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Sharks, In Deep
by Howard Youth
Listen now 26:50
Shoulder-deep in salt water, with chest, hips, knees, and toes completely out of view, who hasn't seriously pondered what lurks beneath the frothy green waves? Marine biologists certainly do—not out of fear, but because so little is known about sharks and because many of these oceanic predators are disappearing in a watery realm now dominated by humans.
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| Humans pose a greater threat to sharks than sharks do to humans. |
Thirty years after their appearance, the popular novel and film Jaws and dramatic "shockumentaries" depicting feeding frenzies still loom large in the public's outlook on sharks. But not everyone views these fish as monsters, including the people who know them best. "The perception has been that sharks bite, bite, bite, that they're mindless feeding machines. You might say humans are the same around the Thanksgiving table," says A. Peter Klimley, a University of California, Davis, marine biologist who has spent much of the last 25 years studying shark migration and behavior.
Yes, sharks do kill a few people each year. But unprovoked shark attacks, while alarming and grisly, are rare compared with other perils. Of the world's approximately 500 shark species, four—tiger (Galeocerdo cuvier), bull (Carcharhinus leucas), oceanic whitetip (C. longimanus), and white (Carcharodon carcharias)—rack up the lion's share of attacks on humans. Shark researchers try to put attack statistics in context: During the 1990s, an annual average of 130 U.S. motorists died after colliding with deer, while an average of 15 people succumbed to venomous snake bites, according to the International Shark Attack File kept at the Florida Museum of Natural History in Gainesville. During the same time period, the annual average number of people killed by sharks in U.S. waters was 0.4.
In 2004, 61 unprovoked shark attacks were documented around the world, including seven fatal incidents. Compare these figures with the estimated 100 to 200 million sharks killed by commercial and sport fishing each year, and it's clear that humanity poses a widespread risk to sharks rather than the other way around.
Shark Basics
The popular image of sharks as dolphin-size, gray, and
torpedo-shaped is accurate for some species, but not
nearly all. In size, sharks range from the banana-size
dwarf lanternshark (Etmopterus perryi) to the
world's largest fish, the whale shark (Rhincodon
typus), which reaches lengths of 40 to 50 feet.
Sharks range in build from thin to squat and in color
from bright blue to zebra-striped. And while all sharks
are predators, what they eat and how they procure it
varies greatly. Some haunt tropical reefs and snap up
small fish; others grab seals in cold waters. Angelsharks
(genus Squatina) lie camouflaged on the ocean
floor, then ambush passing mollusks and fish. Small
cookie-cutters (genus Isistius) wander the
open water, taking bites out of larger sharks and other
fish. The huge whale shark sucks in a large volume of
water then strains it through its gill slits, leaving
behind a hearty serving of plankton, wriggling fish,
or crustaceans.
While sharks are fish, they belong to a different class (Chondrichthyes) than the world's 25,000-plus bony fish species (Osteichthyes). The two classes likely diverged as far back as 400 million years ago. Among other differences, sharks and the closely related rays, skates, and chimaeras lack plate-like bony scales and have skeletons comprised of cartilage, not bone. Also, sharks have five to seven pairs of gill slits, while bony fish have one pair of gill openings.
There are differences you can't see from the outside too. For example, sharks lack the swim bladder that keeps bony fish suspended in the water. Instead, they are kept from sinking or floating by their cartilaginous skeletons, which are lighter than bone, and by oil stored in their livers that is lighter than water. Shark species that migrate across open water also maintain neutral buoyancy—and extract oxygen from the water—by remaining in constant motion.
Sharks have relatively large brains in proportion to their body size and highly developed senses. Keen senses of smell and taste, for example, help sharks locate both prey and mates. And they alone have jelly-filled organs in their heads called ampullae of Lorenzini, which are electro-sensory receptors that may help sharks locate prey hidden in sand and under other cover or detect changes in water temperature. Migrating sharks may also use their ampullae to navigate by orienting themselves via the Earth's magnetic field.
Some sharks lay eggs, while others bear live young that feed on undeveloped eggs or siblings in the womb, or are nourished by a placenta similar to that of mammals. Either way, sharks produce far fewer young and take much longer to mature than many commercially valuable bony fish. The sandtiger shark (Carcharias taurus), for example, bears just two pups born after a gestation that lasts nine to 12 months, while a large female striped bass (Morone saxatilis) produces about 4.2 million eggs, and a female southern bluefin tuna (Thunnus maccoyii) releases about 15 million eggs over several days. Some small shark species start breeding when they are a few years old and may produce more young than sandtigers, but most large coastal shark species, including the notorious white shark, bear only between two and 12 pups. Females of most of these large species don't reach maturity until they are between ten and 17 years old. Some, like the dusky shark (Carcharhinus obscurus) and the white, breed only every two or three years.
