A COMMERCIAL fisherman once told me the reason fish decays faster than beef is because of the fish's death enzymes. You can sneak up on a cow, knock it on the heat and it'll never know what hit it. No physiological stress over dying, therefore the cow releases little of the decay enzyme that causes the meat to break down after an animal is killed. On the other hand, it's rather difficult to sneak up on a fish.

Actually there is more to it than that. According to Spencer Garrett of the U.S. Government's Bureau of Fisheries, fish flesh deteriorates more rapidly than red meat because the decay enzymes which are released when fish are caught and killed continue to work at cold temperatures since fish are cold-blooded. Which means that even when fish are stored at low temperatures the flesh will continue to break down, although at a slower rate.

In addition fish are protected against decay bacteria by a coating on the skin that when damaged, lets in microorganisms. Although fish have a minimal amount of fat, that fat is polyunsaturated and will turn rancid faster than the saturated fat in meat. And fish undergo greater enzyme-releasing physiological stress when caught. Don't feel too sorry for the fish, however, Garrett says they feel no pain.

For all these reasons it is important when buying fish to select the freshest possible. Fish should not smell "fishy." They should have a sweet, slightly marine scent. Shark, skate and ray (not a carnivorous law firm) are an exception and should have a faint ammonia odor. Look at the fish's eyes. The pupils should be black and a bit popped. One trick used by sneaky fishmongers is to cut the gills off a fish. If the gills are not bright red then it's been too long from home. The flesh of the fish should be firm or elastic and come back when poked, the scales shiny and close to the skin.

When buying frozen fish make sure the packaging is not ripped or tattered and there are not icy rimes around the fish. Blood at the bottom of the package indicates the fish has been thawed and refrozen. This does not necessarily mean the fish is spoiled, but the meat will be dry and tasteless.

Store fresh fish covered loosely in plastic wrap and put in the coolest part of the refrigerator.It will be safe fr up to two days. If freezing is necessary, fatty fish like mackeral, butterfish, salmon, bluefish or smelts can be frozen for up to 3 months. Lean fish (snapper, rockfish, bass, perch, porgy, halibut, sole, turbot, flounder, cod) can be frozen up to 6 months.

Knowing what you now know, you can buy excellent fresh and frozen fish at the following Washington area markets: DISTRICT OF COLUMBIA:

Southern Maryland Seafood Co., in the Eastern Market, 225 7th St. SE (546-9135). A wide variety of local fresh fish is available, including fish fillets and shellfish. Open from 7 a.m. to 5:30 p.m. Tuesday through Saturday and until 6:30 on Friday.

Maine Avenue Wharf, 1100 to 1200 Maine Ave. SW. Curtis and Brown Seafood (484-0168), Morgan's Seafood (488-8145) and 10 other companies sell up to 20 varieties of fresh finfish and shellfish from permanently docked boats. Although prices are shown, bargining is not unknown. Most boats sell from 9 a.m. to 9 p.m. seven days a week.

Southeast Warf, 4129 Wheeler Rd. SE (563-3600). If an evening hunger attack for fresh trout hits you, the SE Warf is open from 10:30 a.m. until midnight Monday through Friday and until 2 a.m. Saturday. A larger variety of fish will be available at the end of the month.

D & W Seafood and Poultry Market, 2508 14th St. NW (232-4486). Black bass, flounder, croaker and more types are sold from 11 a.m. to 8 p.m., seven days a week.

Cannon Seafood, 1065 31st St. NW (337-8366). When available Cannon's offers fresh tuna, swordfish, halibut, gray sole fillets from Boston, fresh salmon from the West Coast, rainbow trout from Idaho trout farms. Maine's Baylon oysters at 70 cents apiece are worth it. Open from 7:30 a.m to 6 p.m. bMonday through Wednesday; until 7 Thursday and Friday and until 7 on Friday. Open Saturday from 8 a.m. until 5:30. Closed Sunday.

R.W. Claxton, 240 E St. SW (554-9230). Although Claxton's does not concentrate on retail sales, fresh fish are available in season. Frozen fin and shellfish are also offered. Open from 6 a.m. to 2 p.m. Monday through Friday. Closed Saturday and Sunday.

Larimer's Market, 1727 Connecticut Ave. NW (332-3366). Four to 6 varieties of fresh fish are offered at the butcher counter next to the live-lobster tank. Open from 9 a.m. to 6:15 p.m. Monday through Saturday. Closed Sunday.

