Sea trout: For five years big schools invaded the middle Chesapeake Bay in late summer and early fall, providing big-water excitement for people in small boats.

Then they went away.

Fall trout fishing "sure ain't taken off yet," Said Mike Sullivan, skipper of the Miss Dolly out of Chesapeake Beach and a veteran pursuer of sea trout. "There isn't a hot spot in the Bay right now, as far as I know. If I had to go and catch 10 trout today I wouldn't know which way to point the boat."

The predicted late-summer sea trout-blitz lured one group of fishing regulars to the Bay Bridge area last week for four days in hot pursuit. After three days they'd caught some rockfish, a lot of bluefish, some oyster toads, some oysters, summer flounder, a few perch and some hard crabs. But no sea trout.

On the fourth day, at last, the string was broken when Darrell Lowrance (of he Lowrance depth-finder company) interrupted his mental meanderings to attend to a sudden violent attack on the soft-crab bait he was dragging across the Bay floor.

Up came a nine-pound trout, glistening pink and gold.

Bob Spore, the captain, buzzed his colleagues on the marine radio and shortly the horizon was dotted with the noses of fishing boats headed his way.Rarely has one normal-size fish caused such a stir.

The school of trout, such as it was, showed up as a small hill of scratches on the chart recorder. Spore threw a marker buoy out and every time his 38-year-old Bay-built workboat drifted over the lump there was excitement aboard. "Get your lines down," he would say. And before the day was over two more trout had come aboard. That made for a catch of three, all nice ones, and left Spore's people happy, of all things.

A year ago this time, on a similar expedition, I was aboard a boat where so many sea trout were caught the fishermen were tossing them back by noontime, having filled the fishbox with more than 70. That has not happened this fall.

There are various explanations for the disappearance of the big schools of sea trout. One of the most interesting is the idea that after two straight unusually dry summers, salt water has intruded farther than ever up the Chesapeake, expanding the summer range of sea trout and scattering them over a much wider area.

Indeed, the area in which Spore was fishing, several miles north of the Bay Bridge, is very close to what is rainier years would be fresh water. Yet his catch included 35 small flounder one day last week. Elsewhere in the Bay system, menhaden schools have been wandering up the Potomac as far as Washington, and bluefish are thick throughout the upper Chesapeake.

Ben Florence, a Maryland state biologist specializing in Bay species, said trout concentrations recently were found in water eight to 10 feet deep in the mouth of the Chester River. That's farther north and farther upstream than the traditional range for these fish.

All of which hints that sea trout, responding to salt-water boundaries, may this year have gone beyond their normal late-summer feeding spots and settled in new places where fishermen haven't thought to look for them. If it's so, they have yet to head back down the Bay on the way to their winter homes in salt water. The possibility remains that they'll stop off at traditional feeding holes during that migration.

It would be a pity if they skipped us entirely. Fall sea-trout fishing is the second-trout were caught the fishermen were tossing them back by noontime, having filled the fishbox with more than 70. That has not happened this fall.

There are various explanations for the disappearance of the big schools of sea trout. One of the most interesting is the idea that after two straight unusually dry summers, salt water has intruded farther than ever up the Chesapeake, expanding the summer range of sea trout and scattering them over a much wider area.

Indeed, the area in which Spore was fishing, several miles north of the Bay Bridge, is very close to what in rainier years would be fresh water. Yet his catch included 35 small flounder one day last week. Elsewhere in the Bay system, menhaden schools have been wandering up the Potomac as far as Washington, and bluefish are thick throughout the upper Chesapeake.

Ben Florence, a Maryland state biologist specializing in Bay species, said trout concentrations recently were found in water eight to 10 feet deep in the mouth of the Chester River. That's farther north and farther upstream than the traditional range for these fish.

All of which hints that sea trout, responding to salt-water boundaries, may this year have gone beyond their normal late-summer feeding spots and settled in new places where fishermen haven't thought to look for them. If it's so, they have yet to head back down the Bay on the way to their winter homes in salt water. The possibility remains that they'll stop off at traditional feeding holes during that migration.

It would be a pity if they skipped us entirely. Fall sea-trout fishing is the second-best challenge of the middle Bay, the area of the Chesapeake most accessible to Washingtonians. It's serious fishing, dependent on the skipper's skill at locating schools with his depth-finder and his ability to fish qui etly over a moving school without spooking the fish.

The traditional favored schooling point for sea trout has been around Sharps Island Light at the mouth of the Choptank River, a five-mile run directly across the Bay from Breezy Point. "There's a small bunch there," Sullivan said, "and another small bunch around James Island," a little farther south.

He said the best concentration reported so far was farther north, above Bloody Point, but that's the bunch Spore and his colleagues searched four days to find.

The harder these trout are to catch, the greater the challenge and the deeper the satisfaction when you finally do. I managed to boat two last week, working a small bucktail just off the bottom in 25 feet of water. On light tackle, the trout hit with a shock and fought hard and long. They are also wonderful table fare, and unlike bluefish they do not suffer from a stay in the freezer.

If the trout fishing fails, there are wideranging supplies of medium-size blues throughout the upper Bay, and they are still responding enthusiastically to trolled surgical eels.

Fall is second only to May as the right time to taste the pleasures of the rich estuary that is the Chesapeake. If spring offers the first shot at big stripers and blues as they launch their season-opening assaults, October marks the last chance to intercept fleeing sea-fish.