In the natural scheme of things, slow shark reproduction works fine. While sharks produce far fewer offspring and take longer to mature than bony fish, many are relatively long-lived (dusky sharks, for example, may live 40 years or longer) and their pups are relatively large, standing a better chance against predators than do tiny bony fish eggs and fry. But when people are thrown into the mix, sharks often don't fare so well; many species simply can't breed fast enough to keep up with the relentless pace of today's fishing industry.
Troubled Waters
As shark populations slip downward, our knowledge of
how they live and how best to protect them remains,
in many cases, sketchy. "Part of the reason we know
so little about sharks is that they used to be considered
nuisance species," says Enric Cortes, a research fishery
biologist with the National Marine Fisheries Service
in Panama City, Florida. Cortes and his colleagues work
to assess shark populations in U.S. waters of the Atlantic,
Gulf of Mexico, and Caribbean. It's a tricky business
given vast areas of sea, huge fishing fleets, and fishing
boats' often spotty reporting of which and how many
sharks they catch, either intentionally or as bycatch.
Although public concern about terrestrial endangered species, freshwater pollution, pesticides, and other environmental issues grew to a frenzied pitch during the 1960s, '70s, and '80s, only shark biologists seemed to fret over sharks, and they had minimal support. "There was little or no funding for research," says Cortes, looking back. "We're still suffering from that."
Even the total number of shark species remains hazy, as new species continue to come to light. Fifteen years ago, many experts claimed there were about 350 shark species worldwide, but today, about 460 species is a more common estimate. Marine biologists believe the total may rise to about 500 when poorly known—as yet not formally described—species are included.
The total might also decrease, as more and more shark species experience population declines. The World Conservation Union lists six shark species as critically endangered; four as endangered; 24, including the sandtiger and white shark, as vulnerable; and 59 others as near-threatened. An additional 107 species fall within the murky data-deficient category, meaning there is inadequate information to assess their risk level.
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Although some sharks
look vicious, they rarely attack humans. |
One thing is certain: The fishing industry threatens the ability of many shark populations to sustain themselves. Thanks to better ocean mapping, sonar, and bigger, faster boats, the world's fisheries are more accessible than ever before. The tools boats use to catch fish are also far larger in scale. One open-ocean longline, for example, may stretch 80 miles and have thousands of hooks that grapple fish, birds, turtles, and any other sea creatures that take the bait.
Many sharks are caught intentionally, but just as many fall prey to the wasteful practices of large fishing fleets. Scientists estimate that about half of the 100 to 200 million sharks killed each year are bycatch, caught unintentionally in nets or on hooks set for tuna, shrimp, and other sea life.
A shark's chance of surviving being hauled onto a boat often rests in the hands of fishers, who have to process the hooks or nets quickly. In the U.S., says Cortes, sharks caught as bycatch on lines are sometimes kept by fishers, "but usually they cut them off the line with the hook attached. These sharks may do well or not." In some species, sharks frequently die before they are hauled aboard, perhaps because of stress. Hammerheads and other species that must swim continuously to extract oxygen from the water also tend to die quickly, because the flow of water over their gills is restricted when they are tangled in nets.
Despite sharks' vulnerability to overfishing, "most shark fisheries around the world are virtually unmonitored and completely unmanaged," write shark experts Leonard Compagno and Sarah Fowler in their 2005 book Sharks of the World.
Reaching for Limits
The decline of Canada's porbeagle shark (Lamna nasus)
population sheds light on the dangers of no-holds-barred
fishing. Porbeagles were heavily fished off the Atlantic
coast of Canada for decades; because female porbeagles
do not reach maturity until they are 13 years old and
then typically bear just four young each year, the porbeagle
population could not sustain heavy fishing. The Canadian
government declared its Atlantic porbeagle population
endangered in 2004 after a study revealed a 90 percent
drop in the porbeagle population since the 1960s. Catch
quotas were lowered and breeding grounds closed off.
Porbeagles are not the only species suffering in the North Atlantic. In a 2003 paper in the journal Science, Dalhousie University biologists Julia K. Baum, Ransom A. Myers, and their colleagues reported "rapid large declines in large coastal and oceanic shark populations" in the heavily fished northwestern Atlantic. They estimated that white, scalloped hammerhead (Sphyrna lewini), and thresher sharks (genus Alopias) declined by more than 75 percent in the previous 15 years. "Only in the past half century, as fishing fleets expanded rapidly in the open ocean, have large marine predators been subject to this intense exploitation," they wrote.
