dwindling, fish-lover Jamie Merrill decided to eat only sustainable varieties. But he soon found out it wasn't that simple.

Sink or swim: writer Jamie Merrill with two line-caught Cornish mackerel (David Sandison)

Take me out for dinner and I'll often start with mussels then opt for steamed Thai-style sea bass, while at home some of my favourite dishes are rainbow trout with herbs and spaghetti vongole. All this seafood has been causing an uneasy feeling in my stomach of late, though. Not from a past-its-best piece of plaice or pollack, but from the constant drip, drip, drip of fishy stories.

Like most fish eaters, I've known for some time that the supplies of fish in our oceans are dwindling, but despite the work of organisations such as the Marine Conservation Society, campaigning films such as End of the Line and campaigns by celebrity chefs such as Hugh Fearnley-Whittingstall and Jamie Oliver, I hadn't realised until recently that fish is our last truly "wild" food and that we are rapidly in danger of losing it for ever.

This summer the drip of stories seemed to became a torrent and headlines such as "Fish stocks plundered", "Just 100 adult cod left in North Sea" and "Only 50 years of fish left" got me thinking. Like many dedicated foodies, I'd be the first to boycott a delicious regional produce if it was endangered or damaging the environment, so why was I happy to keep munching my way through my favourite seafood dishes as if cod was going out of fashion?

The scientific consensus is now increasingly clear and ignorance is no excuse. In his recent book, Ocean Life How Our Seas Are Changing, Callum Roberts, a professor of marine conservation at the University of York, explains that many species of fish have been reduced by an astonishing 99 per cent over the past 100 years.

"The advent of the ocean-going engine in the 1900s meant our fishing fleets were able to go further and deeper in the pursuit of fish," says Professor Roberts, who acted as a consultant to the BBC's Blue Planet series and The End of the Line. "This meant mankind gained the ability to change the seas on a planetary level and that fish no longer had anywhere to hide from us.

"At the same time globalisation has shielded us from ever having to confront the reality of what we've done to the world's ocean because we're rich enough to import what we want and pillage the developing world of its fish stocks to get it."

The result of this century of devastation is that species such as Atlantic salmon and cod have become commercially extinct, many other species face annihilation within a generation and we now have to import 101,000 tons of cod and 60,000 tons of haddock (worth a combined £528m) from outside the EU to replace the fish that the Atlantic and North Sea can no longer provide.

With this chilling knowledge, I recently set myself a personal challenge; to only eat truly sustainable seafood. Greenpeace defines this as fish "from a fishery with practices that can be maintained indefinitely without reducing the target species' ability to maintain its population". This seems clear enough, but unlike organic or fairtrade produce there is no nationally recognised standard for sustainably sourced seafood. Seafish, which gives advice, carries out research and provides recipes is sponsored by the UK government and tends to act as the voice of the industry. Whereas the Marine Stewardship Council, an international non-profit that awards certification standards for sustainability, is a far more benign body, but has fallen out of favour with campaigners and some scientists over which fish and fisheries it grants its blue seal of approval.

According to Professor Roberts, the similarly named Marine Conservation Society (MCS) and its well-researched fish-by-fish guide is a good place to start. In the end, though, I stumble across some guidance from Greenpeace and its Seafood Red List. Primarily because it seems the most stringent, I adopt it as my new mantra.

"The most frequent question I get is 'what fish can I eat?'", says Willie Mackenzie, a campaigner for Greenpeace. "There isn't a simple answer, but the best one I can give is that if people are buying their fish from Sainsbury's, Marks & Spencer and Waitrose, they are moving in the right direction." Choosing the right supermarket, I soon find out, is just the start. Greenpeace also recommends several basic rules as such opting for line-caught fish, buying from small-scale fisheries where possible, sticking to fish from the British south-west over Atlantic varieties and avoiding certain species entirely.

Even that isn't the full story, as Professor Roberts explains: "The trouble for concerned shoppers is that there isn't a lot of practical help available." And while some stores are certainly more open than others, it is still hard to find a truly sustainable fillet of fish if you hold yourself to the highest stands and err on the side of caution.

One of my favourite dishes is wild scallops with pancetta. I was pleased to see that Marks & Spencer sells this bearing the MCS logo, but a search on my smartphone shows it's dredged rather than hand-dived. Dredging for scallops causes massive damage to the seabed and the dish falls short of my new standards.

At Waitrose I discover that Heston Blumenthal's fish pie contains Scottish farmed salmon and farmed prawns from Thailand. It's a similar mixed story, with much supermarket rainbow trout and sea bass. I had assumed farmed fish was a safe choice but as MacKenzie explains, it falls short of the mark in several key ways.

"You can't make something out of nothing," he says. "And the biggest problem with fish farming – aside from environmental concerns around pesticides and spread of diseases to wild stocks – is that you are essentially just laundering fish. For example, a farm will take anchovies from the South Atlantic, turn them into fish meal and feed it to captive salmon in Scotland. The environmental economics of this just don't stack up."

All these restraints mean I'm soon eating a lot less fish at home and often end up leaving the supermarket empty-handed – despite turning to the internet and grilling the staff where possible. Eating out should be easier thanks to a website called Fish 2 Fork, which rates more than 500 chains and restaurants on their sustainable credentials. Again, the reality is tougher and I struggle to find staff who can explain their restaurants fish policy with ease of clarity.

In the end I end up having countless exchanges with clueless staff and one awful exchange with a passive-aggressive chef who refuses to explain where his cod is sourced from. My friends soon tire of this and I start to understand how the first progressive wave of 1970s vegetarians must have felt. Even firms such as Yo! Sushi, which scores surprisingly well on Fish 2 Fork, still uses 300m tons of farmed salmon a year and soon becomes off limits to me.

Whether I'm reading supermarket packaging or a restaurant menu, buzzwords such as "farm assured" and "line-caught" are thrown around, but it's hard to track down what they really mean. Perhaps because, as Professor Roberts explains, they are essentially meaningless: "Line-caught rarely described a fisherman sweating on deck with a rod and pulling out a single fish. The reality is that lines are set with thousands of hooks on them and cause enormous collateral damage."

The whole issue is incredibly complex and at times I can't face the battle of working out what is safe to eat, but even experts such as Professor Roberts admit there's no point in trying to stop people eating fish. The developing world depends on it as a source of protein and eventually in a refined form, fish farming will have to be part of the solution.

MacKenzie makes an excellent point for the rest of us. "When I was young," he tells me, "fish such as salmon and shrimp were posh treats, but now they are really common commodities that you can get in a sandwich at Tesco.

"We've cheapened fish over many generations into something people thing is easy to get inexhaustible and of course it isn't." For me it's become a rare treat again and considering the state of the world's oceans, that can only be a good thing.

The Greenpeace Red List: Fish to avoid

Atlantic cod

Plaice

Tuna (excluding Skipjack)

Tropical prawns (farmed and wild)

Haddock (except line-caught Icelandic)

European Hake

Atlantic Halibut

Monkfish

Atlantic salmon (wild and farmed)

Marlin

Shark

Skate

Ray