Kayaking saved me. Living in Oxford without a car, I felt throttled by the ring road, the city's concrete necklace. I was heartsick, dried up, deprived of nature. At weekends I'd explore the city's green spaces or cycle into the countryside, but I found only sterility: pasteurised parks, perfect rows of rape and wheat, woods picked clean by pheasants. Walking by a stream one day, I realised that the land might be dead but the water was alive. I bought an old kayak for a tenner and dragged it down to the Thames. As soon as I sat in it, I felt I belonged there.

Oxford was built on a swamp. Though wrung from the ground, the water is still there, forced into a labyrinth of drains and feeders, most of them unknown, overgrown, blocked by rubbish and fallen trees. I set out to explore them. I pushed through rush-choked channels scarcely wider than my boat. I found backwaters no one had navigated for years. I stumbled across cannabis gardens and camouflaged shelters where fugitives lived. I dragged my kayak out of the water and through the branches of fallen trees. I'd come home covered in mud and duckweed, scratched to ribbons and thrilled to be alive. I saw mink, roe deer, water rails, kingfishers, sandpipers, the debris of fish and clams eaten by otters, all within the bounds of the city.

From the water, everything looked different. Curtained by trees, fish-shadowed, a channel between the park-and-ride and the dump became a tributary of the Amazon. Abandoned behind railway fences, on the edge of playing fields, anonymously skirting business units, I found places I had never imagined possible, a parallel world. In these hidden corners I also saw great shoals of chub and bream, a giant carp slurping at scum in a neglected drain, barbel furrowing away. But I wasn't interested. After years away from the water, I was ready to start fishing again, but I wanted to catch only fish I could eat.

It was fishing that cemented my love of the natural world. As a boy, I'd sit on the riverbank, seldom catching much, gazing at the insects and birds, watching the fish. The thrill of seeing a vast, lazy tail appear beneath a sunken tree, or dark backs of dace flick in and out of the shadows, or the head of a pike emerge from the darkness – this was all the world I needed. While other children fantasised about space or treasure islands, I submerged myself in the dim green cosmos beneath the water, guessed at but never fathomed. Now there was something else I wanted: a way out of the planet-eating food economy.

I love food, but I hate the way it is produced. There used to be a surplus of allotments in Oxford: I took on five and became an urban smallholder. But I had given up eating fish. I knew commercial fishermen possess a mysterious power over governments, which ensures quotas are too high, reefs can be smashed by beam trawlers, dolphins, turtles and albatrosses snared and discarded. I knew if I was to eat fish, I'd have to catch my own.

I was assisted by another environmental crisis. Someone had released red signal crayfish into the Thames and they had proliferated, wrecking the ecosystem. But perch, which have firm, clean flesh a bit like sea bream, love them. In some places, they hung under the trees in great shoals of two- or three-pounders. I bought a tiny telescopic rod and some little gold spinners. Wherever I found a deep pool, I would tie my boat to an overhanging tree and cast into the gloomiest places. I soon began to catch fish on every trip.

One day I was fishing in my favourite spot, but the perch had vanished. I cast beneath the far bank and my spinner became snagged. I tugged and the thing I had hooked cruised off like a tractor: slow but unstoppable. My ridiculous little rod curved down into the water. When the monster's head at last broke the surface, its eyes were six inches apart. Eventually I dragged into the boat the biggest pike I had ever seen, its teeth like daggers. I returned it to the water as quickly as possible.

It was then that I realised what a kayak could do. You can launch it from anywhere and catch just about anything – even tuna, sharks and giant skate. I didn't want to hunt those species, but I did want to become self-sufficient in fish. Three years ago, disaffected with urban life, I moved to mid-Wales. I started to put my plan into effect.

On a good day, a mile out to sea, you can see the whole of Cardigan Bay. It's smooth, shallow and sandy, and almost devoid of sheltered places from which to launch. Apart from a few crabbers and a small but destructive fleet of scallop dredgers, there's scarcely any commercial fishing here.

