Real estate people tell you that the best times of the year for sales are fall and spring. It seems that people, as do the wildlife they share the earth and seas with, are in the mood to move.

While city dwellers are watching the trees and the skies for signs of the change in seasons and suppressing the primeval urge to push on, naturalists are keeping close tabs on numbers and locations of moving wildlife. A glimpse of the mobile flocks, herds and schools can tell a lot about the state of the wild.

This year Maryland marine biologists are scratching their heads about flounder. Netters and even hook-and-line anglers have been landing the delicious flatfish in places in the Chesapeake Bay where they've never been seen before.

According to Joe Boone, fishery biologist for the Maryland Department of Natural Resources, the flounder population in the bay this year is extraordinary. They've been taken in the upper Bay, in the Potomac as far north as Indian Head, in the mouth of the Sassafrass River north of Baltimore and in the Choptank and Chester Rivers.

In the past, the stray flounder that turned up in commercial nets anywhere above the Bay Bridge was a cause celebre. They were never seen in the rivers.

Why flounder this year?

Boone figures it's an indication of an extremely large year class. Almost all the flounder that have been caught in the Bay lately have been around 10 inches in length. That makes them first-year fish, spawned last winter in the deep ocean wintering grounds.

The small try, maybe an inch or two in length, work their way inshore in the early spring, riding the ocean currents, and move up into estuaries where they spend their first summer feeding on the abundant bottom life.

In normal population years the fish find plenty of room to feed at the mouth of the Bay and in the protected waters on the ocean side of the Eastern Shore. But this year they apparently arrived in such numbers that many were forced to move north into the brackish waters of the northern bay.

One of Boone's colleagues, out bluefishing earlier this summer, said land were spitting up baby flounder the big blues he caught off Kent Isby the handful.

And on a bluefishing expedition of my own two weeks ago I learned about flounder in the bay first hand. While the group I was with managed to land not a single decent blue all day, we did haul in two eating-size flounder, and we were fishing cut bait on oversized bluefish hooks. There's no telling how many flounder we might have landed had we known they were there and brought the right gear.

There are other indications that flounder are on an up trend. Last weekend a fishing partner and I spent two days plying the scenic Bay waters of Chincoteague inlet, looking for flounder.

We were not alone. Hundreds of other anglers were drifting with the currents and bouncing minnows off the bottom. This is extremely late in the year for flounder at Chincoteague but we did well, landing close to 50 keepers in about 14 hours of angling. Again, they were mostly 10 to 12 inches with a few larger, indicating a very large year class.

From all evidence, the flounder havent' left the Chesapeake yet, either, but it won't be long before they do.

For varety-starved middle and upper-bay anglers, they could provide a delightful diversion for a day. The best way to catch flounder is to drift for them over dropoffs - places where the water runs from shallow to deep fairly quickly - and where the bottom is reasonably hard. We picked up ours off Sharp's Island Light at the mouth of the Choptank.

Flounder cannot resist minnows and they feed voraciously if you hit them at the right time. One tip - the lighter the line, the better the action. In Chincoteague we fished four lines, one with six-pound test, one with eight, another with 10 and another with 17.

Fully half the strikes we had were on the six-pound line, probably because less weight was needed to keep it down and the minnows swam more freely.

But hurry. It won't be long before the great annual bay exodus is on and flounder will join bluefish, croaker, spot and sea trout on the march to the sea. Soon we will be locked in an easy chair before the fire, sipping brandy and longing for spring.