Humans are taking a greater toll on sharks than sharks have ever taken on humans. Over the last two decades, shark fisheries have grown and multiplied, fueled by an increasing demand for shark meat, fins, cartilage, liver, skin, and teeth. Millions more sharks are also killed by non-shark fisheries, because the nets currently in use catch every creature that swims by. Today, some shark populations are in serious trouble. While many commercially exploited fishes have declined in abundance, sharks have been the most quickly and radically affected by fishing. Why are sharks unable to bear current fishing pressure?
The answer lies in their basic biology.
As top predators in marine food pyramids, large sharks are much less abundant than species at lower levels. So unlike sardines, mackerel, or even tuna, there are fewer sharks to start with.
To compound the problem, sharks grow slowly and can take 10 years or more to reach sexual maturity. Females produce relatively few offspring at most only a few hundred pups in a lifetime. These low reproductive rates make shark populations particularly sensitive to overfishing, and overfished populations are extremely slow to recover.
So what's a hungry world to do? The ocean is an extremely important source of protein, but many marine species are being fished at greater levels than their populations can sustain. Overfishing, coupled with pollution and destruction of marine habitats particularly key nursery areas along the coast have sharply decreased the abundance of many ocean animals, and threaten the health and stability of ocean ecosystems on which we all depend.
Research is an important part of the solution. Only by understanding the biology, distribution, and ecological role of sharks can we construct wise management plans. Will we learn to balance the needs of all living species? Can we cooperate on a global scale to implement sustainable fishing practices? The survival of many species, including our own, depends on swift, affirmative answers to these questions.
Staying in Order
How many kinds of sharks swim the seas today? Living sharks fall into just 8 major groups, called orders.These 8 orders contain about 370 different species. There are probably others yet to be discovered. For example, megamouth sharks remained a secret until 1976! From their origin more than 400 million years ago, sharks have evolved
in many directions. The more than 370 species alive today sport a
variety of shapes and sizes that support widely different lifestyles
from the flattened, bottom-dwelling angel shark, to the ten-inch-long,
glowing pygmy shark, to torpedo-shaped predators like the white shark.
In 1994, California became the second jurisdiction in the world (after South Africa) to protect the white shark.
California State Assembly Bill 522, signed into law by then governor Pete Wilson, stipulated that, with limited exceptions, no white sharks could be taken in California waters for at least five years. This protective legislation which has been extended indefinitely was the result of much hard work by an unusual alliance of legislators, fishermen, surfers, divers, kayakers, scientists, and environmentalists. AB 522 s author was Dan Hauser, former state assemblyman from Humboldt County. His willingness to draft and fight for legislation on behalf of alleged "man-eaters" seems ironic considering that he is an avid sport diver. But his conviction was just one more example of the increasing social consciousness about the importance of conserving wildlife.
White Sharks in Numbers
With release of the motion picture Jaws in the 1970s, the white shark became at once the most celebrated and reviled predator in the world. Trophy fishing for whites became an international craze, and sincere consideration was given by some groups to attempt extermination of the species in locales where they were considered a threat.
In fact, white sharks, typical of top predators, are naturally rare in the environment even under the most favorable of circumstances. There is no reliable figure for the total population of white sharks in California waters, but scientific estimates have been as low as mere hundreds of individuals. Despite their small numbers, it is believed they perform a vital ecological role, holding populations of seals and sea lions in check.
As with other sharks, white shark populations are highly vulnerable to fishing and other forms of mortality. It takes many years for whites to attain reproductive maturity. Very little is known of their reproductive habits, but it is believed that they give birth infrequently and produce small litters or even single pups. When you consider the length of the California coastline (roughly 1,000 miles), this goes a long way toward explaining the sparseness of our white shark population. It may also explain the infrequency of encounters with white sharks by humans. Though millions of people enter the water along the California coast each year, attacks have occurred on an average of just one or two times annually for about the past 40 years. Few encounters have been fatal.Writing with author/illustrator Richard Ellis, Scripps alumnus and white shark authority John McCosker has described the white shark, Carcharadon carcharias, as "the supreme hunter-killer; the largest game fish in the world; the most dangerous of sharks; one of the few animals on Earth that we fear can or worse, will eat us; the stuff of which legends are made." But in spite of the fear these creatures may strike in our hearts, they are, also in Ellis and McCosker s words, "more rare than dangerous." This, combined with their role in the ecology of the ocean, is why they deserve protection.
One of the most interesting and unusual species is the whale shark, named for its enormous size and its behavioral similarity to some whales. Reaching a length of 50 to 60 feet and a weight of 25 tons, the whale shark, Rhincodon typus, is one enormous fish, the largest in the sea.
