Fishermen leave the cold little brooks, the swift rivers, and the freshwater lakes to cast over the big blue water and the shining flats. They hear of and then experience the Barnum-and-Bailey world of great fish that they thought light tackle was never intended for, and often they miss a sort of angler's third world--between the land and the sea.

Florida's southwest coast is an intricate pattern of that third world of brackish water, tritely called "the sea's nursery." There is no more fertile zone than brackish marsh, and here it is dramatized by mangrove forests and endless water routes. It begins vaguely somewhere north of the Ten Thousand Islands, and it ends vaguely somewhere past Florida's southern tip. They call it the "back country."

DAWN IS THE BEST TIME on the roadside canals at the land edge of the big swamp. The traffic has not yet really begun, and the sun comes up red out of Miami a hundred miles away. The formations of wading birds are outward-bound from their roosts or just becoming settled along the ditches, the raccoons are still busy patrolling the edges, and silent alligator heads--three knobs representing eyes and snout--are more plentiful than they will be later.

It is dawn when the canal fish tend to be most active. If the tide is falling, a matter hard to predict that far from the Gulf of Mexico, the freshwater bait will be coming down from the river of grass with its cypress knobs, alligator holes, and little ponds. You hope to be there when the fish meet the bait, and you watch for little showers of it at gaps in the bulrushes, cattails, or saw grass. The fish themselves, snook or bass or tarpon, appear as gentle bulges against the swamp side of the canal or as pistol-shot strikes. The bubbles left by such strikes are what the ditch fishermen look for as they seek a station.

There are times when the streamer must match inch-long "glass minnows" and other mornings when a longer streamer is better, something more desirable to be selected from thousands of lesser things. And there are mornings when a drab little lure, sinking groggily beneath the foam of a strike, will be scooped up somewhere near the bottom. And if it is not too scientific, it is at least a matter of selectivity.

When the sun grows hot and the traffic builds, you leave the ditch and walk to your car across a road shoulder that has been used by the fly fishing famous for some 50 years. Tourists from Cincinnati or St. Paul will see your rod and wonder what can be caught in a roadside ditch, not knowing it is connected loosely to the seven seas.

"Probably bluegills," they will announce. They have never learned that ditch fish have won tournaments.

But in the boat you can be in the back country in seconds with no gradual change from grass to mangroves. The mangroves take over instantly, some of them 70 feet tall, and they have gradually pushed inshore since I first saw them 25 years ago. When man drained off more fresh water, salt water intruded and brought drifting mangrove bulbs and their groping roots with it.

IN THE DEEP MANGROVE CREEKS the outboard motor shuts out the swamp noises and sends many of its residents into hiding, but the anhinga flies ahead of you over a narrow stream along with the kingfisher and the green heron. The creek may narrow until its knitted overhead branches form a tunnel--a tunnel sometimes so brushy you fend off branches with your hands--and thick cobwebs promise that no one has fished there recently.

Then an inland bay opens abruptly, miles from the open Gulf, perhaps so small a bay that it is only a long cast across. Or call it a pond or a little lake. No, call it a bay, for that is how it has always been known by those who hide in the swamp. For the swamp is a hiding place.

The Indians hid there to watch the pirates who hid there after forays in the open sea. The smugglers hid there to avoid the Union navy, as did draft dodgers of two wars. Rum runners also hid there but moved through the outer islands in the deeper channels. Only three miles from the tight little bay with its fallen old buttonwood trees were the anchorages of the world's wealthy who brought their yachts here before a road crossed Florida from Miami and came down from Naples. They fled from the business world and to hide from an intrusive public.

Now the drug runners make their pickups from planes and deep-water boats in the endless outside channels, but they are not likely to come to the little bay where alligator poachers and deer jacklighters have hidden from park rangers.

You hide from all of the outside world and cast the little streamer against an ancient buttonwood log with dangling mangrove roots near it and watch for any silent swell or telltale flash beneath.

After the outboard has stopped its idling sputter, the sounds of the mangrove swamp will return--the caws of crows, the unidentified splashes, and perhaps the bottle-cork pop of a snook's strike so far under the mangroves it cannot be reached. At midday there may be a pressing, humid air-stillness that exaggerates any sound of life.

In the natural human estimate of fish by the size of the water they occupy, the newcomer to the little bay at the end of the mangrove tunnel is never prepared for the occasional presence of a giant tarpon, somehow out of scale. If he strikes and leaps to crash into the mangroves or to leave the fly high in the air over his turgid upheaval, there is no fisherman who can accept the explosion calmly. You are likely to remember the thrown streamer silhouetted against a thunderhead or dropping into a mangrove tangle.

It is more likely to be a snook or redfish that takes the lure, or possibly a five-pound "crick" tarpon, still appearing as a big fish in such confined quarters.

MORE FISHERMEN HAVE COME to the mangrove swamps, and it is different from the times when we stayed on the little houseboat with the skiff tied alongside. At night we heard the intermittent roars of banks of mosquitoes coming and going with the breezes, and in those times the sound of an outboard's singsong as it threaded through rivers and creeks was an event for comment.

In those days you saw no other fisherman for days, and when the porpoise or manatee came by, you felt the houseboat rock gently on quiet nights and grinned as you changed position in your bunk.

There were some occasions when blasting strikes might jar you awake, and the waves from them would move the boat and thump the skiff gently against its side.

