THERE'S A CERTAIN nervous energy around fish markets. Their very perishable product can't languish on the shelves. Like the restless sharks in the deep blue sea which must keep swimming or die, those babies resting briefly on crushed ice must be kept moving.

So it is with three new fish markets which have opened the past year in Northern Virginia. They are operated by young men in their 30s who are hustling for the business of diet-conscious consumers, and their emphasis is not just fresh versus frozen, but really fresh fish.

The three new markets offer three distinctively different styles. Port City Seafood in Alexandria is no-nonsense and clinical. Except for the chill, you wouldn't hesitate to lie on its counters for an appendectomy.

Besides fish, Daniels' Cellar and Sea in Old Town Alexandria offers wine and loaves, specialty groceries and even take-out sandwiches. In wine lingo, its fish could be described as of recent vintage, firm-bodied, with a good color and attractive aroma.

Most creative of all, Old New Orleans Seafood Market in McLean has brought a touch of the exotic to its suburb. It is introducting fresh Louisiana seafoods not only within its store but also from two bright blue vans which set up in remote locations, unfurl a striped awning and hustle their seafood on the street corner. OLD NEW ORLEANS SEAFOOD MARKET 6232 Old Dominion Drive, McLean, Va. 232-7755.

Open Monday through Friday, 9 a.m. to 7 p.m., Saturday 9 a.m. to 5:30 p.m.

"Mon Dieu! You have lotte!" say the French customers when they discover the Old New Orleans Seafood Market closeted in the Chesterbrook shopping center in McLean. Lotte is the French name for the monkfish prized for its lobster-like flavor but found only recently in area markets.

This is not the only surprise. Ceiling fans hum, ruffling the foliage of hanging plants; blue striped awnings brighten the walls. With the tile-patterned floor and the blue wrought-iron Bourbon Street park benches, you are in New Orleans.

Even the fish, glistening on lavish beds of shaved ice, are not the usual stock from Atlantic shores. Redfish (a.k.a. red drum or channel bass) is prominently featured.

"It's the most popular fish in New Orleans," says Moe Cheramie, 36, a Cajun who decided Northern Virginia was ready for fresh Gulf shrimp, Louisiana crabs and crawfish (a colloquialism for crayfish) and a high quality selection of frozen creole specialties. He stocks his counter with uncommon market fish such as the monkfish, tuna, dolphin (the fish, not the mammal), shark, grouper and cobia in addition to the more familiar fish from New England and farther south.

A raw bar has been promised, to allow sampling of freshly shucked shellfish. To wash them down, the soft drink machine is stocked with more Louisiana specialties, Pop Rouge (strawberry soda) and Barq's root beer.

Cheramie, with his gray-eyed Gallic charm, plays host, offering suggestions, steering customers to a new fish, or asking them how they cook mussels, then saying: "Why don't you try this? Slice an onion, add some chopped celery, put in the mussels, add a couple of inches of white wine and steam until the shells open. Delicious!" The dazzled customers depart vowing to scrap their old recipes.

Besides the French who like to chatter in their mother tongue to Cheramie, Finns and Swedes have found the market. To them it is a source for crayfish, a traditional food whose season has now ended, but Cheramie sells it frozen in boilable plastic bags at $4.54 for a 24-ounce package.

The season is just beginning for what Cheramie says is his biggest seller -- fresh Gulf shrimp. Great heaps of them arrive with heads on. If they are to be cooked at home, the heads are snapped off before sale (medium, $7.19 a pound, large $8.39); when Cheramie cooks them as spiced shrimp or a house specialty, barbecued shrimp, he leaves the heads on until after they are cooked and then removes them before sale.

"Cooking shrimp with the head on gives a sweeter, more distinctive flavor," he says, offering a plump morsel to a customer.

Most of the shrimp sold in this area is frozen, but Cheramie meets a plane nearly every day and he has trucks arriving once a week from Louisiana and Maine. He persuaded a cousin to scout for his stock. "I have him running up and down the bayous buying seafood, packaging it and getting it on the plane."

His shelves are stocked with such things as gumbo file' (ground sassafras), the key ingredient for gumbo dishes; creole seasonings; canned turtle soup; beignet mix for French Market doughnuts, and even Camellia brand dry red kidney beans for the red beans and rice crowd. It offers take-out Po Boy sandwiches and New England lobster rolls. Frozen items include southern specialties--crayfish and crab bisques, shrimp and okra gumbo, stuffed eggplant and shrimp. There are frozen frog legs and boudin, a pork and rice sausage, and perhaps eventually frozen New England specialties such as chowders.