I bought a sea kayak specially rigged for fishing and began to investigate. There are plenty of species here – from whitebait to basking sharks – but I wanted to pursue only those whose numbers are high. I might take the occasional bass, bream or plaice, but I would not subsist on them.

Mackerel pour into the bay in summer, and are easy to catch. There are big shoals of herring in winter, though kayak fishing is more dangerous then. The reefs hold plenty of small pollock. The best species to hunt would be dogfish: a small shark whose population has exploded thanks to the offal and by-catch dumped by the fishing industry. But dogfish have green eyes like cats, and you have to hit them again and again to kill them; I cannot fish for them. There is one other species that no one seeks, even though it cooks very nicely. It's the most dangerous animal in British waters. It has formed a fair portion of my diet over the past three seasons – I'll explain in a moment.

My challenge, though, was to find a common fish I can pursue all the year round. There is one obvious candidate: the estuaries swarm with grey mullet. But hardly anyone fishes for them because they are widely considered impossible to catch.

The first two summers were terrible. Rain and gales lashed the coast. Even on rare days when the wind dropped, the swell was often big enough to roll the boat over. This isn't especially dangerous if you're a good swimmer and keep your head, but it makes fishing almost impossible.

I soon discovered the kayak fisher in Cardigan Bay faces three hazards. The first is the offshore wind. You can make way against a force 4 or 5, but not for long and not if it strengthens. I was prepared for this and haven't yet been caught out.

The second is landing. Keeping a surf kayak perpendicular to the waves is easy, but fishing kayaks are much longer: the stern gets knocked around by breaking waves and the boat skates. In even a moderate sea you're likely to get tipped. That's fine as long as you know what to do: duck and flatten yourself on the sand until the kayak washes over you – stand up too soon and the next wave will bring it down on your head. But if you wipe out in a heavy sea, there's no time to duck.

The third hazard is the most interesting, and dangerous. I came across it on my first trip, half a mile off the coast. I had set out to catch mackerel, found them almost straight away and was bringing them up in ones and twos, tiger-striped, fast and stupid. Then I hooked something that felt different.

While the mackerel dashed around crazily, this thing stayed down and shook its head. I could feel the vibrations all the way up the line. I brought it to the surface; it was about 18 inches long and thin, almost eel-like, mottled brown and white. I had no idea what it was. As I lifted it out of the water, it started thrashing madly. I swung it in towards my free hand, but just before I grabbed it some ancient alarm went off. I dropped it and, pulling up my bare feet, studied it as it rattled around the deck. I had never seen anything like it. Fins ran the length of its body, shimmering purple and green. It had a snake's stripes on its flanks, bug eyes on top of its head and a huge, upturned mouth. Suddenly, from some long-forgotten book or poster, the name swam into my mind.

You may have heard of its nearest relative. Every year, a few hundred people have the misfortune to tread on a lesser weever, a small fish that buries itself at the water's edge at low tide. When it feels threatened, it raises its dorsal fin, which contains three spikes charged with poison. The pain is said to be excruciating – rather like a scorpion sting – and can last for days. The greater weever is much the same, but the sting is worse – if you have a weak heart, it can kill you. Most people survive, but if you are stung in a kayak, you're unlikely to make it back to land. The pain and toxic shock would make paddling almost impossible. So now I carry a club; if I catch a weever, I draw it against the side of the kayak and hit it very hard. When it's dead, the dorsal fin relaxes and you can bring it aboard. It makes excellent bouillabaisse and curry: it has firm, white flesh a bit like monkfish.

