by John McPhee
I asked him what happens when shad return from the ocean. When does salinity change occur? “We don’t know,” he answered, explaining that it is not clear whether shad come into their rivers on homing instinct alone, or from physiological necessity as a result of a renewed cellular and osmotic need to be in fresh water. He said, “The fish is doing what it wants to do, and whether it is following its physiological limitations, or whether it’s just innate behavior—we can’t really tease those two apart. To understand the physiology we have to have more controlled conditions, and we haven’t done that at all, with adults. Hormones that are involved in triggering spawning may be involved in triggering their ability to go back into fresh water, but we don’t know.”
It occurred to me, parenthetically, to ask him what we literally mean when we say of someone that he drinks like a fish. In a given period of time, how much water goes into the mouth and over the gills of a shad? He said there had been no studies on this particular fish, but an extrapolation from rainbow trout provides a conservative estimate, i.e., “when swimming fast or recovering from exercise” even a little buck shad will take in ten quarts a minute.
If larval shad are experimentally immersed in sea water, they die. If juvenile shad are removed from a river in summer and put into sea water, they survive. “Right at the larval-juvenile metamorphosis, when they take on the appearance of full-grown shad, that’s the time when they develop their salinity tolerance,” McCormick said. “That’s a couple of months before they normally migrate. It was Joe Zydlewski who determined this, when he was a student in my lab.” Possibly the salinity tolerance develops early because some shad spawn near their river’s salt line, and the young, in high water, could be washed down and go to sea prematurely. With dual capability—salt and fresh—they could get themselves back across the line. Or zone. The fresh-salt change in a river differs with years and seasons. Some rivers have very sharp salt wedges, and in others the change is more gradual. Shad tend to be in larger rivers, where the change is not abrupt.
Salmon of all species can go back into fresh water at any time in their lives. The mechanism is always there. As a result, biologists believe that salmon evolved from ancestors that lived in fresh water, acquiring in their evolution their ability to go into salt water. Shad, in this context, are “almost the reverse of salmon,” McCormick said. “At the end of the outmigration, they actually lose their fresh-water tolerance.” Why they do not retain their juvenile ability to go back and forth between salt and fresh is an as-yetunanswered question. “Since they’re spending three or four years out in the ocean, maybe it’s just not economical for them to keep all those structures developed and sitting there. It’s maybe a waste of energy. That’s what we assume is going on. They don’t keep structures that they don’t really need.”
The structures, principally, are chloride cells in the gills. “We know a lot about these specialized cells,” he continued. “We know the hormones that cause their development and the hormones that turn on ion transport when the shad move into sea water. Hormones control their ability to make the change.”
When the small silver shad cross from fresh to salt downriver, they leave behind them the stream’s iron gauntlet of voracious predators—the black bass, the channel cats, the walleyes, the muskies. However, as they sprint for the blue ocean, bluefish are waiting at the sea buoy. Fish-eating fish go for bright flashing targets. Mackerel scissor into them. Sea trout, stripers, sharks. Tuna eat them by the classroom. If they swim too close to the bottom, flounders suck them in. The juvenile shad that joins the big crowd in the ocean—the yearlings, the twos, the threes, the fours—may swim in the shadow of a six-pound roe, but it is already a stressed, anxious, nerve-shot fish. In the science, a new young fish that survives the trip downriver and emerges into the ocean to join an adult school is known as a recruit.
SIX
A COMPETITIVE ADVANTAGE
Before that recruit sexually matures and returns to its river to spawn, what is its itinerary in the ocean? Until fairly late in the twentieth century, science was unaware of the travels of the species. In 1879, Roosevelt and Green reported “that shad do not roam about the ‘vasty deep’ in immense shoals, making journeys of thousands of miles, and sending off relays to each river they pass, but that they remain quietly near the streams where they are bred till the time comes for them to leave the ocean, seek the fresh water and complete their duties of procreation.” Roosevelt—a novelist and entomologist and a basic figure in American fly fishing—was known in other contexts as Teddy’s uncle. When he wrote the book with Seth Green, he was the Commissioner of Fisheries of the State of New York. Their idea about the schools’ staying close to home was based on the varying abundance of shad in East Coast rivers, and it served as conventional wisdom for three-quarters of a century. The species’ long journeys in the ocean—typically twelve thousand miles from recruit to spawner—were suspected in the nineteenth century but were essentially unknown before 1958, and unclarified before the eighties and nineties.
