We have it on high authority that this should be the peak of the two-week-long yellow perch spawning run -- right now, today.

Commence fishing!

The authority is Jay O'Dell, Maryland state biologist who has charted the progress of these yellow-and-black harbingers of spring for 18 years. His isn't a happy story, overall, with streams close to Washington producing smaller and smaller yellow perch runs every year except for the Patuxent, which continues to flourish.

But perch still mass in rivers farther away, particularly on the Eastern Shore. My favorite is the Corsica River in Centreville, a half-hour drive east of the Bay Bridge, from which on Wednesday I plucked about 50 yellow perch, all but 10 of them too small to keep.

So who cares if they were throwbacks? This is a celebration of the season, yellow perch being the first of a long string of species to invade the rivers to spawn. Few things are sweeter after frigid winter than to watch your bobber dancing with the first bite of the year and to sense first-hand the swarming advance of fish hordes bent on making more fish.

Even with leaden skies and a cold, east wind blowing, you could feel spring bringing life to the Corsica's marshy banks. If you couldn't feel it, you could hear it in the growling harronks of the Canada geese passing overhead, preparing for migration, and the eerie cries of their snowy cousins, the whistling swans.

Or you could listen to Tony Bernacki's patter as the tide came up in the evening.

Perch rode the rising tide up the waterway and bunched in a deep pool. Bernacki, fishing the pool with a small, silver spoon suspended under a bobber, got strikes on every cast.

"I'm not working Friday," he advised his brother, John, who was nearby.

Two fish later, he backed up a day. "Forget Friday," he said. "I'm coming back tomorrow. This is unbelievable."

When the fishing was good, it was busy work and amazingly quiet. Maybe two dozen fishermen were scattered along the banks around the deep pool, but all you heard was the whiz of line flying as they made their casts, the plop of bobber in the water, a grunt when someone missed a strike or a muttered, "Good one," when somebody landed a keeper.

It was a private reverie these folks enjoyed, watching the bobber, twitching it along to excite some passing perch, then setting the hook hard when the bobber went down.

Ten years ago, Washington-area anglers lined the narrow headwaters of the close-by South and Severn rivers to fish this way for spawning yellow perch in March, but the runs in the last several years have been poor. O'Dell blames acid rain. He's measured rainfall as acidic as vinegar in the region, and pH levels of about 5.6 -- strongly acidic -- in the creeks themselves after a hard rain. "Any reading under 6 causes mortality to eggs and larvae," he said.

O'Dell thinks the effects of acid rain on Eastern Shore streams may be offset by liming of nearby farm fields. For whatever reasons, Eastern Shore rivers don't get as acidic and the perch runs continue, although even they aren't what they used to be.

This spring, O'Dell will install automatic liming machines on headwaters of the South and Magothy rivers, where perch runs are all but gone. The gear will dump lime during rainfall to neutralize the acid, which is thought to result from sulphurous smokestack and car exhaust fumes combining with water vapor in the atmosphere.

Here's hoping it works, because after the yellow perch, rivers in the Chesapeake system play host to white perch, herring, rockfish and what's left of white and hickory shad populations in a spawning season that extends through May. Not one of these species is thriving, and it's good to know someone is trying to figure out why.

There is yellow perch fishing on the Patuxent at Wayson's Corner, at the Rte. 4 crossing in Prince George's County. Fishing there reportedly was very good last week. Cee-J's Bait and Tackle, east of the bridge on Rte. 4, has information.

Allen's Fresh, south of LaPlata off Rte. 301, has been slow, according to O'Dell. On the Eastern Shore, traditional perch spots with public access are the headwaters of the Chester at Millington ("red hot," according to sources), the Wye at Wye Mills, the Corsica at Centreville, the Choptank at Martinak State Park and at Red Bridges upstream, the Blackwater at Rte. 335 and a little feeder of the Chester River at St. Paul's Church on Sandy Bottom Road off Rte. 20 near Rock Hall.

Bait? Grass shrimp, if you can get them fresh (try Angler's on Rte. 50 at Annapolis); small minnows, if you can't. Commence fishing!