Over a third of wild fish stocks have collapsed and only 2-3% of the world's big sea fish are left. The state of the oceans is, if anything, worse than the state of the rainforests. And far, far less is being done about it. We are, quite simply, taking out too many fish, for food and for fertilizer, using destructive and wasteful fishing methods. And we have been for over a hundred years. And it's accelerating. The percentage of fish species registered as threatened by the Red List rose from 4% in 2000 to 37% in 2008. And that's only the species they've managed to evaluate.

It's not just the loss of bio-diversity, nor even the thought that your grandchildren might never get to taste anchovies or tuna if we carry on this way. A combination of straightforward pollution, our selective removal of key species from ecosystems, and global warming, is creating vast dead-zones of water where nothing can live, except toxic algae blooms and the occasional jellyfish (the cockroach of the sea as far as indestructibility goes). As we rely on the oceans for more than half of the oxygen in the atmosphere, if we get too many dead zones, we're in even more trouble than if we were to lose all the forests.

It's not just a matter of saying "Well, when they get so rare we that it's no longer commerically viable to fish them, we'll just wait a few years and they'll bounce back". Often, they don't. If you take a species out of a system, the system has perforce to adjust. New species (usually simpler organisms from lower down the foodchain),  move in, and make it exponentially harder for an over-fished species to recover. It is feared that cod, once so plentiful at Cape Cod that they "stayed the ships" with their swarming bodies, have been supplanted in their Atlantic ecosystem since the 1992 crash by jellyfish and are likely never to be able to recover that particular slot in the marine world.

There's lots of information at the Marine Conservation Society (MCS - not to be confused with the MSC, the Marine Stewardship Council) on fish to eat and fish to avoid. But here are some highlights:

Any kind of sole or flatfish - and, notably, monkfish - is overwhelmingly likely to have been bottom-trawled. The supertrawlers (subsidised globally with tax-payers' money) use vast nets, large enough in some cases to engulf cathedrals, to scrape up and flatten miles of seabed (twice the area of the continental United States every year), desolating it and wiping out all the life thereon. This is a bit like bulldozing rainforest so we can eat sloths.

Most prawns you're likely to eat in this country will have been farmed by third-world producers in environmentally-catastrophic conditions. Coastal habitats such as mangrove deltas (one of the richest and most diverse natural habitats) all over developing countries (Thailand, India, Mexico, Malaysia) are being replaced by prawn farms and polluted by the chemicals used to clean the tanks and keep the prawns alive under massively over-crowded conditions. After a 5 year stint has made the local water undrinkable, killed off the local wild-fish populations and put the local people at risk from pathogens bred in the tanks, the farmers (often city entrepreneurs) move on. Of course, the antibiotics, pesticides and colourants in farmed prawns don't make them the best choice for your health, either. Some northern wild-caught prawns, preferably from the North Sea, are a viable option - if you can find them. However, the usual levels of bycatch (juveniles or unsaleable species which are thrown back dead) are horribly high.

More or less anywhere you buy unnamed "fish" in the UK, it's going to be cod. And cod is in trouble. So much so that in 2007 WWF tried to take the European Union to court over their failure to obey their own regulations (to issue a moratorium on fishing where stocks drop below a certain level). The EC First Council have now blocked this.

The collapse of the Atlantic cod stocks is a particularly clear illustration of a major factor that prevents recovery after overfishing: the weakening of the gene pool. Fish, particularly females, tend to get more fertile with age. If you catch all the big fish, not only do the smaller specimens remaining produce fewer young when the breeding season comes round than their larger companions would have done, but you gradually force the species to put more energy into maturing faster and less into building healthy cells - which can lead to genetic weakness in the stock and increase vulnerability to disease. Which, of course, is something that happens anyway if you destroy the bulk of the population. Before industrial fishing came along, an average-sized codfish was a yard long. Now they average 16 inches, because we simply don't let them live that long.

Some supermarkets, notably Sainsbury's, are making an effort: all their salmon, they boast, is farmed. But farming can be even more damaging to wild populations than wild fishing: notably in the case of the wild pink salmon crash in British Columbia. In 2002 there was a 97% drop in the wild population after infestations of sea-lice in farms near their runs. And it takes, at a conservative estimate, around 3 pounds of wild fish, ground into pellets, to make a pound of salmon flesh. Much of that, currently, comes from krill, the base of the northern food chain and the main food of whales. And krill are in trouble because of global warming. You might also want to consider the 23 litres of petrol used to bring each 5 kilos of salmon to the consumer.

Why is so little being done?

