Sex, Death and Fly-Fishing By John Gierach 238 pages. Firestone/Simon & Schuster. Paper, $8.95. Where the Trout Are All as Long as Your Leg By John Gierach Illustrated. 84 pages. Lyons & Burford. $15.95.

In the title essay of "Sex, Death and Fly-Fishing," John Gierach describes what trout-fishing people call a "spinner fall," or the final stage in the mating dance of the adult mayfly, known as a spinner for its swooping and swirling motion.

"A mayfly spinner lies on the surface of the stream in what fishermen call the 'spent' position. To picture it accurately, remember that the insect has just had the first and only orgasm of its life and is now, in the natural course of things, dying from it. His body lies flush with the water, wings spread, legs out flat, tails splayed wistfully. Usually he's limp. If he struggles at all, he does it feebly at best. There's probably a silly look on his face, although it's hard to tell with insects."

Mr. Gierach, who grew up in the Midwest, who now lives on the St. Vrain Creek in Colorado, and who previously wrote "Trout Bum" and "The View from Rat Lake," represents a new generation of fishing writers. As the foregoing passage shows, he's technically and entomologically hip, but not piously reverent about fishing.

As other passages in "Sex, Death and Fly-Fishing" reveal, he is capable of feeling awe in the presence of nature. Of the North Fork of the Cache la Poudre River, in Wyoming, he writes: "The stream itself looked new, in that raw way canyon streams have. There's a sense that the place is still being carved, that it isn't finished and never will be. The boulders that are littered around still have crisp edges free of lichens from where they broke out of the walls and fell in. If you look up on the slopes you see eons of exposed geology, but then some of this looks like it happened yesterday."

But he also laughs at nature in these essays, cataloguing the drawbacks of subsistence living and pointing out that in the Denver area at least, plain old woodsmoke accounts for some of the air pollution, which suggests that the natural way may not always be the healthy way. You might say he's a counterculture fisherman, but he has a sense of humor about that too.

"Sex, Death and Fly-Fishing" was published last year and can be found in piles by cash registers in tackle shops, where it is reported to be selling well. It's just a mite on the wordy and undisciplined side. Any subject serves his muse. He even builds an essay, "The Chairman's Bass," around a huge fish he saw but never even tried to catch. But at any point in its 18 essays, one is likely to find a useful tip, a funny line or a passage that catches the mood of a stream exactly right.

Mr. Gierach's new book, "Where the Trout Are All as Long as Your Leg," is far leaner and sparer in its language. This makes you appreciate all the more the places where he lets himself go a little, like the line about fishing after dark -- "I couldn't see them now, but I could still hear them: rises that sounded like a moose trying to wade quietly in deep, still water" -- or the passage in which a beaver slapping its tail scares the author so badly that he falls over backward in a pond and goes under.

The pieces in "Where the Trout" are about the secret places everyone dreams of where the fish are bigger and the hatches go on forever. In the most haunting essay, the author recalls how as a 5-year-old, he and his gang one day ventured "out there" into nature where the adults had forbidden them to go. They didn't catch anything but they learned an essential lesson about fishing: "that when done properly it is socially unacceptable, and that the farther out there you go the better it gets."

Like all devoted anglers, Mr. Gierach never lets us forget that the secret places are getting steadily scarcer as civilization encroaches. But he puts things in self-ridiculing perspective by pointing out that one of the compensations of ecological catastrophe will be that it will enable environmentalists to say: "There, see? We told you so."

Mr. Gierach is also good at explaining why human beings like to fish. In "Where the Trout," he traces the roots of the hunt back to the beginning of religious ritual. In "Sex, Death and Fly-Flying," he writes: "This fishing business probably has something to do with play -- practicing a highly refined food-gathering technique as if it really mattered, even though we don't need the food and will probably release any trout we happen to catch. Play is what puppies do. It looks like good, innocent fun -- and it is -- but it also develops the predatory skills that will be needed later in life by the serious adult canine. Ever notice how hard a puppy can bite?"

But Mr. Gierach's most compelling story about why people fish doesn't explain a thing. It is told in an essay on the guiding business in "Sex, Death and Fly-Fishing" and describes an aged man who arrived to meet his guide in a chauffeured limousine. The man had barely the strength to wade to the right spot in the stream, just above a bridge. But when he had tottered there, he made a perfect cast and was onto a 24-inch trout.

"The fight was long, and not what you'd call one-sided," Mr. Gierach writes, "but it came to a point where the big, strong trout was below the bridge and the frail old fisherman was above, with some deep, treacherous wading in between where the current sluiced between the pilings.

"The old man started under the bridge, shuffling, leaning into the current, and the guide said, 'Uh, listen, would you like me to run down there and net him for you?'

" 'What I'd like,' the old man said, 'is for you to keep your goddamned hands off my fish.' "

In short, there is sex, there is death, and there is fly-fishing. And the old man had his priorities exactly straight.

Photo: John Gierach