THERE may not be a pot of gold at the end of the rainbow, but you can definitely find big bucks at the end of your fishing line.

The Maryland Department of Natural Resources is sponsoring a "Catch a Fortune" fishing contest worth nearly $500,000 in which several species of fish are tagged and released in the Chesapeake Bay. Twenty fish are being tagged each month, including at least one bluefish worth $25,000, one white perch at $5,000 and one sea trout valued at $5,000.

The contest runs until December 31, 1986 and is open to the public. There is no entry fee but you must have a valid Chesapeake Bay fishing license. All fish must be caught in a sporting manner by hook and line on a rod and reel. In addition, there's a 50 percent bonus if the fish is taken while fishing on a licensed Maryland charter boat.

In cooperation with the Department of Natural Resources, the Maryland Charter Boat Association has put together a comprehensive list of charter boats and the areas they fish. The pamphlet comes with an information package listing the contest rules and regulations. To get it all, call 301/269-3765 or write to: Maryland's Catch a Fortune Contest, DNR, 69 Prince George Street, Annapolis, MD 21401. WHAT'S THE CATCH? WASHINGTON AND VICINITY POTOMAC RIVER -- Large catfish are now roaming the river, says Ray Fletcher at Fletcher's Boat House. Although this year's leader in Fletcher's catfish tournament is still a 13-plus-pounder, larger fish are taken each year from this portion of the river. Cut herring are the ticket to success for the catties. Bass fishing is still improving, and the river is in excellent condition. If you're a white perch fan, there are still enough large ones to keep you interested. Some are 12 inches or so, which can be considered a trophy in any body of water. A bit farther downriver, bass fishing is still holding up well at The Spoils just above the Woodrow Wilson Bridge. Small crankbaits, plastic worms and large, live shiners have been the most productive baits. MARYLAND

POTOMAC RIVER, DAM 4 -- "Lots of bass here, but only a few keepers," reports the Rev. Bill Zimmerman, an avid river fisherman with many years' experience smallmouth fishing in the upper reaches of the Potomac. Using a minnow/jig combination, he's been able to hook up with some dandy smallmouths this spring and says he maintains his success throughout the dog days of summer. His secret, he says, is to work the waters close to shore where the structure quickly drops off to deep pockets. The same technique is used by many bass pros when tournament fishing in the south. TRIADELPHIA AND ROCKY GORGE -- Grape-colored and flavored plastic worms have been responsible for catches of some good-sized bass in both reservoirs, according to Monty Embry at Angler and Archer in Rockville. The best fishing times have been early in the morning and late in the evening. That's when the sunlight angle is best for shallow running lures. Although fishing has been somewhat on the slow side, this is usually the case right after the spawn is over. Within the next week or two, the action should improve considerably.

LIBERTY AND LOCH RAVEN RESERVOIRS -- Liberty has been the hot spot for largemouth and smallmouth bass alike. Some weigh more than five pounds and are taking large, live minnows lip hooked to the back end of small jigs. The best action has been in the upper end of the lakes, according to Doug Lyons at Old Reisterstown Bait and Tackle. Crappies have now migrated back to deeper water, but you can fill a stringer if you fish the deep structure close to shore.

CONOWINGO LAKE -- Crappie fishing has been holding up well for anglers dunking live minnows along the Harford County shoreline. Some of the slabsides have been running upwards of 15 inches, but the average fish is closer to 10. Scattered catches of largemouths and smallmouths are being made in those same areas. Live minnows and small crankbaits have produced the better catches. The upper end of the lake near Muddy Run has been the best spot for smallmouths. Anglers casting a minnow/jig combination have been hooking up with bronzebacks weighing up to two pounds. VIRGINIA

LAKE ANNA -- Big catfish, stripers, largemouths and a few good-sized crappies are what you can expect to catch if you're willing to put forth the effort in this productive lake. Vince Gish of Locust Grove, Virginia, hooked up with two channel cats weighing 11 pounds, 1 ounce, and 8 pounds, 5 ounces respectively. Gish was fishing with a chunk of cut bait. The most successful striper fishermen are now using live gizzard shad and fishing them along the drop-offs, according to Steve Maudre at Anna Point Marina. Earl Tuell, a Gordonsville resident, has caught more than a hundred stripers in the lake since the season began. Tuell uses what he calls "Smith Mountain Lake Tactics" -- fishing deep with live shad.