Cheramie has other innovations. Besides his two mobile units which set up daily in downtown McLean and at the intersection of Springhill Road and Old Dominion Drive, he offers free delivery if orders are phoned in before noon on Mondays, Tuesdays and Wednesdays. PORT CITY SEAFOOD 1108 Oronoco Street, Alexandria, Va. 684-0090.

Open Tuesday through Friday 10 a.m. to 6 p.m., Saturday 10 a.m. to 5 p.m. Closed Sunday, Monday.

C. Derek Walker, 33, and George Mayo, 32, the young owners of the bustling Port City Seafood market, like to turn over their inventory every two days. Having both a wholesale and retail operation helps. They think they get better prices from their sources and they are freer to tell a supplier to come back and pick up a shipment that they feel isn't up to their standards.

"We change our prices every day," says Walker, "reflecting both downturns and upturns in prices."

Walker was prepped in the seafood business at Cannon's in Georgetown. It shows in the Alexandria operation. There is the same clinical starkness in the shiny stainless steel bins with red snapper glinting in the crushed ice and heaps of croakers, sole, dressed baby rainbow trout ("This week's special, $3.25 a pound").

But there is a difference. At Cannon's, customers have to ask about prices; Port City posts them on a large blackboard. "We see people study the price list before they take a number to be served," says Walker.

The staff willingly answer questions, though if they don't know an answer or are busy, they may hand you a copy of Time-Life's "The Good Cook" series on fish. Walker experiments cooking fish several nights a week and encourages his staff to acquaint themselves with the shop's products.

Besides the usual stocks of New England and mid-Atlantic fish, Port City has several kinds of smoked fish. Smoked bluefish from Cape Cod is a specialty ($5 a pound). Smoked brook trout, whitefish and cod are all $4.25 a pound. Nova salmon is $11.75 a pound.

In Port City's 14 months its restaurant and retail trade have developed slowly. "It was a rough time to open," Walker says, looking back. "But we've had steady growth and we're here to stay." DANIELS' CELLAR AND SEA 102 South Alfred Street, Alexandria, Va. 836-1790.

Open Monday through Saturday, 10 a.m. to 7 p.m.

Situated in a former row house just off King Street in Old Town Alexandria is a cozy, seafood boutique. Old Towners out walking the dog can drop in, pick up swordfish steaks, add a container of crab claws for an appetizer and gossip a few minutes with Dan Coakley about the perfect chardonnay to wash it all down.

Daniels' gets it name from the two "Dans" who are the owners. Dan Coakley, 32, oversees the retail operation, while Dan Grubaugh, 38, handles the wholesale business which includes a warehouse in Fredericksburg.

Grubaugh, who previously owned restaurants in Yorktown and Gloucester Point, Virginia, is on the road every day--60 hours a week--finding fresh seafood. One week a huge whole halibut reclined on a bed of ice with an equally enormous swordfish.

Coakley was a wine salesman and a beverage manager for a hotel chain before the two joined forces and decided they had the combined talent for more than just an ordinary fish market.

Their single case of seafood carries a fairly basic stock of fish fillets and a few newly popular fish, such as monkfish and grouper. They also have shellfish, crabmeat and scrod fillets.

The wines include a wide range of prices and labels, as well as weekly specials on inexpensive wines--1979 Trakias from Bulgaria, Bertal bordeaux, at $2.79, or $30 a case, for instance.

For the customer buying swordfish, Coakley will recommend a nice sauvignon blanc or chardonnay. (His own choice would have been Concannon Sauvignon Blanc--"delightful, almost a buttery texture.")

There are bristling bouquets of long french bread loaves and fat round loaves of Italian breads, all from the nearby Bread and Chocolate shop. They are sliced up for shrimp and fish salad sandwiches that Coakley sells to the lunchtime business crowd.

In his deli case are sides of smoked Nova salmon, smoked in Brooklyn, N.Y. and priced at $15 a pound, the price decreasing to $14 for orders above a pound. He also has the hard-to-find Knorr-Swiss fish bouillon cubes.