I have painted a grim picture so far, but don't let it put you off. To fish from a kayak is to become an animal: calm, cunning and free. On the first day that photographer Dominick Tyler joined me, we were idling about not far from the shore, looking for mackerel, when he pointed to a disturbance in the water. Seven finned backs rolled through the surface like greased wheels. The usual collective nouns – pod and school – strangely compressed and buttoned down, are all wrong. This was an exhilar­ ation of dolphins. We followed them for two miles along the coast. They came up behind us and exploded from the water; they leapt together and crossed in midair. We never lost touch with them: even when they dived, we could see the smooth scars of turbulence they left on the surface.

Tracking fish means following birds. Gannets never lie. I have seen them hang against an emerald sky at twilight, white crucifixes shot with the last of the light, then fold their wings and fall like darts into the water. Sometimes, surrounded by a flock of living thunderbolts, I have felt the spray on my face as they plumed into the sea. They always find the fish.

Shearwaters are less reliable. They skim along the coast, working the water all day, just above the waves, as they can land only at night. It's taken me three years to decide that the shearwaters and I have been chasing each other, each convinced the other one knows where the fish are.

Sometimes the birds take me far out to sea. Three miles off the coast, hearing just the cry of gulls and the tipping of the waves, I find the place of comfort I have always sought. This is my altar, my sanctuary. Here at last I can live the wild life of the spirit.

When I catch mackerel, I always eat some on the beach. The best way to cook them is not to. Raw mackerel straight from the boat is the best fish I have ever tasted. The second best way to cook mackerel is as follows. Land them; gut them; stuff them with wild thyme from the shingle slacks behind the beach; and roast on a driftwood fire. Sometimes I throw beach parties where I bring a grill and nothing else. We take turns to set out through the waves and catch fish for the barbecue.

Fishing like this is hard. Sometimes the catch scarcely replaces the energy I've used. Occasionally I find a monster shoal, so dense I can fill my bags in half an hour. But self-sufficiency means taking fish throughout the year. The mackerel leave in October and don't return until May. I freeze some of the catch, but it doesn't last beyond December. I will eat fish for only part of the year unless I can find a way to catch grey mullet.

Mullet live in the estuaries, so you can fish for them on days (and this means most of the winter) when it would be too dangerous to take the kayak on to the open sea. You can also use a canoe, which is easier to anchor than a kayak and has more room for tackle and spare clothing. The only problem is working out how the hell to get them. I know of people who have resorted to crossbows and shotguns. But I did my research, and eventually I learned about an ancient technique that scarcely anyone uses. I'm not going to tell you what it is, in case you buggers spoil the fishing, but it involves a mobile lure you have to make yourself.

I launched my canoe into the Dyfi estuary with Dominick just after low-water slack. The Dyfi at low tide is a sandy desert split by a thousand channels. Some lead into a wilderness of mud and cockleshells. Others eventually wind into the main river. In the canoe, you have no idea where you're going: the only clue is the taste of the water.

In a channel somewhere among the sandbanks, we found what I had been looking for. Along the far bank, the surface was oddly riffled and chopped. Whenever a bird flew over, it exploded in spray. Very slowly, keeping low, we edged across the channel until we were just 10 yards from the bank. Then we shipped the paddles and I slid the anchor into the water. I paid out enough warp to bring us level with the shoal, carefully picked up the rod and cast. The fish ploughed across the surface as the bait hit the water, but immediately regrouped. I tied on a lighter lure and cast again. I started to wind, and immediately the rod tip went down. To my intense disappointment it was a bass. I put it back and tried again.

This time the rod banged over more persuasively. Even before I got the net under it, I knew what it was – or so I thought. As I unhooked my first grey mullet, too small to keep, I noticed a spot of brilliant gold on its gill cover. Only later did I discover this meant it wasn't a grey mullet but a golden one. I fished on but caught two more bass, so I packed up before I did any more harm.

So here's the score so far. Plenty of mackerel, though only sometimes. Too many greater weevers. The odd pollock, whiting, gurnard and bass; no grey mullet. Lots of energy expended; one or two near-death experiences. A tough way to feed myself. But very much alive.

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