RAPPAHANNOCK RIVER -- Smallmouth action in the river is holding its own, and lots of stripers are now in the river, reports Carl Gentry at Chesley's Sporting Goods in Fredericksburg. If you happen to catch a striper in tidal water, it must be released until the season opens June 1.

SMITH MOUNTAIN LAKE -- Big stripers and walleyes still dominate the action in the upper end of the lake, according to Sonny Davis at All's Hunting and Fishing in Salem. During the past two weeks, Davis has hooked up with 14 big walleyes and a few good-sized stripers using Redfins cast along the shore at night. Fishing guide John Jones says bass fishing has been a bit slow , but a few late spawners are still on the beds and he expects conditions will improve by the weekend.

JAMES RIVER -- Smallmouth fishing has been hot and cold, says Earl Coppage at Timberlake Sporting Goods in Lynchburg. Some days you can hook loads of fish using any lure in the tackle box, while on the bad days, the fish completely ignore your best plug. As the water warms, fishing should improve here also with good catches of smaller bass and catfish. CHESAPEAKE BAY SUSQUEHANNA RIVER -- The white perch have migrated downriver and are now schooled near Lapidum Landing, says Terry Rhudy at Conowingo Bait. Although this year's white perch run hasn't been outstanding by any means, a number of large fish have been checked in for citations. Catfish have been plentiful throughout the entire river and some have topped 15 pounds. Cut bait, nightcrawlers and chicken livers have accounted for fantastic catches of these fish. Bass anglers are having a difficult time locating smallmouths since they completed their spawning cycle, but a few good-sized bronzebacks have been taken at the mouths of the creeks. Small Sassy Shad, cast in fast-water areas, have been the top-producing lures.

SUSQUEHANNA FLATS -- Fishing conditions on the Susquehanna Flats have been poor because of the volume of water running at Conowingo Dam. Muddy water and fairly brisk winds kept most anglers away from the shallows where bass fishing is usually outstanding. However, some anglers have taken small boats and fished in protected coves near the marina piers and hooked up with some fair-sized largemouths using small crankbaits and plastic worms. Some good-sized perch are being taken in some of the deeper pools by bottom fishermen, but only one fish in four could be considered a keeper, according to Herb Benjamin at Herb's Tackle Shop in North East.

GUNPOWDER AND DUNDEE RIVERS -- The mouths of the Gunpowder and Dundee rivers have been a perch fisherman's paradise for the past few weeks. Several citation-sized white and yellow perch have been checked in at Gunpowder Bait and Tackle. According to shop owner Sue Demaf, the perch are taking grass shrimp fished on the bottom; mixed with the perch are some large catfish. Bass fishing has dropped off considerably since the largemouths have completed spawning and moved into deep water.

BAY BRIDGE AREA -- Blues weighing up to 18 pounds are now being caught at the Bay Bridge by trollers using Crippled Alewives and large Tony Acetta spoons. Trollers are working these lures in about 35 to 40 feet of water along the main shipping channel with a fair degree of success. Chummers, however, are outfishing trollers by an eight to one ratio in shallower water near Hacketts Bar. The first blues taken by plug casters in the shallows have been confirmed, reports "Fishing Charlie" Ebersberger at the Angler's Sport Center on U.S. 50 in Annapolis. Although the choppers are not being caught in big numbers yet, they more than make up for this with their size. The average fish has weighed nearly 19 pounds, and they go berserk when they're hooked in shallow water.

