Geoscience Reference
In-Depth Information
The American shad spawn in rivers all along the Atlantic coast. The translucent larvae spend their first sum-
mer in the river, feeding on planktonic crustaceans, and in the fall, when the temperatures drop below 15°C
(59°F), the 10-centimeter (4-inch) fry descend to the sea.
Just where in the sea they go was a long-standing mystery in fishery science. Studies of Connecticut River
shad first showed that they made impressive coastal migrations, moving northward as summer advanced along
the coast. Shad with origins as far south as Florida were found in the Gulf of Maine. These migrations take
place in response to changing water temperature, with shad seeming to prefer to stay in an envelope of water
(known as their isotherm) with a temperature range of 13 to 18 degrees. This predilection for warm water leads
them into the Gulf of Maine and ultimately into the Bay of Fundy, the only places along the Atlantic coast with
that specific temperature range during August and September.
During the five years that shad normally spend at sea, they may travel 20,000 kilometers (12,400 miles), ap-
pearing as far north as Nain, Labrador, in summer, and as far south as Florida during January and February. All
shad from the eastern seaboard enter the Bay of Fundy at least once during their life cycle, and tagging experi-
ments in the inner bay have shown returns from every river from Florida to Labrador that contains a known
spawning population of shad.
It is estimated that there are 10 million adult shad and many more juveniles in the Bay of Fundy every sum-
mer—half the North Atlantic population. While in the bay, they feed on mysid shrimp and other small crusta-
ceans that, in turn, are nourished by the detritus-based food system originating in the salt marshes.
In addition to shad, a parade of southern fishes, including striped bass, alewives, sturgeon, and dogfish,
come up the eastern seaboard and into Fundy's tidally dominated migratory circuit to exploit this summer
feeding ground. As well, in season, the tides become thoroughfares of indigenous fish life, as anadromous and
estuarine species, such as tomcod and smelt, make their spawning runs into the tidal rivers at the head of the
bay. Other deep-water species—cod, pollack, mackerel, and halibut—also visit Fundy's shallow upper reaches
during certain months to feed on the offerings of mudflat and salt marsh.
IN JULY AND
August, the inner basins of the bay also play host to another remarkable influx of life—feathered
migrants from the north. Some 2 million shorebirds converge at a select number of sites to feed and fatten dur-
ing their journey from their Arctic breeding grounds to their wintering grounds in South America. Of the
thirty-four species documented in the inner bay, by far the most numerous is the semipalmated sandpiper. Up
to 95 percent of the world population of these sparrow-sized “peeps” depends on Fundy's inner basins for the
energy required to make the epic migration.
There is no more awe-inspiring spectacle in Fundy than the flight of sandpipers over the vast mudflats,
sometimes in massive flocks of a half million. These brown and white birds perform an aerial ballet as they
bank and turn in unison, flashing white and dark like two sides of a mirror. It is a living light show, as the birds
fly wing-to-wing in an impossibly intricate choreography.
Most of their time in Fundy is not spent in flight, however, but on the mudflats, where they follow the ad-
vance and retreat of the tide line, obsessively feeding with their characteristic sewing-machine motion. Their
prime prey is the scavenging amphipod Corophium volutator. Commonly called the mud shrimp, Corophium is
a tiny, lipid-rich crustacean that feeds on the ben-thic algae which overspread the mudflats like a living mem-
brane. Common to the European coast, it is found on this continent only in the Bay of Fundy- Gulf of Maine
region. (The organism may have been an early invasive species that arrived in ballast during the Age of Dis-
covery. That speculation raises the question, however, of how a native shorebird could have evolved such a de-
pendency on an invasive species in just a few hundred years.) In Fundy, the number of mud shrimp in a partic-
ular mudflat is directly related to the amount of very fine sand. Corophium builds its U-shaped burrow with
seemingly biblical wisdom: too much fine sand and its burrow collapses, too little and the mud is too thick to
dig. Where substrate conditions are ideal, the density of Corophium peaks at an astronomical 63,000 per
