It's that time of year when the thoughts of fly fishermen turn to pure and cold mountain streams, tumbling waters, quiet woods-and trout. With the April winds come the first mayflies - and winter-starved trout can be expected to gorge themselves.

Fly fishermen hope their fur and feather flies will tempt the fish just as much as nature's own and, in anticipation of one subtle rite of spring, they begin to flex the rods, oil the reels and patch the waders.

Fly fishing is often called an art. It's not. At its best, fly fishing can represent how a person relates to nature, and through an understanding of the aquatic biology he imitates he captures, examines, ofter releases and sometimes kills. But enough of philosophy; on with the news.

The most important regional news is that the Virginia Commission of Game and Inland Fisheries has, after three years of work, completed a statewide biological survey of all its cold-water (trout) fisheries.Such surveys are going on in many states in response to pressure from organized fly fishermen, biologist, Trout Unlimited, Federation of Fly Fishermen, and the American League of Anglers, because hatchery stocking programs have forced much of the country's wild trout populations to the edge of extinction. Larry Mohn, a biologist with the Virginia Commission, said last week that Virginia will probably report about 1,800 miles of wild trout water. Almost all of the most productive streams are on the east side of the Blue Ridge from Shenandoah Park south to the southwestern corner of the state.

Nearest to Washington and within the Park, where only flies and single-hooked lures are permitted, are the Rose, Hughes, Robinson and Conway rivers, as well as the Rapidan and Staunton rivers that are no kill wild brook trout streams where all fish must be released unharmed. Get a topographic map, since these streams, as well as most of the remaining 1,800 miles of native trout water are accessible only on foot. The best flies in April will be quill gordons, size 12, fished on the riffles and behind boulders on which the natural quill gordons emerge.

Mohn also expects Virginia to develop five management classifications for its wild trout water this year, and by July 1980 we should see some very exciting special regulation areas created that will enhance wild trout fishing for Washingtonians. Several fertile limestone streams, which are the current rage trout fishermen, showed up in the survey with good native populations. They were all on private line and are not open now, but the state may work out access agreements, according to Mohn.

Unfortunately, Maryland has not fared so well in its statewide biological survey. Almost all remaining wild trout water, and there isn't much, is too small to be considered fishable. The best fly fishing in Maryland will be for smallmouth bass in the Potomac from Harpers Ferry to Brunswick (Mike Farnham, 937-4391, guides this water) and from Hagerstown to below Shepherdstown, West Virginia. Fish the holes below riffles with black, yellow or brown streamer patterns early and late in the season or dry caddis patterns and nymphs when the flies come off the water in midsummer, Maryland trout anglers might look to Morgan Run, which flows from the west into Liberty Reservoir west of Baltimore; Hunting Creek in Catoctin National Park, north of Frederick, Patuxent River near Damascus and Savage River in the western part of the state. All are no-kill stocked trout areas.

Pennsylvania is engaged in its own coldwater survey. Charlie Fox, the state's present piscatorial patron saint, writing in Fly Fisherman, attacked the Fish Commission's survey as an attempt to do away with existing special regulation areas. I wouldn't be surprised to see the state drop some of the no-kill streams that have failed to achieve self-sustaining populations. Fox has put the region on notice that ending some no-kill areas will politically require the creation of more promising no-kill areas. Increasingly, fish commission personnel seem to be deciding that there are more tourist dollars in a big, wild, challenging trout that must be released unharmed to be caught again and again than there are in a nine-inch hatchery product that's bred for resistance to disease, tolerance to stress in the holding tanks and fast growth on grain pellets, but that can't survive or reproduce in the wild.

Because of the survey in Pennsylvania-or as Fox would say the state's favoritism for hatcheries, bigger budgets, more dwarf dead fish in the creel and opposition to no-kill regulations - the controversial moratorium in Pennsylvania on additional special regulation areas will continue this year, longer than planned, while the survey continues. If it continues at snail's pace, then Charlie's suspicions may well be right.

For years southern Pennsylvania's "famous four" streams - the Letort south of Carlisle, Big Spring southwest of Newville, Falling Spring east of Chambersburg, and the Yellow Breeches south of Carlisle - have been praised and over-publicized, and now they suffer from too much weekend pressure on the no-kill sections. If you must fish those streams this month, here are some recommendations. Try little olives, size 18, on the Letort during midday, particularly if it's cloudy and a bit windy. Sulphur nymphs will work on Falling Spring during midday through May. Try dark streamers in the holes on lower Big Spring for rainbows and browns or muskrat nymphs for native brook trout in the headwaters. But, for more trout and less crowds, I recommend Clarks Creek northeast of Harrisburg and Penns Creek east of State College, and for some of the best small mouth bass stream fishing in the East I'd look to the Juniata River below Lewistown.

Nationally, two events during the winter are worth comment. In december, the Montana Board of Natural Resources and Conservation, in a landmark ruling, granted two "in-stream flow" water appropriations, for fishery and for health purposes, on the Yellowstone River, which faced excessive water demans from industry, agriculture and mining interest. The Yellowstone is the longest free-flowing river (600 miles" in the lower 48 states and is among the blue-ribbon national trout waters. A great sport fisher, still pure and unpolluted because of the bordering National Park, has been saved.

The other good national news comes from Connecticut, which reports that 87 Atlantic salmon returned to the Connecticut River in 1978 for the first time in more than 160 years. Connecticut River salmon are extinct, and Connecticut, Vermont and New Hampshire have for about 10 years been trying to develop a strain of wild Atlantic salmon that would survive and return to the cleaned-up river. A fishery is still years away. Fewer fish or more could return to the river this year, but at last there is hope. CAPTION: Picture, SOME LIKE IT MIXED: A WHITE PERCH BEING TAKEN ON A FLY ON CHESAPEAKE BAY.