CHESAPEAKE BEACH -- Calvin Tyler at Tyler's Tackle Shop has had his smoker cranked up for loads of big blues. One captain brought in more than 200 pounds of blues to be smoked with Tyler's recipe. Captain Shaker Black at The Rod N Reel Dock says the blues are scattered and charter boats running out of that facility have been working for every fish. The big news here has been catches of sea trout that have shown up at The Gooses. The weakfish are taking trolled spoons, but the better catches are being made by bucktail trollers.

DEALE -- Captain George Prenant on the Stormy Petrel has been loading up on lots of big chopper blues while trolling near the main shipping channel. Prenant is also reminding folks that the Annual Deale Fishing Tournament is under way through November. The entry fee is $5 dollars per boat, and each month a cash prize based on the number of entry fees is split betwen the lucky angler and a local charity. This month's prize money will go to Childrens Hospital in Washington.

SOLOMONS ISLAND -- Captain Bill Meadows on the El Toro reports his bucktails have been bringing in some huge sea trout, running up to 12 pounds, just out from Solomons, along with lots of huge chopper blues. Captain Lou Snyder on the Julie Lynn spent Memorial Day weekend fishing the same area and also hooked up with loads of good-sized trout and blues. The fishing in this area of the Bay has been excellent for the past several weeks.

POINT LOOKOUT -- The smaller blues have arrived in huge numbers inside the mouth of the Potomac, says Captain Phil Langley on the Tracy Ann. Breaking fish weighing from one to five pounds are taking trolled surgical hose eels, small spoons and bucktails worked close to the surface. Captain Bruce Scheible at Scheible's Fishing Center reports every boat in the fleet has been loading up on all the blues it can handle. Captain Eddie Davis on the Miss Valerie II reports his parties are catching lots of blues also and an occasional trout weighing up to eight pounds. Captain Doug Scheible on the Bay King II has been chumming at the Middle Grounds for larger fish and reports excellent fishing for blues weighing from three to 15 pounds along with a few trout. It appears that everything has busted loose earlier this year.

CAPE CHARLES -- "We are now seeing the best run of black drum here in over ten years," says Captain Otis Asal on the charter boat Bucaneer. Asal's wife Brenda has brought aboard the largest drum of the season -- an 84-pounder that took a whole sea clam. Captain Don Stiles on the Elizabeth, running out of Kings Creek Marina, reports most of the boats are returning to the dock earlier than usual with their limit of 12 drum. In addition, the sea trout run is now in full swing and those who don't mind making the run to the Bay Bridge Tunnel Complex can hook up with all they can handle. The same area also holds lots of good-sized flounder, tog and shark. Bottom fishing with crab baits will take these fish on the calmer days. ATLANTIC OCEAN

CHINCOTEAGUE -- Big blues are now at the 21 Mile Hill, reports Captain Steve Finchbaugh on the Canyon Connection. Some weigh more than 15 pounds. They're taking cut bait trimmed with red and white skirts. Lots of bonita are in the same area, but catching them has been tough because of the chilly water. Inshore anglers are managing to hook up with some good-sized flounder at Queen Sound using live minnows and squid strips for bait. At the mouth of the inlet, you'll find a few gray trout mixed with snapper blues, each weighing up to four pounds.

OCEAN CITY -- Although the blues are scattered at the offshore lumps, trollers using Hoochy Trolls in red and chartreuse have managed to hook up with some wild fish. The choppers are averaging about 14 pounds and really put up a battle on light tackle. The only problem facing anglers here lately has been a constant northwest wind that has prevented small boats from venturing out. Inshore, at Assawoman Bay, flounder fishing has been hit or miss. The best place for the flatties has been just above the U.S. 50 bridge close to the sand islands.

DELAWARE BAY -- Fourteen Foot and Brandywine lights are where the fleet has congregated for the past several weeks. However, the sea trout fishing in both places has been sporadic, with only scattered catches of weakfish. According to Captain H.D. Parsons at Fisherman's Wharf in Lewes, you have to get right over the fish and let them have it with both barrels in order to do well. Bucktails trimmed with squid strips have been the best baits. Flounder fishing has also been hit or miss, but those knowledgeable about the area can usually fill a cooler with enough fish to make several good meals.