The Most Important Fish in the Sea
Obliteration of the world's great shark species may be upsetting the balance of entire ocean ecosystems
WEARING SHORTS and wet suit booties, marine ecologist Charles "Pete" Peterson wades carefully through a bed of sea grass looking for scallops. This shallow bay on North Carolina's Outer Banks was once studded with the animals--colonies so dense "you couldn't set a foot down without stepping on three of them," he says. But on this July morning, Peterson searches for an hour and finds only two.
The rest have been wiped out by ravenous bands of cownose rays that are swarming the Outer Banks in record numbers, says Peterson, a professor at the University of North Carolina's Institute of Marine Sciences. Not long ago, sharks would have eaten many of the rays before they could plunder the scallops' beds. But today, large sharks are rarely found here. "This is what happens when species like the ray are released from their controls and allowed to have a population boom," says Peterson, surveying the ruined scallop beds, still lifeless despite a three-year effort to restore them by closing the fishery. "It shows why we should all care about what happens to the apex predators of the sea."
What has happened, in fact, is a global collapse of great shark populations worse than any in the known history of these ancient predators. Since the 1960s, nearly all large shark species have suffered steep declines--in some cases by more than 99 percent--due to overfishing and wasteful seafood harvesting practices by humans, the only species that has managed to usurp the shark's place at the top of the food chain. Along the U.S. East Coast, some once-dominant species, including dusky, tiger and scalloped hammerhead sharks, are now considered "functionally extinct," says Julia Baum, a marine ecologist at Dalhousie University in Halifax, Nova Scotia, and coauthor with Peterson of a recent study documenting the far-reaching effects of the predators' decline. "The loss of sharks is triggering changes that cascade through the food web."
But unlike other imperiled sea species such as whales and manatees, or highly sociable creatures such as dolphins and orcas, sharks suffer from an image problem. They evoke little sympathy from most people, thanks to fictional movies such as Jaws and occasional real-life attacks on swimmers. Scientists such as Peterson and Baum say it's time that people began to appreciate the positive attributes of sharks. And they believe a two-inch-long scallop may be the place to start.
In Bogue Sound, just west of North Carolina's Cape Lookout, the daily ebb tide lowers the water line to reveal sand flats and vast shallows where the water barely covers the sea grass. Unable to traverse farther by kayak, Peterson gets out and begins to wade, sending small crabs and fish scurrying ahead of him. If any rays are lurking nearby, he hopes to scare them off as well, or at least avoid encounters with their stingers. "Incredibly painful," he warns, keeping his eyes trained on the sea bottom.
Finally, a black shape the size of a bathtub mat materializes from the muck and darts away. This ray is early. Many more--tens of thousands more--will follow in late summer and fall as the fish swarm into the bays during their southerly migration, looking specifically for bay scallops. Even before the onslaught, only a few scallops survive here. Peterson finds a small one and picks it up, studying the grooved grayish white shell and row of purple eyes on the outer mantle. "After these are gone, the rays will find other prey," he says, "and that means clams and oysters"--shellfish whose populations already have been reduced by overfishing, disease and pollution.
With its elongated snout, the cownose ray has a particular talent for crushing and eating scallops and other shellfish. Since 1970, the number of rays cruising these waters has increased about 20-fold to an estimated 40 million. Peterson says the surge in cownose rays is a primary reason for the disappearance of scallops in coastal North Carolina, which until recently was home to the nation's second largest scallop fishery after Massachusetts.
Did the shark collapse trigger the surge in hungry rays? To find out, Peterson and his team analyzed decades of population data for 25 species that live in East Coast waters. First, they measured changes in the populations of 11 large shark species--including bull, dusky, black tip, tiger, sandbar and scalloped hammerhead sharks. Then they examined population changes of 14 so-called "mesopredators," mostly rays and small sharks that occupy a middle rung in the food chain and are themselves preyed upon by large sharks. The results: All of the large shark species experienced sharp declines. But among mesopredators, the trend ran in the opposite direction. All but 2 of the 14 species witnessed a population boom, just as the big sharks were crashing.
The research has generated controversy among marine scientists, some of whom argue there are not enough data to reliably gauge the extent of the sharks' decline and how it affects other species. But experts all agree on this much: Most large shark species have suffered devastating losses, which are now being felt across the food chain, in every part of the world. "Some populations have been absolutely creamed," says Jack Musick, a Virginia-based professor of marine science who has tracked changes in shark numbers over four decades. According to his own surveys, common Atlantic species such as the sandbar are down 50 percent in 30 years, while occasional visitors such as the tiger shark have disappeared altogether.
PERFECT EATING MACHINE
In a room marked "Fish Autopsy," Musick opens the lid of a large freezer chest and pokes around until he finds the specimen he is looking for. It is a juvenile mako shark, about three feet long, caught years ago by a fisherman and donated to the Virginia Institute of Marine Science in Gloucester Point, where he works. The dead fish, one of thousands in the institute's collection, is partially gutted and reeks of an alcohol preservative. But Musick is enthusing about its features as though it were a rare sports car. "Look how aerodynamic--the head is like a bullet," says Musick. "Check the low aspect-ratio of the tail fin."
One of the more interesting features can be felt but not seen. The shark's skin feels as rough as sandpaper, a phenomenon Musick says is caused by thousands of raised, scalelike features called denticles. For years, scientists debated whether denticles had a modern function or were simply a primitive form of fish scale. The answer came from NASA, which discovered that the tiny bumps give sharks a speed advantage in the water by reducing drag, or resistance. Today some competitive swimmers wear bathing suits made of material designed to mimic shark skin. "Nature figured it out millions of years ago," says Musick.
