What makes this fecundity all the more striking is its placement directly alongside so much seeming lifelessness. The sea itself lies in a deep cleft produced by the southern extension of the San Andreas Fault millions of years ago. It stretches for about 700 miles north to south, and is bordered on both sides by the hot Sonoran desert. Hike inland a couple of miles and you can find a Baja wilderness of dryness, muted color and eerie silence. But along the shore mudflats and shallows in the north support a wealth of animal and marine species. Steep undersea canyons farther south (some of them reaching depths of more than two miles) produce rich swellings of nutrients that anchor a food chain rivaling the tropical rain forests in diversity. Almost 900 species of fish live in the Sea of Cortez, along with some 30 species of marine mammals.  There's plenty to catch from the beach in Baja -- inshore species like roosterfish and sierra will fight harder than any trout you've hooked in Montana. But the deep-water game fish are what dreams are made of. Finding them requires motoring out past a shelf where the water changes from pale green to deep blue and then trolling lines patiently (oh, so patiently) until the fish turn up. The first bolt of lightning comes when a dorado or tuna charges across the water to hit one of the flies. Once it does, the lucky fisherman fights the fish while the other one reels in and starts throwing handfuls of live sardines into the water. If the caught fish is part of a school, the sardines will lure others to the boat. Then you cast -- sometimes for an hour -- to a swirl of fish riled into a feeding frenzy.  Often, though, the fish will surprise you.  On our second day of fishing, we were trolling with fly rods rigged for dorado when a huge striped marlin surfaced suddenly at the back of our boat. The captain, Rosario Cota Rochan, known as Chayo, let out a yell and all at once the mate was rigging a conventional rod with a mackerel and feeding it out toward the marlin. He motioned for me to grab the rod and for the briefest of moments I thought about refusing in favor of continuing to troll with my fly line. When I came to my senses the marlin was already hooked and I spent the next 45 minutes fighting it for all I was worth.  Let me just say that fly or no fly, it was wonderful. Every last minute of it. When the fish jumped we saw that it was huge -- 200 pounds or more -- and when it dove toward the bottom it was all I could do to stay on the boat. ''The Old Man and the Sea'' and other fishing clichés swam through my head as my back began to ache and my hands threatened to slip from the rod. It was just me and the fish, mano a pescado -- if you don't count the six-ton cruiser that Chayo was deftly maneuvering back and forth to help me win the battle.  When the fish finally approached the boat wearily, Chayo asked if we wanted to keep it. We said no and he seemed happy to let it swim away after it had regained its strength. This is something relatively new in the Sea of Cortez. Not so long ago, marlin were routinely butchered, if only to supply the fisherman with a dockside photo to take home. This still happens all too often today, but efforts to encourage catch-and-release fishing for billfish are making big strides.  And this is crucial. For all of its seeming abundance, the Sea of Cortez is indisputably under siege from gill-netters, long-liners and poachers. Fish stocks are nothing compared to what they once were. But there seems to be a growing realization in Mexico that protecting the environment is as crucial as development to Baja's future.  The most promising project is a proposal by a coalition of Mexican government and nongovernment entities -- in conjunction with Scripps and other organizations -- to set up a system of marine reserves in the Sea of Cortez aimed at protecting important spawning grounds and underwater habitats. Getting the reserves established will not be easy politically, and protecting them from poachers would be harder still. But the effort remains a big step forward.  ''The Mexicans involved in this process are nothing short of heroes,'' said Paul Dayton, a professor of oceanography at Scripps. Take a boat out fishing when the sun is rising over the Sea of Cortez and you'll know exactly what he means.  Snorkel Above a Reef, Take a Desert Hike  Of the unspoken satisfactions of calling yourself a salt-water fly fisherman is that you can lord it over all those other guys who catch fish using conventional tackle and bait. Never mind that bait fishermen tend to hook more and bigger fish. The important thing is that you've chosen the road less traveled by opting to stalk your prey with a whippy little rod and an impossibly thin leader. This experience, though, doesn't come cheap. Gary Graham (www.bajafly.com), a guide and outfitter, charges as much as $550 a day. If you don't need the expertise of a fly-fishing guide, renting a boat from your hotel can be less expensive. The Web site for the Hotel Buena Vista Beach Resort has details (www.hotelbuenavista.com). Meanwhile, there are plenty of other things to keep adventurous souls occupied in southern Baja. Here's a sampling.  Desert wilderness is never far away in Baja and the only way to enjoy it is to head out into it. You can hike into the hills right out of Los Barriles or Buena Vista. Maps are available at most of the area's hotels..  The Cabo Pulmo marine reserve south of Buena Vista offers some of the best snorkeling and scuba diving in Baja. You can swim with sea lions and turtles. Tropical fish are abundant. All-day tours from the Hotel Buena Vista cost about $70 for snorkeling and $121 for scuba diving and include all equipment. A few miles north, Punta Pescadero also offers great snorkeling, but the half-hour cliff-side drive to the town will make your hair stand on end.  The secluded bays and beaches of southern Baja are perfect for sea kayaking. Daily rentals are available through the hotels in Los Barriles and Buena Vista. But for a real adventure, try a guided multi-day trip out of La Paz, an hour's drive north. Baja Expeditions (800-843-6967; www.bajaex.com) is a good bet, with half-day trips from $35 to $60, full-day trips from $65 to $75.  Steady winds and relatively smooth, warm water make Los Barriles a prime sailboarding destination. Vela Windsurf Resorts (www.velawindsurf.com; 800-223-5443) organizes hotel and lesson packages at the Hotel Playa del Sol. A seven-day package at the hotel -- including all meals -- is $838 a person based on double occupancy. A two-hour beginner's lesson is $55.  Photos: CHALLENGE -- Flies like the ones above are increasingly being used in the Sea of Cortez instead of the more usual deep-sea tackle. Right, JesÃºs Cota, a Baja fishing guide, displays a needlefish that a client brought in with a fly rod. (Photographs by Gerard Burkhart for The New York Times)(pg. F4); EARLY START -- A sport-fishing boat, below, is already out as the sun rises over the Sea of Cortez, off the Baja Pensinsula. From left, a shop in Los Barriles, the scene along the road between Cabo San Lucas and Buena Vista, and the Cabo bar owned by the rock singer Sammy Hagar. (Photographs by Gerard Burkhart for The New York Times)(pg. F1) Map of Baja highlighting Sea of Cortez. (pg. F4)
AT 6:30 in the morning on a fishing boat in the Sea of Cortez off Mexico, all is wonder and sweet anticipation. To the west, stars still hang over the dark spine of Baja California desert. Pale blue dawn creeps its way up the sky to the east. Yesterday's wind has died to a whisper, leaving the offshore swells to roll by unruffled and rhythmic. And as your barefoot captain points the bow toward the deep underwater canyons where big fish are chasing small ones, the morning breaks into the magnificent spectacle of a sunset in reverse -- a hot red ball melting upward from a glass-flat sheet of shimmering gold.     All of that is fine, but right now, there are more pressing matters. There are knots to tie and flies to choose; lines to keep straight and reels to check. This, after all, is a fly-fishing expedition and you're stalking big game in these waters. Dorado -- powerful, iridescent fish better known in the United States as dolphin fish or mahi-mahi -- are said to flash toward a fly in changing shades of yellow, glowing purple and aquamarine. Yellowfin tuna thrash the water chasing bait as they swim alongside vast schools of leaping, acrobatic porpoises. Marlin, sailfish, wahoo and roosterfish have all been known to charge a hook covered in feathers. To anyone used to catching a 12-inch trout on a good day, latching onto something so exotic seems about as wild as stalking lions in Africa.