Saltwater is big water, from the Atlantic Ocean to the salt bays behind the dunes to the vast expanses of the Chesapeake.

Because it's big it's slow to react to the first warm hints of spring. While freshwater lakes and streams have already lit up with early fishing activity, time remains before the action begins on the salt side.

First to arrive are mackerel, flooding north in great schools off the Atlantic Coast. When they hit, they ofter captured five and six at a time on hook-and-line rigs called mackerel trees, which makes for exciting sport. They were due last week, but arrived a litte late: The first big catches were reported by Oceans City headboats on Monday.

"We picked up a few over the weekend," said Lloyd Lewis at Talbot Street pier. "Everybody caught 'em good on Monday."

The mackerel run, which had also been eagerly awaited at Lewes, Dealware, and Chincoteague and Wachapreague, generally lasts at least two or three weeks. Then come summer flounder, invading the sandy bays where they feed voraciously on minnows and bottom life.

Later still come bluefish and sea trout, tuna, sharks, marlin, croaker, jumbo spot.

All are accessible either in the backwaters, surrounded by salt marsh, or by casting from the beach or in reasonable boat distances offshore.

The Chesapeake, in Washington's back yard, is responding to springtime in its lazy way. Water temperatures last weekend were in the middle 40s, and when they reach 50 it will signal the time when striped hass, called rockfish hereabouts, begin responding to anglers' baits and lures.

Rockfish arrive in the Bay from two directions. They come south through the Chesapeake and Delaware Canal and they sweep north through the mouth of the Bay. They come here because this is the traditional spawning area for most of the stripes on the East Coast.

Their goal is to make it to fresh, fast flowing water in the rivers, where their eggs can develop into fingerlips. Then the mature fish can return to the sea while the juveniles develop to adulthood in the protected waters of the Bay.

Rockfishing isn't what it once was, for reasons that are very much in dispute. To protect reproducing stocks of large females, no fish over 32 inches may be kep until May 1, and after that each sport fisherman is permitted to keep only one so-called "over-sized" striper.

But the real lord of the Bay is the bluefish, which has ruled the Chesapeake for the last 10 years - there just isn't much room for anything else.

Last week the first menhaden arrived in the middle Cheasapeake. These small bait-fish swarm through Bay waters, where they serve as something of a mobile feedbag. The blues won't be far behind.

Bluefish thrive by swarming after the swarms of menhade, and it's a happy circumstance for sport fishermen across the Bay. Generally the first blues arrive around mid-May from their winter homes in deep water. The biggest come first, ravenously hungry as rising water temperatures raise their metabolism.

As the summer wears on the big blues are crowded by smaller and smaller blues, until by July and August there is a superabundance of mostly two and three-pound bluefish.

Some years back bottom fishing was an other popular pastime in the middle Bay, but after Hurricane Agnes pushed the saltwater threshold south the middle Bay became almost barren of saltwater bottom fish. They've never returned in numbers.

But Tangier Sound - between Crisfield, Maryland, and Tangier Island, Virginia - remains an incredibly bountiful bottomfishing hole for sea trout, flounders and croakers, all of which take their sustenance from tiny crustaceans and bait fish that live near the bottom.

Tangier is hard to get to but worth the effort.

It's a three-hour drive to Crisfield or a much longer boat ride from most Maryland points to the sound. But the abundance of fish from June through September is predictable and often flabbergasting.

Because so many people enjoy freshwater fishing before they ever discover saltwater, there is a preponderance of underequipped anglers on the Bay and ocean. Saltwater fishing requires stout rods and heav line - at least 17-pound test for starters.

Later there is time to fool with light tackle, but when learning the ocean game, err on the heavy-tackle side. One may be fishing for croackers, but one never knows when a 60-pound striper, a 70-pound drum or even a 300 or 400-pound shark might pick up the bait.

Which is what makes salt water so appealing in the first place. CAPTION: Picture 1, SALTWATER FISHING CAN BE DONE BY DAY, FROM A BOAT ON THE BAY,...; Picture 2, ...OR BY NIGHT, FROM THE SHORE IN THE SURF. Photo by Gerald Almy.