It's much harder to police the sea than the land - it's bigger, for one thing, and there are no visible borders, for another. Plus there are parts where anyone can legally fish. And while there are many individual fisherman and monster companies who are happy to exploit the oceans to the last sprat if there's money in it for them, there is no organisation with any real power to protect stocks on an international level. Japan, Iceland and Norway have made a joke of the whale-protection laws; in 2006 Russia and Canada opposed a global petition by 1000 scientists to end high seas bottom trawling; French fisherman this year are staging port blockades to have their fishing quotas actually increased, and EU politicians are ignoring their own scientists' frantic advice to reduce quotas all round and stop fishing altogether on some species. Did you know some of your taxes are going to fund the building of new fishing boats? All this is in addition to the pirate fishing vessels from all nations, including our own.

The French fisherman, quite reasonably, argue that if they don't take the fish, someone else will. And it's hard enough for small businesses to compete with the monster fishing fleets. At the top of the industry, fish are rounded up by FADs, frozen into blocks at sea, transported to developing countries for cheap filleting, re-frozen and sent round the world again to be processed further. 

Also, even if one country tightens up its quotas and makes it harder to land over-quota fish, the catch is already on board - black market ships can just mosey off to the nearest country that doesn't care, and land and process it there.

So no one's actually responsible for looking after our oceans. There's no global organisation, currently, that can force countries and corporations to be sensible, to use sustainable methods, to obey scientists' moratoria, to reduce bycatch or to fish with respect for breeding seasons and life-cycle.

And, of course, the other reason: our willingness to buy cheap and dirty. This puts pressure on supermarkets and restauranteurs, which in turn puts pressure on suppliers, who, even where legislation exists, are then tempted to mislabel.

What can we do?

• Educate yourself. "Yeah, I hear the fish are in trouble" carries a lot less weight than "Is your cod caught in the Atlantic or the Pacific?" For a brilliant if traumatic in-depth analysis of the industry problems and some positive ideas for action, try Taras Grescoe's excellent book Bottom Feeder, a Bill Bryson-esque tour of fishing communities and companies. The man loves eating fish and the book is an intelligent, forceful and entertaining investigation of how he and we are to continue doing so.

• Get picky, buy ethically. Don't buy fish if you don't know where it comes from, because if they're not making a fuss about it, it won't be sustainable. Suppliers in Europe are not yet required by law to say, for example, what species of tuna is in the tin. Some supermarkets such as Sainbury's and M & S and Asda are starting to sell sustainable fish in their own brand products, but while you can still buy John West's anonymous tuna from the same shelf the gesture seems to me a bit hollow. But this is exactly where the consumer can make a difference. They won't change until a) we make it profitable for them, or b) it's too late. Your choice.

• Avoid plastic waste. A single one litre drinks bottle could break down into enough small fragments to put one on every mile of beach in the entire world. The tiny pieces of plastic, apart from actually smothering some areas (did you know there's an area of plastic rubbish in the Pacific two-thirds the size of the United States?) chemically attract toxins, get into the food chain and cause a whole lot of problems for creatures like albatross, turtles and humans.

• Ask in restaurants. And if you don't like the answer, or there isn't one, act upon it. And, more importantly, if you find a restaurant serving something ethical, praise them and order it! It's a lot of effort and expense for them to go to in a difficult industry - there's no legislation ordering it or tax breaks for doing it - and they need all the encouragement they can get. 

Here are a couple of very short lists. See MCS for longer and continually updated info. You'll need to watch out for fishing methods: even if the species is OK and in season, it may have been caught by a method generating unacceptable levels of bycatch.

Buy:

• Herbivorous, organically farmed, freshwater species, such as carp (if you can find it). Organically-farmed trout, while carnivorous, is probably the only viable farmed option round here.
• Small fish such as sardines (though no longer the delicious anchovy - stocks have recently crashed, probably due to over-fishing for fertilizer)
• Anything low down on the food chain such as oysters, scallops etc- if sustainably harvested.
• MSC certified Alaskan pollock
• MSC certified Cornish mackerel (in season)

Boycott:

• All prawns, especially tiger prawns (except the northern species pandalus borealis, which is currently doing OK).
• Atlantic cod
• Atlantic halibut
• Chilean sea bass
• North Atlantic, Mediterranean or Bay of Biscay anchovy
• Shark (down, it is estimated, to 1% of the natural population levels)
• Monkfish. Sorry.
• And any flatfish. Anything bottom-trawled, basically.
And be very, very careful about tuna.

The End of the Line, coming soon to cinemas near you, demonstrates the extraordinary fact that governments have to wait for the activists to excite film makers to spread the message to the public before they'll exercise their basic duty of care, not just for the fish but for the planet, which, hello, includes us!