As early as 1937, the distant recovery of one tagged shad was noted in the scientific literature. Over the next nineteen years, the U.S. Bureau of Fisheries and its successor agency the U.S. Fish and Wildlife Service tagged more than seventeen thousand shad netted in various places. Reporting the results in their 1958 paper “Atlantic Coast Migrations of American Shad,” Gerald B. Talbot and James E. Sykes mentioned “very little evidence … as to where shad spend the winter months” but a very distinct discovery that shad native to every river congregate in summer in the Gulf of Maine—that is, in the Atlantic bight north of Cape Cod, an area of some thirty-four thousand square miles.
The idea that shad in rivers are functioning within a frame of temperatures had been around since 1884, when it was proposed by Marshall McDonald in a contribution to G. B. Goode’s “The Fisheries and Fishery Industries of the United States.” Now, after Talbot and Sykes, a specific isotherm began to be seen as the controlling factor in shads’ migrations in the ocean as well. They were keeping themselves in a thermal window. As the sea warmed up in spring and summer, they were moving north to stay in that window. In 1972, William C. Leggett, of McGill University, and Richard R. Whitney, of the University of Washington, published a paper called “Water Temperature and the Migrations of American Shad,” in which they gave the isotherm as thirteen to eighteen degrees Celsius and tracked it to the Gulf of Maine in July and August and back to the mid-Atlantic bight—Cape Cod to Cape Hatteras—in October, November, and December. They pointed out that ninety per cent of the run at Bonneville Dam occurs when the water temperatures in the Columbia River are between sixteen and nineteen and a half degrees Celsius, and that the water temperature at the peak of the shad run in Holyoke had averaged nineteen and a half degrees Celsius for fifteen years. “The precise correlation between temperature and the timing of the spawning migrations of the shad places the maximum number of adults on the spawning grounds when the temperature is optimum for the survival of eggs and young.” These temperatures, they added, apply to most but not all shad. Fourteen shad were once caught by an Atlantic trawler fishing in four-degree water.
Six years later, Richard J. Neves, of the University of Massachusetts, and Linda Depres, of the Northeast Fisheries Center Woods Hole Laboratory, presented the results of a series of bottom-trawl surveys in a paper called “The Oceanic Migration of American Shad, Alosa sapidissima, Along the Atlantic Coast.” In ten years of surveys, they said, not so much as one shad had been picked up in autumn months south of New York. Yet the schools were found at that latitude in winter, and off Hatteras in spring. In a general way, Neves and Depres confirmed the thermal window but observed that shad are vertical migrators as well as horizontal migrators, moving through a variety of temperatures in search of planktonic food. In the ocean, they primarily consume large copepods, euphausids, mysids, and zoobenthos. The zoobenthos and the mysids speak of greater depths. Shad feed anywhere, high or low, migrating from bottom to surface w
ith the presence of zooplankton. But they’re closer to the bottom during daylight hours.
In the middle nineteen-seventies, a young Canadian ichthyologist named Mike Dadswell—aware that a corporation set up by the Nova Scotia government and supported by Ottawa planned to exploit the tides of the Bay of Fundy by sending them through an epic barrage filled with hydroelectric turbines—began to wonder about the effect it would have on creatures that might pass through the barrage with the water, especially shad. Dadswell worked for the Canadian government but he was wondering on his own. He worked in New Brunswick at the St. Andrews Biological Station of the Department of Fisheries and Oceans, and he went up to Fundy’s northern end whenever he could, to set out gill nets from boats, chat up local fishermen, and learn to deal with quicksand holes if he was walking the flats in waders. (“Flip off your shoulder straps and swim out onto the mud.”) Eventually, he got government support for research in both the Cumberland and Minas basins—the uppermost reaches of Fundy, where the tidal range is the most dramatic. When the tide is out, the exposed mudflats, brick red, can be a thousand metres wide; when the tide is in, the same muds are under seven fathoms of water (the height of a four-story building). The Bay of Fundy is finger-shaped and a hundred and twenty miles long. The tide range increases from the open to the closed, northern end. A good spring tide in the Minas and Cumberland basins can bring on a difference approaching sixty feet. Atmospheric high pressure can move low tide down by as much as a foot, making the flats even larger. This is the scene where the unwatered seafloor reaches out to the horizon, the focal point a fishing boat, lying on its side. Dadswell has spoken wistfully of the “three-inch tides of Tahiti.”
