Last year I began to wonder, this year doubt is seeping away, to be replaced with a rising fear. Could it really have happened? Could the fishing industry have achieved the remarkable feat of destroying the last great stock?

Until 2010, mackerel were the one reliable catch in Cardigan Bay in west Wales. Though I took to the water dozens of times, there wasn't a day in 2008 or 2009 when I failed to take 10 or more. Once every three or four trips I would hit a major shoal, and bring in 100 or 200 fish: enough, across the season, to fill the freezer and supply much of our protein for the year. Those were thrilling moments: pulling up strings of fish amid whirling flocks of shearwaters, gannets pluming into the water beside my kayak, dolphins breaching and blowing. It was, or so it seemed, the most sustainable of all the easy means of harvesting animal protein.

Even those days were nothing by comparison to what the older residents remembered: weeks on end when the sea was so thick with fish that you could fill a bucket with mackerel just by picking them off the sand, as they flung themselves through and beyond the breaking waves while pursuing their prey.

Last year it all changed. From the end of May to the end of October I scoured the bay, on one occasion paddling six or seven miles from land – the furthest I've ever been – to try to find the fish. With the exception of a day on which I caught 20, I brought them back in ones or twos, if at all. There were many days on which I caught nothing at all.

There were as many explanations as there were fishermen: the dolphins had driven them away, the north-westerlies had broken up the shoals, a monstrous fishmeal ship was stationed in the Irish Sea, hoovering up 500 tonnes a day with a fiendish new vacuum device. (Despite a wealth of detail on this story I soon discovered that no such ship existed. But that's fishermen for you).

I spoke to a number of fisheries officials and scientists, and was shocked to discover that not only did they have no explanation, they had no data either.

So I hoped for the best – that the dearth could be explained by a fluctuation of weather or ecology. When the fish failed to arrive at the end of May I told myself they must be on their way. They had, after all, been showing off the south-west of England – it could be only a matter of time. I held off until last weekend.

The conditions were perfect. There was no wind, no swell, and the best water visibility I've ever seen here. I looked at the sea and thought "today's the day when it all comes right."

I pushed my kayak off the beach and felt that delightful sensation of gliding away from land almost effortlessly – I'm so used to fighting the westerlies and the waves they whip up in these shallow seas that on this occasion I seemed almost to be drifting towards the horizon. Far below me I could see the luminous feathers I used as bait tripping over the seabed.

But I could also see something else. Jellyfish. Unimaginable numbers of them. Not the transparent cocktail umbrellas I was used to, but solid, white rubbery creatures the size of footballs. They roiled in the surface or loomed, vast and pale, in the depths. There was scarcely a cubic metre of water without one.

Apart from that – nothing. It wasn't until I reached a buoy three miles from the shore that I felt the urgent tap of a fish, and brought up a single, juvenile mackerel. Otherwise, though I paddled to all the likely spots, I detected nothing but the jellyfish rubbing against the line. As I returned to shore I hooked a greater weever – which thrashed around the boat, trying to impale me on its poisonous spines. But that was all.

Is this the moment? Have I just witnessed the beginning of the end of vertebrate ecology here? If so, the shift might not be confined to Cardigan Bay. In a perfect conjunction of two of my recent interests, last week a monstrous swarm of jellyfish succeeded where Greenpeace has failed, and shut down both reactors at the Torness nuclear power station in Scotland.

The Israeli branch of Jellyfish Action pulled off a similar feat at the nuclear power station in Hadera this week.

A combination of overfishing and ocean acidification (caused by rising concentrations of carbon dioxide in the atmosphere) has created the perfect conditions for this shift from a system dominated by fish to a system dominated by jellyfish.

If this is indeed what we're seeing, the end of vertebrate ecology is a direct result of the end of vertebrate politics: the utter spinelessness of the people charged with protecting the life of the seas. In 2009 the Spanish fleet, for example, vastly exceeded its quota, netting twice the allowable catch of mackerel in the Cantabrian Sea, and no one stopped them until it was too late.

Last week, the European commission again failed to take action against the unilateral decision by Iceland and the Faroes to award themselves a mackerel quota several times larger than the one they agreed to, under their trilateral agreement with the EU and Norway. Iceland and the Faroes have given two fingers to the other nations, and we appear to be incapable of responding.

The mackerel haven't yet disappeared from everywhere, but my guess is that the shoals which, since time immemorial, came into Cardigan Bay, were a spillover from the mass movements up the Irish Sea. As the population falls, there's less competitive pressure pushing them towards the margins. Without data, guesswork is all we've got.

I desperately hope it's not the case, but it could be that the fish that travelled to this coast, in such numbers that it seemed they could never collapse, have gone.

monbiot.com