Chevy Chase Seafood Market, 5509 Connecticut Ave. NW (686-1068). Twelve to 15 types of fish including salmon, swordfish, halibut, gray sole and fresh and frozen shellfish are sold from 10 a.m. to 7 p.m. seven days a week.

Giant. All D.C. Giants have a fishmonger except the Connecticut Avenue store and will sell you 10 to 15 varieties of fish from 9 a.m. to 8:30 p.m. All area Giant stores offer self-service fish. Call the Giant consumer service at 341-4322 for information on Maryland and Virginia branch fishmongers.

Carolina Fish Market, 1309 5th St. NE. (546-4848). Only local fresh fish (croaker, trout, flounder, black bass, bluefish) are available from 9 a.m. fto 5:30 p.m. weekdays and 7 a.m. to 6 p.m. on Saturday. Closed Sunday and Monday.

Ware's Fish Market, 725 20th St. NE (398-6000). Local fresh fish such as croaker, trout, porgy and rockfish are sold from 8 a.m. to 6:30 p.m. seven days a week.

Zoltrow's Live Fish Store, 3539 Georgia Ave. NW (829-1122). Ling fish from New Jersey and croaker, perch, porgy, sea trout (weak fish), black bass, rockfish and butterfish are sold in Washington's oldest fish market in one location. Open since 1935 from 8 a.m. to 6 p.m. Tuesday through Saturday, Closed Sunday and Monday. MARYLAND

Atlantic Seafood Market, 7932 Johnson Ave., Glenarden (773-6550). Fresh croakers, trout, spot, black bass, ocean white perch, flounder, butterfish, mullet (with and without roe), rockfish, red snapper, bluefish, ling, whiting and porgy are sold from 7:30 a.m. to 8 p.m. Monday through Thursday and until 7:30 p.m. Friday.

Dock of the Bay, 413 South Frederick Ave. Gaithersburg (840-2246). Five varieties of local fresh fin and shellfish are offered. Open from 11 a.m. to 7 p.m. Tuesday through Thursday, until 8 p.m. Friday and Saturday and until 7 p.m. Sunday. Closed Monday.

A & B Shellfish Co., Inc., 12209 Nebel St., Rockville (881-3732). A variety of fresh and some frozen fish is sold from 10 a.m. to 7 p.m. daily. A clambake-in-a-can for two to bake at home (including lobster, clams, corn, potatoes and sausage) is available for $23.

Flying Fisherman, 8411 Old Marlboro Pike, Forestville (568-1010). Trout, croaker, porgys and shellfish are sold from 10 a.m. to 10 p.m. daily and until 9 p.m. on weekends.

Gloucester Lobster Co., 3630 East St., Landover (773-7747). Over 20 varieties of fresh and frozen fish are sold. See the Largest inland lobster pool -- 24,000 gallons. Open daily from 11 a.m. to 6 p.m. Monday through Thursday, until 5 p.m. Sunday and 6:30 p.m. Friday and Saturday.

Good Luck Seafood Co., 9035 Lanham-Severn Rd., Lanham (449-4500). Four or 5 varieties of fresh fish available from 10 a.m. to 9 p.m. Monday through Saturday and until 7 p.m on Sunday.

Ted's Seafood Market, Rt. 301, Waldorf, Md., across from the Red Fire Lounge (843-2033). Up to 10 kinds of fresh fish sold, depending upon availability, Monday through Thursday from 10 a.m. to 7:30 p.m.; Friday and Saturday until 9 p.m. VIRGINIA

Vienna Seafood Market, 328 Maple Ave., West Vienna (938-0736). Fresh fish delivered daily, New England clambakes and shellfish available and delivered if order is over $100. Open Monday to Thursday from 8 a.m. to 7 p.m.; Friday until 8 p.m.; Saturday from 10 a.m. to 8 p.m. and Sunday from 1 to 6 p.m.

Washington Fish Exchange, 1200 1st St., Alexandria (549-4000). Northern Virginia's largest seafood specialists sell fresh and frozen flounder, rockfish, snapper, croaker, cod, haddock and salmon from Alaska. Open from 6 a.m. to 4 p.m. daily. Closed Saturday and Sunday.

Vance's Seafood and Vegetables, 2406 S. Shirlington Rd., Arlington (920-7799). A small variety of fresh fish sold from 8 a.m. to 11 p.m. weekdays and from 10 a.m. to midnight on weekends.