Fisheries Don’t Welcome This Whelk
Alarmed, Roger Mann, a marine biologist at the VIMS, contacted area fishermen. That’s when he got the really bad news. Local watermen told him they had been catching the huge—often softball-size—mollusk off and on for upwards of 7 years. They easily remembered the creature, whose shell is broader than a native whelk’s, contains a novel edging of tiny "teeth," and can have a brightly red-orange inner surface.
Mann now suspects that larval whelks may have been dumped into the Chesapeake along with ballast from cargo ships from the Sea of Japan—or even the Black Sea, where this invader is credited with wiping out native oysters.
Evidence that this alien whelk has been grazing on Chesapeake shellfish is also apparent—provided, Mann says, you know what to look for.
"Most predators leave a characteristic signature on the shells of their prey," Mann observes. Ironically, the Rapa whelk’s signature is the absence of any mark. An adult whelk’s mouth is so big it can easily engulf a clam. Once inside, the clam opens its shell, allowing the whelk to easily suck out the soft-bodied inhabitant. Then the whelk spits out the shells.
"What you end up with," Mann says, "is a perfectly clean set of shells stuck together, with no cracks or breaks—precisely what hard-clam harvesters have increasingly been hauling up over the past few years.
To discourage Chesapeake fishers from returning any netted Rapa whelks to local waters, Mann began offering a $5-a-whelk bounty. "I figured I would be lucky if I had to pay out for 100," he recalls. In fact, he’s now received more than six times that many—and he has had to cut the per-whelk bounty to $2.
Their large size suggests that some of the invading whelks in the Chesapeake "must be at least 10 years old," Mann says. With the exception of about 15, he notes, "all of the 650 or so [whelks] that we have received have come from a swath of water about 11 kilometers long and 5 km wide." He said that officials might consider dredging this relatively circumscribed area to cull the noxious whelks if this area didn’t also encompass almost the entire local hard-clam fishery—a resource valued at about $1 million a year.
However, he now seriously doubts that this strategy will work in the Chesapeake. Though he knows right where to look for the whelks, poor visibility—even with the aid of a flashlight—in this murky water almost precludes seeing a hand in front of your face. "So, the chance of divers eliminating them is just about negligible," Mann says.
His laboratory studies with whelks hauled in by bounty hunters show these animals happily survive the Chesapeake’s temperatures. Indeed, they are laying mats with thousands of eggs each up to 10 times a season. Unlike the parasites plaguing California’s abalone fishery, which had to crawl across the sediment to find a host, tiny larval whelks float in the water column. This permits passive dispersal, perhaps for many kilometers.
"Equally frightening, "is that these [floating young] can be sucked back up into a ballast tank" of some passing freighter, Mann says, "and then redistributed all up and down the East Coast."
Norfolk, a port adjacent to the whelk’s new home, is a major terminal for container ships plying the waters of the Eastern United States. Indeed, Mann says "there’s almost a shuttle-bus system" of ships that go from various ports in Europe and the Orient to a host of U.S. destinations after leaving Norfolk—such as Boston, Hoboken, Philadelphia, Charleston, and Miami. Moreover, he points out that Norfolk is home to the largest naval base in the Western Hemisphere.
Because of that fact, Mann worries, "trying to eradicate this whelk could be all but impossible."
Armand Kuris, an ecological parasitologist at the University of California, Santa Barbara, is rather frustrated that "the whole history of marine pests has been one of complete fatalism. These new species are detected, reported, and studied for their impacts. Then, people throw up their hands and say: ‘Woe is me.’"
He’d like to see some creative brainstorming for strategies to rout these pests. On occasion, this may actually lead to victories, as he’s just reported for the abalone worm (SN: 9/4/99, p. 151). However, Kuris concedes that even where control strategies look promising, there may be political reasons for not employing them—such as threats of temporary damage to local fisheries or high costs.
Alien invasions—a big and growing problem
The problem with all of them is that when they emigrated, they left behind those predators that had evolved to keep their populations in check. Now, living in an environment largely devoid of natural controlling agents, they are free to spread like the proverbial weed.
Currently, some 6,500 species of alien animals, plants, and microbes have established self-sustaining populations within the United States, according to a new survey of U.S. biological resources (SN: 9/25/99, p. 199). In a few cases, horticulturists intentionally imported these species to beautify the landscape, to improve the quality of food supplies, or to prey on previously introduced aliens. In other instances, hobbyists imported fish, birds, reptiles, or other pets that eventually got loose.
Scientists, too, are responsible for a notable disaster here and there. For instance, the infamous gypsy moth now defoliating forests throughout much of eastern North America trace to a couple of escapees from a late-19th century research lab. There, a breeder was investigating the insects as a source of genes for a better silk worm.
However, "there have been far, far, far more invasions in the oceans" than in the terrestrial environment—and than most people realize, notes James T. Carlton of the Maritime Studies Program at Williams College & Mystic Seaport in Mystic, Conn. The reason, he argues, is that marine invaders stay effectively hidden from the human world until they are fairly well established.
He also cites a few other factors that disguise the real magnitude of marine invasions:
The bottom line, Carlton contends: "It is and will remain far easier to take a multi-billion dollar picture of the surface of Mars—or hold a piece of the moon in our hands—than to effectively control most marine invasions."
Prepared by Janet Raloff, senior editor of Science News.