Migration, the coming and going of fish and fowl, suits the Chesapeake Bay well. The bay itself has, in a sense, been "coming" for several thousand years, since the influence of the ice age last waned. High seas fed by melted polar ice flooded the narrow, ancient valleys cut by the Susquehanna and James rivers, forming our present-day broad estuary. Such warming periods are brief, geologically speaking, and last for only tens of thousands of years. It will not be long, on planetary time scales, before the glaciers advance, and this bay, too, ebbs surely as the tide.
On a smaller, daily scale, coming and going is the bay's nature. Tides surge up and down the estuary, rising and falling every six hours or so like the respiration of a great sea creature.
So it seems especially fitting that our bay is embroidered fantastically with nature's cycles of migration. We tend to celebrate the bay for the quantity and diversity of life that is here; yet fully as enriching are the near-constant leavings and arrivals: The passage of wild geese in autumn and the appearance of soft crabs in spring reassure us and reset our clocks, if only briefly, to a vaster frame of reference.
There is scarcely a month or a week on the Chesapeake Bay when something is not moving in or out, or preparing to do so.
The great blue herons come early to nest, bringing welcome reassurance of spring. Once on a February day, I was hiking a bay marsh on one of the nastiest, windiest, sleetiest days I can recall. The winter had been hard, and looked as if it had staying power. Then overhead, I heard a guttural croaking. A pair of great blues circled on the storm, heading for a clump of tall trees where last summer's nests were piled high with snow and ice. What powerful urges must lure them from the balmier Carolinas and Florida at such times. They are certain, even as winter grips hardest, that warmth and food aplenty will soon emerge to ensure the survival of another generation of Chesapeake-born chicks.
By St. Patrick's Day, the ospreys have begun to show. Like the herons, they do not come and go in the spectacular masses of the waterfowl; but they are nonetheless great travelers. These large fish hawks, whose nests crown thousands of bay channel markers and buoys, commonly live for 15 years. In that time they may travel more than 100,000 miles. Eastern Brazil is the commonest wintering destination for Chesapeake ospreys, but some go as far south as Uruguay and Chile.
The rivers start filling with traffic. Most of the anadromous fishes -- those that run from the sea up rivers to spawn -- are now schooling in the river mouths. White perch, yellow perch, hickory and American shad are all pushing to get on with it.
My personal pick is the common river herring, Alosa pseudoharengus, a silver, 10- to 12-inch specimen, too bony for anything but pickling. The little alosa thrashes its way to the outer ends of the bay's tributaries to lay its eggs. I can't think of a better place to be in April than camped beside a herring stream when squadrons of alosa begin finning across the sun-spangled sand and gravel shallows.
The alosas' appearance represents the end of a journey that began with the herring somehow homing from hundreds of miles out to sea onto the precise little stream where they were born.
Busy, busy, busy, but I'll pick the rockfish anytime as May's migrant. A rockfish can weigh more than 75 pounds and attain lengths of several feet. They are, except for the rare stray shark, the biggest sea-goer that journeys up the bay's tributaries. It is an awesome experience to be afloat in a canoe on a quiet, dark reach of a river and hear the silence of a May evening shattered by the rolling of a huge cow striper, attended by a dozen smaller, leaping males.
Pollution and overfishing in the bay have caused a serious decline in anadromous species over the years, but the worst of the fishing excesses have been stopped: The rock are recovering nicely, the herring are hanging in there (though at much-reduced levels), and the shad may be mounting a slow comeback. There is even an experimental effort to bring back the nearly exterminated sturgeon by releasing baby sturgeon into the wild from hatcheries.
Throughout the months of June, July and August, some of the bay's most important migrations take place in a few days or hours. Summertime seems reserved for the short-trippers, whose navigational efforts are no less remarkable for not involving great distances.
Diamondback terrapins, a species spread throughout the Chesapeake and its rivers, appear to live and die within a small radius of where they were born. Maybe it is one reason they live so long -- more than 40 years in many cases.