What is it about a teenager that can frustrate as well as rejuvenate an adult who admits to being over 39?

Take Mark Gay, a Montgomery County youngster who'd rather fish than eat. Smack in the middle of sweltering weather and foreboding reports that freshwater sport angling was all but down the tube, Mark checked in by telephone. "I did it," he said. "Finally got a smallmouth bass worth talking about -- a four-pound, nine-ouncer out of Triadelphia Reservoir."

Such a smallmouth is, indeed, worth talking about. Especially when one considers that upper Potomac River waders are quite satisfied with anything approaching legality. Good show, young man.

Elsewhere, Eastern Shore river rats ought to remember that summer bass-fishing can bring surprises. Ask Greenbelt's Mike Roach who latched onto a trophy striper in the Nanticoke River last Saturday. His lure: A plastic worm. MARYLAND RIVERS AND LAKES WSSC LAKES -- Young Chris Ciliberti scored nicely on keeper largemouths along with Mark Gay at Triadelphia Reservoir in Montgomery County. Gay's big smallmouth met his end on a black jig trimmed with black pork rind. The depth: No more than 10 feet, in rocky terrain. Scattered catches of bass, sunfish, catfish and carp during the cool hours of the day are the rule at Triadelphia and sister lake, Rocky Gorge. POTOMAC RIVER -- Evenings are great for catfish hunters using liver or worm baits across the river bottom. Productive areas include Fletcher's Boat House, Roosevelt Island, Hains Point and practically all of the river from Montgomery County to Western Maryland. Early or late hours have been good to smallmouth-bass waders around Knoxville (off Route 340). Try loud surface buzz lures when the sun isn't up. NANTICOKE RIVER -- The bass-boat set has always favored this Eastern Shore tidal jewel. Greenbelt's Mike Roach tried the river's Broad Creek tributary last weekend and came up with a 25-pound rockfish, a 31/2-pound chain pickerel and a number of keeper largemouth bass -- all on 10-pound line and purple plastic worms. Rockfish, by the way, are not unusual in the Nanticoke system during tidal changeovers. DEEP CREEK LAKE -- To the reader who asked for details: Deep Creek Lake covers 3,900 acres and sits in the middle of Garrett County; motor boats are permitted; and, of course, a state fishing license is needed. Drive west of Cumberland on Route 48, then turn south on Route 219 to the lake. Currently, the crappies are a good bet in deep, structured water. Live minnows (try Johnny's Bait House on Route 219) or 1/8- to 1/16-ounce white jigs will get them. A few bass are hooked as well. SOUTHERN MARYLAND LAKES -- It's nip-and-tuck at St. Mary's Lake (south on Route 5, through Leonardtown, to Camp Cosoma Road). Bluegills are willing if you can reach deep, brushy water. The bass like black or chocolate-colored plastic worms. Be prepared to find less action than you did in the spring. Gilbert Run Lake (Route 6, east of LaPlata) provides early and late hour sunfish and catfish, maybe a bass or two. EASTERN SHORE PONDS -- Bad news for Johnson Lake (Salisbury, Route 13) night-fishing fans: A chain went up at the entrance and anglers may use the lake only between the hours of 7 a.m. and 10 p.m. Town residents don't even want you to park there waiting for the turnkey. Nearby Leonard Lake is wide open. Both impoundments show decent crappies on jigs and some bass activity off plastic worms. Urieville (Route 213, Kent County) offers big sunfish on flyrod poppers. CHESAPEAKE BAY Thanks to a worried Southern Maryland charter skipper, our Weekend fishing report passed on the news about commercial bluefish netting operations inside the Virginia state line by the end of June. Now it seems everybody who's read a paperback edition of "Chesapeake" talks about it. Charter and private skippers do sing the no-blues blues. But as a Chip Brown story (Post Metro editions, August 1) mentioned, the bluefish problem cannot entirely be blamed on the seiners. Bluefish, as all wild creatures, go through population cycles, and it happens that our region has had a better-than- average decade of bluefish. All the same, we're happy that Virginia is willing to do something to halt the netting. Meanwhile, scattered bluefish are found from Southern Maryland and Eastern Shore waters up to Herring Bay and a few points north. Breaking fish can be spotted now and then around Point No Point, Buoys 50-54, Middlegrounds, Kedges Straits, The Targets and Hooper's Island. Sea-trout fishing may be more productive with peeler-crab baits at night in the same areas, as well as inside Tangier Sound. (From time to time we drop a word about charter captains, and here's another: Greg Morgan, a youthful Tangier skipper whose specialty is anything available in the sound. He can be reached at 301/968-1825.) Elsewhere, spot and some white perch can be found inside the Western Shore rivers, with bloodworms the prime bait. VIRGINIA RIVERS AND LAKES LAKE ANNA -- Deep-fished dark-plastic worms can bring trophy bass. You may have to get down to as much as 30 feet, however. Occasional stripers are taken on lipped lures and catfish may take up the slack when nothing else will. OCCOQUAN RESERVOIR -- A few northern pike have been hooked, one reaching 15 pounds. Generally, the bass chances have dipped unless you're a deep- water specialist. Sunfish are willing. RAPPAHANNOCK RIVER -- Slow going for all hands. Catfish in the Fredericksburg area are a good bet. In the upper river, smallmouth action has been hampered by thunder squalls. CHICKAHOMINY RIVER AND LAKE -- Plenty of youthful bass are jumping onto plastic worms and grubs in the Walker Dam vicinity. Sunfish and catfish are never hard to locate. The lake shows more sunfish catches than any other species. BACK BAY -- Insiders to this weedy monster will catch bass around duckblinds and grassy island edges, but many Virginia Beach visitors who try Back Bay for the first time go home skunked. SMITH MOUNTAIN LAKE -- Dark hours provide a bit of striper action on deep-running lipped lures or jigged metal spoons. That's it. LAKE GASTON -- Very little bass action. Catfish and crappies may make up for it near bridge pilings and waterlogged brush. OCEAN AND INLETS MARYLAND -- Maryland officials want to drive this point home: The minimum size for flounder (anywhere in the state) is 12 inches. Flounder anglers may be checked in the backwaters of Ocean City and the fish measured from tip of lower jaw to the end of the tail. This fish is an important vacation resource for the state. Let's help and make sure the little ones grow up to respectable size. Headboats, meanwhile, are finding tautogs and seabass. Bluefish trollers' successes are only so-so. White and a few blue marlin are taken over the canyons. Sharks are the mainstay for many boaters. Small bluefish, kingfish and flounder continue to offer a bit of fishing fun in the Assateague surf. VIRGINIA -- Last weekend a sudden offshore run of tuna developed again. The yellowfins may provide the only decent bluewater action. Carl Hoke, of Hyattsville, reports a fine outing with Wachapreague charter captain Ray Parker. Hoke and party had a number of yellow and bluefin tuna up to 38 pounds. Billfish crash baits occasionally over Norfolk Canyon. Eastern Shore flounder action is fair from Chincoteague to Oyster. Hot weather takes a toll. Remember that. Inside the Chesapeake's mouth a few sea trout, flounder and bluefish will bite during the evening and night hours at the Bridge-Tunnel islands.