To Musick, the discovery was a reminder of how much humans still have to learn about the shark. Evolutionary forces over a 400-million-year history have shaped this fish into a near-perfect eating machine. Its all-cartilage frame and large liver give it buoyancy and speed. Its sensory system includes a kind of radar that locks on its prey's chemical scent and movement. Some sharks can even detect faint electrical fields, an ability that enables them to find hidden prey.
In recent years, scientists also have begun to appreciate the role sharks play in keeping ecosystems healthy. Sharks clean up aquatic neighborhoods by eating carcasses of dead whales. They keep a lid on the population growth of species such as tuna and seals, which have few other predators. And they contribute to the long-term health of countless species by culling the ailing, deformed and unfit.
Yet sharks themselves have a major weakness: They are slow to reproduce. Like humans, the animals reach sexual maturity late--at up to 20 years for some species. They also have long gestation periods and produce relatively few offspring compared with bony fish. For eons, this mattered little, until a new predator, Homo sapiens, arose to challenge the shark's dominance of the seas.
The blockbuster movie Jaws may have scared beachgoers out of the water when it appeared in 1975, but it also sparked the beginning of an epic disaster for sharks worldwide. Even as the film was playing, recreational fishing clubs, particularly in the United States, began hosting tournaments and competitions specifically targeting sharks, often offering prizes for the biggest fish landed. Thousands of sharks were killed, and nearly as many trophy photos taken. And in nearly every case, the carcasses were simply wasted.
By 1985, shark numbers in U.S. waters had already declined so steeply that recorded landings by recreational fishermen had dropped 45 percent compared to a decade earlier. But demand for sharks truly exploded when commercial anglers discovered the lucrative market for shark fins. Overnight, U.S. fishermen from Florida to New England were engaged in a practice known as "finning," in which sharks are caught, their dorsal fins removed with knives, and the live fish dumped back into the sea to die. The dried fins command high prices in Asia, where they are used to make shark-fin soup, a delicacy. Today finning claims as many as 73 million sharks each year, as fishermen from Europe, Asia, Central and South America and the Persian Gulf supply fresh fins for the growing Asian market.
Although scientists and conservationists have been sounding the alarm about shark fisheries for a decade, governments have been slow to respond. A few countries, including the United States, have banned finning and placed severe restrictions on other kinds of shark fishing. But in most countries, and in international waters, few limits exist. And strict rules in U.S. waters--notoriously difficult to enforce--do little to protect many large sharks that migrate thousands of miles a year.
Still, even here, sharks continue to evoke more fear than sympathy. Each summer brings a new shark scare, and in recent decades the number of annual shark attacks has actually gone up, a fact that would seem to suggest there are more sharks in the water, not fewer. In reality, it's the beach-going crowds that have changed. "There are a hell of a lot more people going into the water," explains George Burgess, director of the Florida Program for Shark Research at the University of Florida.
Burgess, who tracks shark attacks worldwide, says most of the roughly 60 incidents that occur each year are not true attacks but accidental encounters that occur when sharks mistake human limbs for fish. Yet even though far more people die annually from bee stings and lightning than shark bites, modern humans find it difficult to deal rationally with a creature that could, in theory, devour them, he says.
The real story, says Burgess, "is not shark bites man, but rather man bites shark." When people kill sharks, he says, they are destroying not an enemy, but a pillar in an ocean ecosystem that helps sustain life on land. And one doesn't have to like sharks to appreciate that simple truth. "If we wipe out sharks," Burgess says, "we're going to wipe out everything else."
Joe Dupree, a freelance writer based in Northern Virginia, wrote about endangered coral reefs in the June/July 2007 issue.
Shark Attacks: The Warning Signs
Shark attacks on people may be rare, but an increasing number of them do occur--primarily because "there are a hell of a lot more people going into the water," says George Burgess, director of the Florida Program for Shark Research at the University of Florida. Burgess, who monitors shark attacks, says that about 60 are reported worldwide each year. Although the majority are not true attacks (they are technically accidents that happen when sharks mistake human limbs for prey) at least a handful are--which makes a recent study of shark threat displays useful reading for anyone planning to spend much time in shark-inhabited waters.
The study, by late biologist Aidan Martin of the University of British Columbia in Vancouver, describes 29 potential threat displays reported in 23 shark species. According to Martin, the most common threat display—observed in all 23 species—is when a shark points its pectoral fins downward. Others include: a hunched back, gaping jaws, body shiver, tail popping and exaggerated lateral swimming something like an eel. Martin's study was published last year in the journal Marine and Freshwater Behaviour and Physiology.--Laura Tangley
Sharks and Energy Conservation
Shark scales, called denticles, give the skin of these fish a sandpaper-like feeling. Scientists have discovered that denticles also help sharks swim faster by reducing resistance, or drag, possibly because the animals are able to stick the scales out when they swim at high speeds.
Now a research team at the University of Alabama is using a model of shark skin to explore ways of reducing drag on airplanes and ships. Funded by the National Science Foundation, the project specifically is investigating flow over the boundary layer--the one closest to the surface--of a material that mimics shark skin. "We hope to explain how a shark's skin controls the boundary layer to decrease drag and swim faster," says lead scientist Amy Lang, an assistant professor of aerospace engineering and mechanics. "If we can successfully show there is a significant effect, future applications to reduce drag of aircraft and underwater vehicles could be possible." Such reductions would save considerable energy and money. Researchers estimate that a 1 percent decrease in drag, for example, would save an airline up to $200,000 and at least 25,000 gallons of fuel per aircraft per year.--Laura Tangley