Episode 27
Copyright © John Page Williams, Jr. all rights reserved.
This is John Page Williams with another reading from Chesapeake Almanac. The month is September and this chapter is called "Cues of the Season: Daylight and Water Temperature."
It's late summer. Our ospreys are not as obvious as they were even a month ago. Their young have learned to fly and fish. The focal points of the early summer, the nests, are largely abandoned, and the birds have dispersed. Soon they will leave the Chesapeake for their other summer in Central and South America.
The shorebirds that breed in the marshes of the mid- and lower Bay, like willets and oystercatchers, are already moving south. The shorebirds that breed on the Arctic tundra, like sandpipers and plovers, are moving through on the way to the equator and beyond. So are the early waterfowl, like blue-winged teal.
In the Bay itself, fish are beginning to feed more heavily. The most obvious are the rockfish and bluefish that are becoming bolder by the day at driving schools of bait to the surface in spectacular displays of predatory frenzy. Flounder, spot, gray and speckled trout, and Spanish mackerel are also gorging themselves, though generally in less obvious ways.
Somehow these birds and fish have realized that the seasons are changing. They're picking up environmental cues and acting on them. Fall is on the way, though we humans sense it primarily in calendar pages and school reopenings. Shortly after Labor Day, the first of a series of cold fronts will blow through, with clear blue skies and cool northwest winds that will remind even city dwellers of what is ahead. Even so, we will only be catching up to what the birds and fish already know.
Late summer weather, then, poses a riddle. What specific cues are these birds and fish responding to? Air temperature is not reliable. It fluctuates too much, and there's still plenty of hot days ahead. Instead, these animals must be depending on indicators that change gradually and predictably.
For most of the birds the cue is photoperiod, the length of daylight. Their internal biological clocks sense the shortening days, even at different latitudes. Thus, "our" ospreys are noticing the change here and getting ready to head south, while "our" Canada geese, up near the Arctic Circle on Quebec's Ungava Peninsula, are getting ready to come here. The same goes for egrets, shorebirds, gulls, and turns headed south from the Chesapeake, and the other waterfowl and loons headed here from Canada.
Biological clock mechanisms have been studied by researchers for years but, they are still only poorly understood. No matter how they work, though, they're for real—extraordinary adaptations to help these birds make efficient use of widely divergent habitats that in some cases are continents apart.
For fish, the cue is the factor that dominates their lives: water temperature. Fish are cold-blooded, so their body temperatures—and thus their metabolic rates—are dependent on the water around them. Water has a high specific heat. That phrase means that it must absorb a great deal of heat to raise its temperature 1 degree, or it must lose a lot to drop a degree. Thus large bodies of water change temperature only slowly, reflecting seasonal trends rather than day-to-day fluctuations. When the temperature in the open Bay begins to fall, even from the high 70s to the low 70s (that would be in Fahrenheit degrees), it's a reliable indicator that the season is really changing.
Toward the end of this month, after the autumnal equinox, cold fronts will cool the shallows in the rivers and creeks, chasing upriver predators like white perch and young rockfish, and forage fish like menhaden, silversides, and anchovies, out to deeper water in the trenches of the ancestral Susquehanna channel in the upper Bay. As fall proceeds and the water temperatures drop further, blues, spot, trout, flounders, Spanish mackerel, and menhaden migrate inexorably down the Bay and out into the Atlantic onto the continental shelf. Hence the spectacular fall of fishing each year at junction points like the Southwest Middle Grounds where the main Bay and the Potomac meet, and the Chesapeake Bay Bridge-Tunnel, between the Virginia Capes. For us, this change of the seasons is perhaps the richest time of year. The variety of birds and fish that are present is greater now than at any other time, for there are lots of comings and goings. Because animal behavior is geared to processes that change slowly—photoperiod and water temperature—this is a long season, lasting from now until Christmas. Don't put your boat to bed yet. Go and be part of it.