From these three new markets, here are recipes they suggest to use seafoods to advantage: SCROD PORTUGAISE (4 servings)

Dan Coakley of Daniels' Cellar and Sea uses scrod -- young cod or haddock -- but the recipe will work for any fish fillet. If you leave out the tomatoes and make it with flounder, it's sole bonne femme. 2 shallots, finely chopped (or, 1 clove garlic minced; or 2 tablespoons onion, chopped) 2 tomatoes, peeled and chopped 1/2 cup dry white wine 2 tablespoons fresh parsley, chopped Juice of 1/2 lemon Freshly ground black pepper 4 fillets Boston scrod, about 1 3/4 pound total 2 tablespoons each flour and butter Lemon wedges for garnish

In a heavy skillet, place shallots, tomatoes, wine, parsley and lemon juice. Pepper fillets generously and add to skillet. Bring to a boil over high heat. Immediately reduce heat to a simmer and cover. The fish should cook 10 minutes per inch thickness of fillet (approximately 7 to 8 minutes for scrod). Lift fish carefully and remove to warmed serving dish.

Make a beurre manie by kneading together the butter and flour to form small balls. Add these to the pan juices and swirl over medium heat, stirring constantly, until mixture thickens and the raw flour taste has cooked away. Pour this sauce over fillets and garnish with lemon wedges. BLUEFISH WITH TARRAGON GLAZE (2 to 3 servings)

C. Derek Walker of Port City Seafood says this is perfect with bluefish or sea trout. It tames the bluefish and enhances the more delicate trout flavor. 1 pound bluefish or sea trout fillets 1/2 cup good-quality mayonnaise 2 tablespoons dijon-style mustard 2 teaspoons tarragon leaves

Combine mayonnaise, mustard and tarragon and spread generously on fillets which have been placed in a single layer in a shallow baking dish. Bake at 425 degrees until fish flakes easily when tested with a fork. Slide the dish under a broiler briefly until top is lightly browned. BARBECUED SHRIMP (4 servings)

This is a specialty at the Old New Orleans Seafood Market in McLean, Va. where it sells for $11.79 a pound. "It's very messy," warns proprietor Moe Cheramie. Eat it with plenty of french bread to capture the sauce. 1 cup butter 1 cup vegetable oil 2 teaspoons garlic, finely minced 2 bay leaves, crushed 2 teaspoons rosemary leaves, crushed 1 tablespoon paprika 3/4 tablespoon coarse black pepper 1 teaspoon lemon juice 1/2 teaspoon each of dried basil, oregano, salt and cayenne peper 2 pounds large shrimp

In a large heavy skillet heat butter and oil. Add remaining ingredients except shrimp. Cook, stirring until it comes to a boil. Cover and simmer for 7 or 8 minutes stirring frequently. Remove from heat and let stand 30 minutes. Remove cover and add shrimp. Cook over medium heat 6 to 8 minutes until shrimp are pink. Place pan in a 450-degree oven and bake for 10 minutes. LOTTE AMERICAINE (6 servings)

Julia Child says this is a splendid combination, usually associated with lobster, but traditional in France with monkfish, which should be cooked slightly longer than other fish. The fish is done, she says, when it has turned from springy to gently soft. 1 large yellow onion, sliced thinly lengthwise 2 or 3 cloves garlic, pureed 3 1/2 pounds monkfish, cut into 3/4-inch-thick chunks Salt, pepper, flour, olive oil 1/4 cup cognac 1 cup white wine or vermouth 2 cups fresh tomato pulp 2 tablespoons tomato paste 1/2 teaspoon tarragon Parsley for garnish

Film a skillet with olive oil and saute onion slices and garlic for 5 or 6 minutes. Set aside.

Remove gray membrane from fish if this was not done by the fish market. Season fish with salt and pepper. Lightly flour, shaking off excess. Film a second skillet with olive oil and heat over moderate heat until very hot but not smoking. Saute' fish chunks in a single layer for 2 minutes on one side, turn and saute' 2 minutes on the other side. Add cognac, let it bubble, then ignite with a lighted match. Flame for a few seconds, then douse with white wine. Add the saute'ed onion and garlic. Spread tomato pulp and paste over the fish. Add tarragon. Cover and simmer 10 minutes. Remove fish to a warm platter. Boil down sauce to thicken it. Taste for correct seasonings. Spoon over fish and garnish with parsley.

Adapted from "Julia Child and More Company"