He is now a professor of biology at Acadia University, in Wolfville, on the shore of the Minas Basin, and his home is across the peninsula, on Nova Scotia’s open-Atlantic coast. He raises scallops there, and is busiest in September, when they spawn. The device he uses for collecting scallop larvae is a mesh bag containing a tumbleweed mass of monofilament fishing line—sort of a bagful of backlash—set in the ocean. Scallop larvae settle onto the monofilament, stringing themselves like pearls. He attaches many such bags to a long-line, and the sheltered waters of Mahone Bay wash through them for a year. The scallops grow, and when they are about a half-inch across, they drop off the line—but they can’t get out of the bag. In this way, Dadswell has developed a marine farm of half a million scallops. It is no longer clear whether his aquaculture supplements his income as a professor or his academic income supplements the farm. About fifty, he has short graying black hair and a neatly trimmed black-and-white beard. He wears bluejeans and T-shirts with fish on them. He and his family own eight boats, from canoes upward. In J24s, he was once a competitor in the Canadian sailing championships. Because his father was a career officer in the Royal Canadian Air Force, he moved around a lot when he was young, but grew up primarily in Huntsville, Ontario, more than four hundred miles from the ocean, playing lacrosse. To teach his four courses in marine biology and supervise the work of graduate students, he commutes fifty miles or so, across Nova Scotia. He commutes in a pickup with his gun and his dog. Being a professor, he is not always pressed, and sometimes stops to hunt.
Dadswell knew the literature, of course, about the summer congregations in the Gulf of Maine, when he began his studies of shad in Fundy’s upper basins, and he was fairly sure they were not local but had travelled long distances to be there. Shad populations had been rising sharply in U.S. eastern rivers. They were also on the rise in the Bay of Fundy. If he could relate these two facts, then he could demonstrate that hydropower’s potential effect on the shad was an international matter, and an ecological question more than two thousand miles long. Over seven seasons in the Cumberland and Minas basins, Dadswell and various colleagues and assistants tagged thirteen thousand five hundred and eighty shad. The tags resembled small plastic arrows and were inserted just under the dorsal fin. Science swims on. Each tagged fish carried its own serial number and promised the finder a reward.
In the first summer—1979—they tagged five hundred and fifty shad, and waited. September. November. January. No returns. Dadswell, at the St. Andrews Biological Station, was beginning to wonder what could have happened, and he was still wondering, one day in February, when his telephone rang and a male voice addressed him: “Howdy. Ah’m from Welaka, Florduh. Ah got wunna yawls shad down hyuh.” Dadswell enjoys reproducing the moment, largely overcoming the balsam in his drawl.
Over the years of the study, he would hear from eight hundred and twenty-seven people, nearly all of them shad fishermen. He would hear from Verdell Felter, of Lehighton, Pennsylvania, who caught MD06318 “on a black-and-white-with-white-tail shad dart” at Smithfield Beach on the Delaware River. Dadswell sent Felter three dollars. He sent three dollars to the Atlantic Seafood Company, of Mayport, Florida, which had processed and sold MD01604. Ronda Armstrong, of Atlantic Beach, Florida, got six dollars for reporting two fish, and Lee Walley, of Birdsboro, Pennsylvania, got three dollars after catching and reporting a sevenpound roe shad, more than two feet long, sporting the fluorescent orange. MD00558 was caught by a New Jersey man who wrote, “I was very happy to have caught a fish with it’s history it may have. Also I beleave NJ Division of Fish and Game would appreciate this data also.” One of Dadswell’s shad was netted by Fred Lewis at his commercial shad fishery in Lambertville, New Jersey. In Canada, Dadswell has since caught shad tagged by Lewis. MD05533 was caught in a gill net in Palatka, Florida, on the St. Johns River, by a commercial fisherman named John J. Sullivan. And in February, 1980, after Irvin Stanley, also of Palatka, reported catching MD01625, Dadswell wrote to him, in part, “Dear Mr. Irvin … The fish you captured traveled about 1700 miles since last September.”