Since the 1990s, the United States has been one of the few countries trying seriously to manage its shark fisheries. Fishing quotas and bans on scarce species in eastern U.S. waters may be helping some sharks recover somewhat, but not enough. According to Cortes, a regional assessment of large coastal sharks in the Atlantic and the Gulf of Mexico in 2002 "indicated that the biomass hasn't reached a sustainable level—essentially that there is overfishing." But before a U.S. shark fisheries management plan and ban on shark finning—lopping off fins and tossing the rest of the sharks overboard—in the 1990s, the situation was worse. "What drove the declines occurred mostly in the late 1970s and 1980s, although sharks are still not to their pre-exploitation levels," says Cortes.
Beyond coastal purview, sharks swim the dangerous gauntlet of the global commons. Wide-ranging blue sharks (Prionace glauca), for example, constantly cross boundaries and oceanic expanses, migrating seasonally or following currents to reach areas of greatest prey abundance. Blue sharks also segregate by sex and age, with mature males and females meeting only briefly to mate. Understanding this species' complex life history is critical for setting sound shark-fishing guidelines. Yet even if strict guidelines were drawn up, it would be hard to gauge their success given that no one really knows how many blue sharks are caught in most areas. "There is a lot of uncertainty about the levels of landings and bycatch. …We have much more work to do," says Cortes.
Finning is a wasteful practice that has been hard to monitor on the high seas. Fins are prized in Asia for use in shark-fin soup, a delicacy that can fetch $100 or more a bowl in increasingly affluent regions of that continent. After more than a decade of outcry by conservationists, the first international ban on shark finning—effective only in the Atlantic—was adopted in 2004 by more than 60 member nations of the International Commission for the Conservation of Atlantic Tunas. The United States, Brazil, Canada, the European Union, Namibia, and South Africa have also enacted finning bans in their own waters.
In 1999, the Food and Agriculture Organization of the United Nations instituted an International Plan of Action for shark conservation, but few of the countries that signed on have crafted the requested national plans to monitor and control their shark fisheries, the United States being one rare exception. While awareness of the need to conserve sharks is growing worldwide, the determination to stem their decline is not keeping pace. Already, there are signs that the seas are suffering as a result.
Bites Out of the Food Chain
Marine biologists are now convinced that what they feared
might happen over the last 20 years has come to pass:
The widespread capture of sharks for fins and meat and
as bycatch is changing ocean ecology. A study by Myers
and researcher Peter Ward published in the April 2005
issue of the journal Ecology found that in
the tropical Pacific Ocean between Hawaii and Fiji,
larger sharks, along with other top predators such as
tuna and marlin, are being replaced by smaller fish
species such as pelagic stingrays. According to Myers,
"the main consequence of many years of industrial fishing
in the area is a reordering of the ecosystem, thus increasing
the number of small fish that we don't like to eat."
The size of sharks still being caught in the region
has dropped dramatically: In the 1950s, blue sharks
caught there averaged 115 pounds, while those caught
in 2000 weighed on average 49 pounds. During that same
time period, catches of blue sharks dropped more than
85 percent.
"When you chop off the top of the food chain, you get a lot of destabilization of the system," says Samuel Gruber, a professor of marine and atmospheric science at the University of Miami and director of the Bimini Biological Field Station in the Bahamas. "Certain fish that sharks normally keep down are then free to eat other fish that keep algae down on the reefs, and so on." The loss of algae-grazing fish may leave reefs vulnerable to the rapid expansion of competing algae, which may slow the coral's development or overtake the ecosystem.
Gruber, who has studied lemon sharks (Negaprion brevirostris) at a nursery at Bimini, has watched their numbers dwindle over the past 20 years. "We went from catching ten sharks per 100 hooks every 24 hours—to be tagged and released—to just less than one per 100 hooks. This is happening on a global basis," says Gruber. "They're just running sharks into oblivion. It's no longer survival of the fittest because nobody's as fit as humans."
Discovering Sharks
Beneath the water's glassy surface lie many unanswered
questions about the basic life history and distribution
of sharks, but the last few decades have yielded exciting
discoveries. One of the biggest catches of the late
20th century occurred off Hawaii in 1976, when a 14-foot-long,
soft-skinned shark with a melon-like head got snagged
on the anchor of a U.S. Navy vessel. Dubbed "megamouth,"
this shark was formally named Megachasma pelagios,
which means "great sea cavern" in Greek, in 1983. The
megamouth was so dissimilar to other known sharks that
scientists created a new genus and family for it. To
date, megamouths have been sighted 27 times, with reports
scattered across the Pacific, Atlantic, and Indian oceans.
One was tagged and tracked in open water. It was recorded
at 45 feet below the surface at night, then dove down
to 450 feet below the surface during the day, likely
following the plankton it feeds on.