Whale sharks defy the stereotypical image of sharks as ferocious hunters. Instead, these gentle giants are filter feeders that feast on swarms of tiny, floating animals called zooplankton. Though the massive jaws are equipped with thousands of tiny teeth, whale sharks neither bite nor chew their food. To feed, the shark cruises slowly near the ocean surface with its mouth wide open, or orients its body vertically below the surface and sucks in vast volumes of water. Food particles are collected on a sievelike mesh over the gills and "coughed" forward into the throat, then swallowed. The basking shark (up to 33 feet in length) and the megamouth shark (currently known from only 12 specimens up to 17 feet in length) are also filter-feeders.
Originally thought to be so different from other sharks that it was classified in a family of its own, recent studies place the whale shark in the same family as zebra and nurse sharks, based on their anatomical similarities. This family, together with four other families of carpet sharks and wobbegongs, constitutes one of the eight orders of living sharks called the Orectolobiformes. Most are bottom dwellers, with the exception of the whale shark.
Only recently have we had a glimpse into whale shark reproduction. Like their nurse shark cousins, whale sharks are live bearers. A pregnant female killed in Taiwan was found to have 300 pups inside. It is thought that whale shark babies develop like nurse shark babies inside thin, membranous egg cases which are retained in the mother s uterus. The embryos hatch and complete development in the uterus before being born. We still do not know where and when whale sharks give birth, or what early life is like for the pups.
Whale Sharks Around the World
Roaming tropical seas worldwide (between approximately 30 degrees N and 35 degrees S latitude), whale sharks are most often seen in blue water, but sometimes swim close to shore. They seem to be opportunists, congregating at certain times and locations to take advantage of seasonal food blooms. The annual springtime congregation of whale sharks at Ningaloo Reef, a 161-mile stretch of fringing reef off northwestern Australia, coincides with full moons that trigger mass spawning of corals. Other species also spawn and aggregate at this time, creating a bountiful food supply.he regular arrival of whale sharks in certain areas, combined with their docile, surface-dwelling behavior, has allowed the development of dive tours that provide the opportunity to swim alongside these amazing animals. The predictable aggregation of whale sharks also facilitated the development of regional fisheries for the species over the past decade. Fueled by a growing foreign demand, mainly in Taiwan, an estimated 150 to 450 whale sharks were slaughtered annually in the Philippines for their meat and fins. Numbers of this species declined dramatically in response to the harvesting, and despite increasing effort, fewer and fewer sharks were landed the classic sign of overfishing which has too often been ignored.
The whale shark story shares some interesting parallels with the north Pacific gray whale. Following a predictable migration schedule and nearshore route to calve in the shallow lagoons of Baja California, gray whales were easy targets for hunters. Only government protection saved them from near extinction.
In 1998, the Philippine government banned the killing and sale of whale sharks. In the course of their research in the Philippines, Scott Eckert of Hubbs-Sea World Research Institute, Gerry Kooyman of Scripps Institution of Oceanography, Scripps graduate student Louella Dolar from the Philippines, and Bill Perrin of the National Marine Fisheries Service were instrumental in achieving this protection. Calling attention to the fact that whale sharks are highly migratory, and that unregulated harvesting by one nation adversely affects others that rely on these animals, these scientists continue to work for international cooperation and sustainable harvest.
-- Debbie Zmarzly, Science Specialist
Fun and Frequently Asked Questions
Do sharks have a rib cage?
No. Sharks may have some short ribs, but the ribs don't form a protective cage around the internal organs as they do in most other fishes.
Why is shark skin so rough?
A shark's skin is embedded with millions of tiny, sharp, toothlike scales, giving it a texture like sandpaper.
Do sharks have a tongue?
Most sharks do. The tongue is formed by a heavy fold of tissue from the floor of the mouth, supported internally by cartilage.
How is a shark like a cat?
The eyes of both have a unique, mirrorlike, reflecting layer (a tapetum lucidum) behind the retina, which increases their ability to see in dim light.
What organ can be so large in a shark that it occupies as much as 90% of the body cavity?
How does a shark's skin help it swim?
The special alignment and grooved structure of the toothlike scales embedded in a shark's skin work to decrease drag, thereby greatly increasing swimming proficiency. This arrangement is so effective that scientists are experimenting with sharklike skin surfaces for airplanes and boat hulls.
Must sharks swim to breathe?
True for most sharks, but not all. Some can pump water over their gills
What is the world's largest known fish, living or fossil?
The whale shark.
What is the world's second-largest fish?
The basking shark.
Do sharks have small brains?
Some shark species have small brains, but many have large brains relative to their body size, well within the range for birds and mammals which have the largest brain-to-body ratios.
Can sharks sun tan?
During a research study in Hawaii, the amount of melanin (the pigment responsible for tanning) in the skin of scalloped hammerhead sharks increased greatly when the sharks were placed in shallow ponds and exposed to more sunlight than normal.
Why would a shark eat a tin can?
Sharks have a special sensory system that helps them detect the weak electrical impulses given off by animals. In seawater, certain metals undergo a chemical reaction that produces an electrical field, so a shark may become confused and mistake a metal object for prey.