Dadswell heard from Nain in Labrador, from the Pee Dee River in South Carolina, from the Ogeechee River in Georgia, and Rivière des Prairies behind Montreal Island. He heard from the St. Marys River (the Atlantic boundary of Georgia and Florida), the Satilla River, the Altamaha River, the Savannah River, the Edisto River, the Santee River, the Cape Fear River, the Neuse River, the Pamlico River, the Roanoke River, the Chowan River, the James River, the Pamunkey River, the Rappahannock River, the Potomac River, the Susquehanna River, the Hudson River, the Connecticut River, the St. John River, the Miramichi River, the St. Lawrence River. Among others. The shad he tagged in summer in the Cumberland Basin and the Minas Basin of the Bay of Fundy had gone out to and were reported from every historical shad river in North America. He and his colleagues had established something previously unknown: that the American shad as an essentially entire and convening species not only migrates to the Gulf of Maine in summer but keeps on going into the Bay of Fundy and circulates through its uppermost basins.
Dadswell has a friend named Peter Hicklin who tagged semipalmated sandpipers in Chignecto Bay and the Minas Basin—special-performance birds that come down from Arctic Alaska and Arctic Canada in mid-summer, and stop over in upper Fundy to stuff themselves with mud shrimp and build the fat that will carry them nonstop to South America. One sandpiper can eat forty thousand mud shrimp in one day. The bird’s weight doubles in ten days. A few become so bloated they can’t fly. You see hundreds of thousands packed together near water’s edge at high tide, and out on the flats near the line of receding water. Hicklin’s story not only parallels Dadswell’s but equals it in political pertinence, because any human artifact that would damage the shad’s ecology would surely decimate the fuel that supplies the birds’ intercontinental flights. Hicklin tagged many hundreds of them, Dadswell told me, and “never got a tag back.” Eventually, he persuaded the government to send him to the mouth of the Orinoco River, where he went from island to island visiting the homes of fishermen, and found a favorite souvenir—the bands he had attached to the legs of the birds.
“In the northern hemisphere, if you stand facing the sea, the major current flow in estuaries always comes in on the left side and goes out on the right side,” Dadswel
l remarked one summer day, in a lecture I heard him give in Digby, beside the Annapolis Basin. The Bay of Fundy is a large estuary. As you face down it toward its open end, its residual currents come in on the left, and run northeast up the Nova Scotia side, then swing through the complicated upper end always in a counterclockwise direction, and exit on the right, down the coasts of New Brunswick and Maine. The counterclockwise motion is a result of the Coriolis force, which derives from the planet’s rotation and spins northern waters counterclockwise not only through estuaries but also down the drains of bathtubs and sinks. Shad swim with the residual currents—in spring and early summer on the Nova Scotia side, and in late summer and early fall on the New Brunswick side, going the other way. The shad use the current to guide them around the bay. Shad on their way to spawning grounds up Fundy rivers are the first to appear in the bay in spring, followed by the great nonspawning migration. In Dadswell’s studies, he has not come upon a single exception to the counterclockwise route.
Of all the possibilities between the north coast of Labrador and the St. Johns River in Florida—historically, the thermal brackets of the species—why would the American shad be collectively drawn, as to nowhere else, to the Bay of Fundy? Dadswell clicks onto a screen a photographic slide of a steep-rising Fundian coast, cobble beaches exposed at low tide. In the large zone between high and low tide, rockweed grows in dark-green profusion, and it grows in exceptional abundance in the Bay of Fundy because of the vast acreage of the intertidal zone. It is the familiar form of seaweed that has little floats spaced along it in pairs. These are air bladders that lift rockweed in the water, where it bobs slowly, undulates, and sways. At low tide, it lies on the beach, a wet mat. It has no predators. It is slightly toxic to small creatures that might otherwise graze it bare. It grows, and goes on growing, and nothing stops it. Where does all the carbon go that is locked up in this plant? The tops of the rockweed are broken off by the motions of the water. The loose pieces float in great quantity in the bay. Washed ashore, they lie piled at the high-tide lines as wrack—in effect, a compost heap. Fly larvae chew on it, and reduce it to SLS.