When the fluorescence made its night imagery in the water, you could throw bread crumbs from the houseboat's stern and stare at the graceful futuristic shapes of gafftopsail catfish turning in eerie illumination as they fed silently, and when one broke the surface you were startled.

But when the National Park Service marked the boat route from Chokoloskee Island and the outside channels all the way to Flamingo, the headquarters at the south end of Everglades National Park, there was a change. They opened up some of the creeks so that bigger boats could go through, and anyone with good eyes could steer those inside waters all the way. Gone was the bit of pride that went with knowing 70 miles of unmarked channels where the propeller cleared the oyster bars by inches, and there was no reason for anyone to be lost anymore.

The bigger boats that everyone seemed to buy were fast, and there was no longer much excuse for living aboard down in what they called the Shark River country. It is not unusual to run more than two hundred miles for a day's fishing and spend the night in a modern motel.

But while the bigger, faster outboard opened up the main routes, it had a different effect on the little creeks that went back in erratic patterns to other little creeks and led to little bays and to broad tidal rivers. A canoe or slender johnboat and a machete could go places the fishing machine could not reach, and although the Park Service might frown on it, the Swede saw and hand ax could break way into waters unfished for years.

The mangrove everglades change; new islands have grown and old ones have disappeared. Another form of timelessness is in the ceaseless movements of tides, even in the little closed creeks, and a professional guide and I once came upon some giant cut timbers lodged barely afloat in a creek just wide enough for us to get through.

"I'll be damned," he said. "Those old sticks have floated around that country for 30 years. They were here when I was a kid. I guess they never will find their way out."

The old timbers had wandered about through two or three hurricanes, noted casually by new generations of fishermen and gradually riding lower and lower in the water. I haven't seen them now for 10 years but I keep expecting to.

LIKE THE HUNTER WHO FINDS vine-covered headstones in what he thought was virgin forest, the fisherman finds human traces at strange points in the mangrove country. Some 30 miles from a dock there are canals, arrow straight and slowly being reclaimed by the persistent mangroves. They were dug in the 1920s by developers who saw them as a simple means of draining the everglades, a preposterous idea viewed with wonderment by anyone with elementary engineering knowledge. Yet some of the land-to-be was sold before that dream burst.

In and near the old canals are remains of sunken barges and draglines. Even more provoking are the gradually disappearing boilers of a "shark factory" that thrived briefly upon a great platform on a Shark River island, producing all of the things to be made from sharks.

And to one who has seen only a little recent history of that strange half-land in a quarter-century of fishing across it, there is a special feeling at the sight of the banana trees where the hermit once lived, his little house more nearly hidden each year. He has been dead for years now.

And I know there are the scant remains of an old dock at the end of a cluttered creek, nearly filled with the rotting trees of some vicious storm that struck unseen and unheard by humans a long time ago. The "farm" served by the dock had disappeared long before the storm, and the farm story is doubted by a fisherman who truly believes he is at the end of the world. The tarpon used to wait for us in that creek, rolling indolently. I know the farmer's nephew and I know people who were raised at Chatham Bend where the old houses are now crumbling. And to prove my right to tell of the "old days," I once saw a map with a muddy bay named after me.

WHILE THE ROADSIDE DITCHES ARE BEST AT DAWN, the back country is best as evening comes on, especially when the thunderheads pile high and the tide runs hard in the narrow creeks, gurgling against the roots with an urgency that tells of rain in the grass country. Rain has ended the smell of smoke and the flying particles of ash carried from the saw grass fires, part of the ecology of the region.

There will be the distant rise and fall of sound from an airboat far off in the open glades and the beginning of night noises in the swamp. Perhaps there will be the frantic screeching of a raccoon fight somewhere nearby. And as you carefully apply the streamer to a little eddy below a dead tree, you hear a quick, soft, swishing sound and see a little flock of white ibises flare above you, startled to find you blocking their low-altitude flight along the sheltered creek on their way to roost.

There is something below the dead tree and you sense it somehow. Perhaps there was a barely perceptible swell to the water, or was there a little flash somewhere deep in the dark, stained creek?

"Hold it!" you hiss to whomever is handling the little boat. As night came on you had consciously reduced your movements to stealth and lowered your voices almost to whispers. The fellow on the oars churns a little too much in stopping, and you wish you had the electric motor instead.

You look behind you, for the backcast must slip between the columns of trees that line the creek. Your companion crouches a little, partly in fear that you might drag your backcast, but mainly to become invisible--as if a fish behind a log in a 30-foot-wide creek would be checking the cargo of a 14-foot boat.

YOU EXAMINE THE LEADER and wonder why you are so sure you saw something, but when you cast again there is no doubt. There is a bulging swirl, pushed upward by some broad tail and heavy, irritable form, but the streamer comes in without hesitation and you decide to change it. Yellow instead of red and white this time. Or possibly one of those colorless things the man sent you in the mail. Make it a white-and-blue bucktail.

As you tie it on, you wonder what it could represent, and you work the line out with a few false casts. But you have only 30 feet to throw it, and this time you decide to let it sink before you begin the retrieve. That means it must come very close to the old log. It does.

It sinks rather fast with its heavy hook, and when you start to bring it in with a twitching retrieve, it stops and you set the hook violently against something solid below the log. It starts down and away, and for the moment you had better give line.

My God! It could be anything!