Scaup: Searching For A Lead
November 03, 2010
While Populations Continue To Decline, Biologists Search For Answers In The Smallest Places
Sometime in 1986, a freighter docked at the Port of Detroit, unloaded her containers, and flushed her ballast tanks before heading back to sea. There was nothing to distinguish her from hundreds of other ships that run the St. Lawrence Seaway into the Great Lakes every year--except the passengers she carried. Her ballast water had come from one of Europe's major rivers, and with the water came a few tiny, free-swimming creatures, the larval form of the zebra mussel.
By now, it's an old story in the New World. We bring in a new animal or plant, and it transforms the continent. The introduction may be intentional as it was with the English sparrow, German carp, and purple loosestrife. Or accidental, as it was with American chestnut blight, cheat grass, and kudzu. Either way, the invader finds a new niche and out-competes everything else in the vicinity. The ecological shock wave ripples out into the environment in ways that are as complicated as life itself.
Within a few years, the zebra mussel was well on its way to remaking the Great Lakes. While the individual mussel is seldom much larger than a penny, the species makes up for its tiny size with a mighty breeding potential. A single female can produce up to a million eggs in a single season, and the resulting horde of mussels will attach to any firm surface. There are reports of zebra mussel colonies with 200,000 individuals in a single square meter.
The mussels are happy to anchor on native clams, often smothering them in the process. They filter the water for floating food, and their cumulative appetite can actually make a body of water clearer as they suck up uncountable trillions of tiny plants and animals. This voracious feeding can deprive many other small aquatic animals of a meal, eventually shifting the species composition of entire underwater communities.
And their effect may not stop at the water's surface. There is evidence that the zebra mussel and another invader, the quagga mussel, may be at least partly responsible for the decline of North America's scaup.
The last 15 years haven't been kind to the great and lesser scaup. In 1983, biologists estimated that there were about 7.1 million breeding scaup in North America, and the number has been going down almost ever since. While many other duck populations rebounded in the wet years of the late 1990s, scaup hovered around four million birds. This year, the estimate stands at 3.7 million, barely half the 1983 population.
The introduction of the zebra mussel to the Great Lakes could have been good news for scaup. Both species dote on small shellfish, snails, crayfish, and insect larvae, diving up to 40 feet to hunt on the bottom of larger lakes, major rivers, and as far as 10 miles out into the ocean.
The Great Lakes have always been a major stopping point on the annual scaup migration. Before the zebra mussel showed up, these resting birds focused mainly on snails. Since the zebra mussel explosion, the number of scaup on Lake Erie around Long Point, Ontario, has increased by as much as tenfold. Surveys of zebra mussel populations in some of these areas suggest that mussel numbers may be dropping, possibly because of the pressure from hundreds of thousands of hungry scaup.
Unfortunately, the fact that scaup like zebra mussels doesn't mean that a zebra-mussel diet is good for scaup. Colonies of zebra mussels filter huge quantities of water to strain out tiny plankton; in the process, they filter many other things out of the water, including pollution. The process is known as bio-concentration. It was responsible for the problems bald eagles and peregrine falcons had with DDT in the 1950s and 1960s, and it may be contributing to the troubles scaup are having today.
Researchers on the Canadian side of Lake Erie have been testing for contaminants in scaup fat over several years. Their work indicates that levels of PCBs and DDT metabolites in the birds are relatively low and probably have little effect. The same can't be said of the element selenium.
A tiny bit of selenium is crucial to the health of most living things. More than that tiny bit is poison. Twenty years ago, California's Kesterson National Wildlife Refuge made headlines with its selenium problems. Kesterson received large amounts of irrigation runoff from the selenium-rich soils of the San Joaquin Valley. Concentrated in soil and food, the selenium caused catastrophic birth defects in thousands of young birds. Tests on mallards have shown that concentrations of selenium above nine micrograms per gram (dry weight) in the liver can impair egg laying and fertility. Levels above 33 micrograms per gram can impair the health of the adult bird.
Scaup taken from some parts of the Great Lakes through the early 1990s carried high levels of selenium in their livers. Ten lesser scaup from Lake Ontario averaged 22 micrograms per gram of liver. Ten lessers from Lake St. Clair carried an average of nearly 17 micrograms per gram. Ten lessers from Lake Erie carried an average of nearly 13 micrograms per gram. Thirteen greater scaup averaged 28 micrograms per gram. According to Dr. Scott Petrie, lead researcher on the project, "One hundred percent of adult female Greater Scaup and 77 percent of adult female Lesser Scaup that we have analyzed had selenium levels above the level at which reproductive impairment could be expected." If a substantial part of the scaup population is carrying this much selenium, it's no surprise that numbers are falling.
Lesser scaup killed along the Mississippi Flyway in the last six years show a wide range of selenium concentration. More than half the birds taken in Louisiana and Illinois carried at least 10 micrograms per gram of liver. Thirty percent of the Minnesota birds and 20 percent of the birds killed in Wisconsin and Manitoba had elevated selenium levels. Christine Custer of the U.S. Geological Survey and her colleagues concluded, "high selenium concentrations in lesser scaup livers. . .seem to be confined to the industrial portions of the flyway." Samples taken in California suggest a similar pattern on the West coast where another exotic, the Asian mussel, has established itself.
Heavy industry has existed on the Great Lakes, up and down the Mississippi valley, and down much of the Pacific coast for more than a century, and scaup have used the shallows in all these places for much longer. If food from contaminated water was enough to drive scaup numbers down, then why didn't the populations crash generations ago?
Biologists aren't sure. Petrie and his research group are testing scaup that were killed on the Great Lakes before 1985 to find out whether selenium levels in the ducks were lower before the zebra mussel invasion. If they are, it suggests that zebra mussels are somehow concentrating selenium better than the native animals the scaup were eating before exotic shellfish showed up.
That possibility isn't too far-fetched. Samples taken from Lake Erie in 1973 and 1974 show that bottom sediment contained about one microgram per gram. Tin
y zooplankton in the water averaged a little more than two microgram per gram while fish averaged about three micrograms. In the year 2000, Petrie tested zebra mussels from the lake and found that they contained an average of about eight micrograms of selenium per gram of tissue. It appears that filter feeders like the zebra mussel may accumulate more selenium than other aquatic species. The snails that kept scaup fed before the zebra mussel aren't filter feeders--they graze on algae or eat dead plant and animal matter. This subtle difference in diet could have led to lower concentrations of selenium in the native shellfish, which would have made a world of difference to the scaup.
There are other possible explanations for the decline among scaup. Research in Minnesota shows that birds on spring migration are carrying less fat than they were 20 years ago. Is there a food shortage down the flyway? No one knows. Poor nesting success in the prairie parklands and boreal forest may be due to deteriorating habitat--we know that it's been an unusually warm 20 years in those latitudes. Still, the selenium connection is probably the strongest lead we have.
Identifying an ecological problem of this size and subtlety is a significant challenge in itself, but if selenium proves to be the cause of scaup declines, we will face a far more daunting challenge--finding a solution. Can we reduce selenium emissions from industry? Can we keep selenium out of our waters? Can we eliminate the zebra mussel, the quagga, and the Asian mussel from their beachheads in America? Nothing we try will be easy.
Years ago, some anonymous conservationist said that natural systems are not only more complex than we do understand; they're more complex than we can understand. The strange case of the scaup and the zebra mussel seems to support that observation. A clam the size of your thumbnail fastens itself to a rock in Lake Erie, and the shock is felt all the way to the Bering Sea.
It's a hard lesson. I wonder if we'll learn something from it.