I took myself to the Brazilian Amazon, on a mission to catch an arapaima which is often said to be the largest freshwater fish in the world.

I'd done what research I could, but I had no clear destination. It hadn't been possible to narrow my search down to a specific place. In the end it wasn't quite sticking a pin in the map, but it wasn't far off.

My plan was to go as remote as possible, beyond the reach of the subsistence fishermen. To help me navigate the human environment, I had done three months of intensive language study - a solid three hours every day.

And I was well prepared for the rigors and uncertainties of traveling that lay in wait, having made three expeditions to the much tougher Congo rainforest in equatorial Africa.

So I was confident of success. My three months in Brazil brought me a grand total of zero arapaima. I finally got a small one, four feet long and weighing maybe forty-five pounds, six years later, after going back every year in between.

Most of the Amazon, it turns out, is nothing like what we see on nature documentaries, where it seems you can hardly move without tripping over boa constrictors and jaguars.

Unlikely as it sounds, there is a huge commercial fishing industry, much of it to feed the two million inhabitants of Manaus, the improbable city at the heart of the region. Countless boats, carrying tons of ice, make voyages of hundreds of miles up the winding tributaries- especially the Purus - catching fish and buying fish from locals. Back then, lack of ice and refrigeration in the remoter parts was no help to the arapaima, thanks to the market for their dried, salted flesh and the general lack of other ways for people to get cash.

I remember asking José where I could find a lake where the arapaima fishermen didn't go. He snorted at my naivety and said, "They get everywhere."

They beat the water with sticks to drive the fish into the meshes. They use harpoons to probe for fish that had learned to lie low and stay quiet.

Fishing this way, if a lake is small and not too deep and snaggy, a team of nets men can completely empty it of arapaima, everything apart from the baby two- and maybe three-footers, which slip through the mesh.

It is tactical and ruthless, and it has to be seen to understand why a lone angler with a rod just can't compete, and why any rare escapees can be almost supernaturally wary.

There was hard evidence for their existence in the past. But arapaima hunters don't catch and release. The fact that a lake, or a region, held arapaima in 1910, or even 1970, means nothing today.

These two experiences, with carp and arapaima, taught me a lot about the importance of research. I could have saved myself a lot of time, trouble and disappointment by only fishing waters where there had been documented carp captures.

But on my arapaima hunt, there were no documented rod-and-line captures to go on that was my whole reason for wanting to catch one.

Looking back, conventional wisdom says I should have bailed out of the Purus sooner than I did, and tried elsewhere. Certainly when I did so, I found that arapaima in less pressured waters are normally much easier to tempt. But I find I have (almost) no regrets, because in this case my inefficient angling brought a level of understanding that I wouldn't have come to otherwise.

There are times, it turns out, when angling is not the be-all and end all, but a door to something else.

It's also true, of course, that it's necessary to experience some failure to calibrate our appreciation of success.

Not enough failure should prompt as many questions as not enough success.

Nowadays, though, I don't have the luxury of time. If I don't get a fish I risk being out of a job. No fin no fee, as somebody, maybe me, once quipped.

And that really concentrates the mind: I can't afford to go to the wrong place. Of all the things I have to consider for the kind of fishing that I do, to bring about the magical convergence of right bait, right place and right time, place is the component with the widest range of possibilities. There's a lot of water out there, way too much to explore in a single lifetime, so it's a process of narrowing down.

I start by researching the geographical range of my target species, as far as this is possible, in books and online, long before I go anywhere near the water. But this is only the start.

Most big freshwater fish, in most parts of the world, have all but disappeared from most places where they used to live. As with arapaima, the main reason is over-harvesting, but there are other factors too. Dams block the migration routes of many fish, so they disappear from the water above the dam - or even altogether, if breeding grounds are cut off. Draining of floodplains, cutting off backwaters, competition from invasive species and pollution also play a part. And sometimes it's just willful slaughter, as was the case with North American alligator gar in the early 1900s, thanks to the incorrect assumption that killing these predators would boost populations of game' fish. Consequently it's a sad fact of life that most big fish now tend to be restricted to isolated pockets here and there, which can be hard to find

To locate these special places, it's a matter of keeping one's antennas active, of tuning in to the buzz and whisper of the water people. It's a world of tip-offs and informants, of reading between the lines, of triangulating one source with another, to weed out the wishful thinking, the exaggerations and misinformation. It's about correlation and extrapolation, about building a meaningful picture from mere scraps and fragments. In other words, it's an extension of what I used to do when I was fishing in England: the same principles but with less information. Except sometimes there's so little information that it seems to shift into something else, more like astronomy. When I first considered going to Zaire, now Democratic Republic of Congo, in search of goliath tigerfish, the place was an information black hole. In the end there was no alternative to going in blind. It was a miserable two-month blank trip, but an invaluable recon. (I ended up in an area where it can be both dry season and flood season at the same time - work that one out! - with the fish dispersed throughout the forest and even netsmen going hungry. In other words: right place, wrong time.) Now though, after more than three decades of doing this kind of thing, in various far-flung corners, I'm in the happy position of having done the leg work, and the paddle work, and knowing the basic lie of the water. With this mental framework in place it's easier to extract meaning from new wisps of intel, and to set off with some degree of confidence. Once on location, the narrowing down continues. I talk to people, and try to find reliable sources, to take me to a deeper layer of detail. So important is this that, even if I have a tranllator, I try to learn some basics of the language in advance, at least some greetings and key fish-related vocabulary. A little of this can go a very long way, thanks to the fact that fishing itself is a universal language.

In short, then, finding the right place is all about good information, about tapping in to local intelligence - particularly if time is limited. Only right at the end - the final zeroing-in of the cross-hairs and divining the coordinates in three dimensions - does it change, sometimes, into something else. But up until that point it's information, information, information, whatever kind of fishing you are doing. Or you can ignore everything I've said and fish for rumors. A good angler studies the rules for the very purpose, sometimes, of breaking them. In fact there can be special delight in tearing up the rules and scattering them to the wind, because fishing is also about the power of hope, sometimes against all odds. And sometimes it is about the miraculous.