Megamouths and many other sharks do not limit their movements to vertical jaunts in the water column—some also migrate great distances. Just how far they travel is now coming to light, thanks to satellite tracking and other research efforts. In October 2005, a study published in Science documented dozens of salmon sharks (Lamna ditropis) migrating from summering areas in Alaska to wintering spots off Hawaii and Baja California, Mexico. Many other species are known to migrate, including the whale shark. One was charted traveling 8,000 miles in one direction over three years.
The longest journey detected to date was the trek taken by a female white shark that swam 12,400 miles, from near South Africa to Australia and back, in under nine months. This discovery marks the first concrete evidence that South African and Australian white shark populations are not isolated from one another.
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| Many shark behaviors, including migration, are not fully understood. |
"Finding that they make long-distance movements, that's interesting," says Klimley. "But why [do they do it]? That's what I'd like to know." Klimley and others have studied white sharks for years and thanks to their work, this species is one of the best understood. But that doesn't mean it's fully understood. "Nobody knows how many white sharks there are per area. That's important for protection interest. Do they arrive in groups? Do they depart in groups? How often do they feed? How extensive are their movements? That's what we want to find out."
The answers may take years to find. Gruber, for example, spent 30 years studying juvenile lemon sharks in their shallow lagoon nursery habitat, but wanted to find out how and where the adults bred. Then, in 2005, he and his colleagues began studying a large group of adult lemon sharks in the waters off Jupiter, Florida. Gruber now thinks they finally understand how adult lemon sharks get together to breed—females return to their birth areas and release pheromones into the current. The pheromones attract nomadic males. He thinks other shark species could have a similar game plan, but that remains to be seen.
Cold Water, Warm Sharks
Perhaps no shark species has stirred more public emotion,
and been more misunderstood, than the white shark, which
is the villain of Peter Benchley's novel Jaws.
In light of recent studies, Benchley acknowledges that
his past writings painted a less-than-accurate picture
of the white shark in action. In his 2002 nonfiction
book Shark Trouble, Benchley writes:
The new knowledge we've gained since the mid-1970s has convinced me that while almost all of the great-white-shark behaviors I described in Jaws do, in fact, happen in real life, almost none of them happen for the reasons I described. …We knew so little back then, and have learned so much since, that I couldn't possibly write the same story today.
These days, white sharks' behavior in general and feeding habits in particular are less of a mystery, thanks to scientists like Klimley, who has spent countless hours watching white sharks hunt seals and sea lions. "They investigate almost anything at the surface. They can't see well," he says. "They get close enough to see, and make a decision if they're going to eat." Klimley and others believe that white sharks often take sample bites and will quickly abandon food with low fat content, as they typically do with birds and humans.
Immature white sharks have different feeding preferences than mature white sharks, which fascinates Klimley. "The little ones seem to hang out in warm water and feed on fish, which is a low-energy food, while big ones live in cold water and eat marine mammals—high-energy food. They're almost like different species," he says.
Klimley believes that a high-fat diet of flabby sea mammals enables white sharks to live in cold waters where many other sharks don't venture. "Why should a white shark be a picky feeder, and why show a preference for fatty prey?" writes Klimley in his 2003 book The Secret Life of Sharks. "The answer may be that surplus energy is needed to keep the shark's body warm." Klimley once touched an adult white shark as it swam past his boat. "Its body was warm, around 75 degrees F in contrast to the cold water, often in the 50-degree range off Northern California," he writes, adding that "the white shark derives thermal energy from the same insulating fat that enables seals to survive and thrive in cold waters."
Some sharks frequent even chillier coastal waters than those favored by white sharks. Off Alaska, salmon sharks—stout, speckle-bellied fish in the same family as white sharks—maintain a constant core temperature of around 79ºF, which is 36 degrees higher than the surrounding water. Salmon fishers consider them competitors, and off parts of Alaska the sharks' distribution closely matches key salmon concentrations. Both salmon sharks and Pacific sleeper sharks (Somniosus pacificus) increased dramatically in Alaskan waters during the 1990s, for reasons that keep scientists scratching their heads.
Why are salmon and sleeper sharks on the rise off Alaska? Did more sharks move to the area due to water temperature changes or changes in prey distribution, or both? Or is there a general increase in their populations? These are but a few of the questions still circling around sharks and their ecosystems.
While we may continue to fear them for what they are—supremely adapted predators—we are also learning to appreciate sharks. For life without them means oceans with a lot less bounty, and a lot less mystery.
—Contributing editor Howard Youth's recent
ZooGoer articles explored border parks, eastern
coyotes, Florida's exotic reptiles, and Spain's Canary
Islands.
ZooGoer 35(2)
2006. Copyright 2006 Friends of the National Zoo. All
rights reserved.
