Sharks: Dangerous or Endangered?
Mention the word sharks, and what goes through your mind? The movie Jaws? Ferocious giants of the sea ripping up helpless swimmers, surfers, and boaters? The word is even applied to people who engage in extortion, preying on others through deceptive practices.
Yes, sharks are predators that occupy the top layer of the marine food web, and many are large and powerful enough to be capable of harming a person. Even so, only certain species pose a definite danger to people who venture into their habitat, while most are inoffensive. On a worldwide scale, the number of shark attacks on humans amounts to about 100 per year, of which only 5 to 15 are fatal. In most cases, the attack ends after the initial contact and the shark does not kill or eat the victim.
By comparison, many more people die each year from water-related activities that do not involve sharks. Even the number of casualties from lightning strikes is much higher. Unfortunately, each shark attack is sensationalized by the media, and these stories then shape public perception of sharks.
On the other hand, human activities exert a key influence on shark survival. In recent years, as fisheries relying on other types of fish have declined, shark captures--both intentional and accidental--have greatly increased. According to an estimate reported online by NOAA Fisheries, over 100 million sharks are killed each year. Consequently, populations of many species have been dramatically reduced.
To gain a better perspective on these conflicting issues, it is important to understand sharks in terms of their biological features, life cycles, dietary habits, and role in marine ecosystems. In addition, we need to consider how they are affected by commercial and recreational fishing.
Characteristics of sharks
Sharks can be found in all the world's seas, from the equator to the polar regions, in shallow as well as deep waters. We are currently aware of about 480 species of sharks, occurring in a variety of shapes and sizes. Some species, labeled benthic, dwell mainly on the seafloor. Others, known as pelagic, spend much of their time navigating the open seas, although they may also be found on the ocean bottom. Some species inhabit rivers and lakes.
One important distinguishing feature of sharks is that their skeletons are made of cartilage, while those of most other types of fish are made of bone. Other cartilaginous marine animals include rays, skates, and chimaeras, and they--as well as sharks--are grouped in the class Chondrichthyes. Bony fishes, on the other hand, are placed in the class Osteichthyes.
Generally speaking, a shark has a streamlined body with a long, flattened snout, a parabolic mouth on the ventral (lower) side, and eight fins: two pectoral and two pelvic fins, and one each of the first dorsal, second dorsal, anal, and caudal (tail) fins. The upper lobe of the caudal fin is noticeably longer than the lower one. Furthermore, the male's pelvic fins are extended to form a pair of organs called claspers, which are used to impregnate females.
The body shape, however, varies according to the way of life of the species. For example, fast-swimming mackerel sharks have a large, conico-cylindrical shape; benthic cat sharks have long, slender bodies; and benthic angel sharks and saw sharks are highly flattened in shape. Hammerhead sharks have unique, rectangular heads.
The world's largest fish is the whale shark (Rhincodon typus), which may reach 65 feet in length. The great white shark (Carcharodon carcharias) can attain a maximum length of more than 20 feet [see "Marine Predator Extraordinaire," The World & I, November 1994, p. 218]. Most shark species, however, are under 5 feet. The smallest of them is the dwarf lanternshark (Etmopterus perryi), only about 6 inches at maturity.
Sharks differ from bony fishes in several ways, in addition to the differences in skeletal composition. For instance, most bony fishes have a swim bladder in their upper body cavity. It is a gas-filled sac that lends buoyancy by offsetting the weight of bony tissue. Sharks lack this bladder, but they are only slightly heavier than seawater because of their light, cartilaginous skeleton and huge, oily liver. Moreover, the gills of bony fishes are covered by a flap called an operculum, while sharks' gill slits are uncovered.
A shark's skin is covered by small (usually microscopic) toothed structures known as dermal denticles or placoid scales, which help reduce friction while swimming. These structures generally point toward the tail (though in some cases they are vertically oriented), so that rubbing the skin from tail to head gives a sandpapery feeling.
A shark's mouth contains 5 to 15 parallel layers of teeth so that when the front teeth break off, as happens frequently, new ones quickly take their place. Depending on the species, a shark may shed 10,000 to 50,000 teeth in its lifetime. The teeth vary in shape, according to the animal's diet. For instance, the great white shark has sharp, wide teeth, suitable for shearing or sawing pieces from large animals. The shortfin mako (Isurus oxyrinchus) has narrow, curved teeth adapted for seizing smaller, fast-moving schools of fish. The common smooth hound (Mustelus mustelus) has smooth, flattened teeth that enable it to crush hard prey such as mollusks and crustaceans.
Sharks have a highly developed nervous system, with several types of senses that are mainly used to find and catch prey. These senses include (1) smell and taste (chemoreception); (2) vision (photoreception); (3) hearing, touch, and a lateral line system (mechanoreception); and (4) ampullae of Lorenzini (electroreception). The lateral line system--consisting of a series of sensory canals just beneath the skin, running lengthwise on each side of the body--enables the animal to detect vibrations and changes of pressure in the water. The ampullae of Lorenzini are receptors (on the shark's head) that can detect weak electric fields, such as those produced by the muscular contractions of prey.
Reproduction and feeding
Compared with other types of fish, sharks have a slow growth rate. As a result, their sexual maturation takes a long time--from 2 to 20 years, depending on the species. In addition, they produce relatively small numbers of young--the litter size usually ranges from 2 to 20. In many cases, litters are produced only every alternate year.
The reproductive methods used by sharks may be classified into three types, in each of which the eggs are fertilized within the female. In one group of species, described as oviparous, the females lay horny egg cases, each containing an embryo nourished by a yolk sac. The females of a second group, termed aplacental viviparous, give birth to live young that were sustained in the uterus by a yolk sac. In a third category, called placental viviparous, the females produce live young that were nurtured by placenta in the uterus.
The average gestation period is 9--12 months, but it takes up to 22 months for the piked dogfish (Squalus acanthias). When the pups are born, they are fully formed and ready to feed. The maximum life span of a shark ranges from 10 to 70 years or more, but members of most species live 20--30 years.
The diverse species of sharks specialize in pursuing specific groups of prey, but humans are not on the menu of any species. Some of the giant species--namely, the whale shark, basking shark (Cetorhinus maximus), and megamouth shark (Megachasma pelagios)--are content to feed on planktonic organisms, including small crustaceans, fish eggs, and fish larvae. The diet of other species includes other types of fish, mollusks, crustaceans, worms, echinoderms (sea stars and sea urchins), pinnipeds (seals and sea lions), cetaceans (whales and dolphins), marine turtles, sea snakes, and seabirds. Most sharks feed mainly on live prey, but some are scavengers, feeding on dead animals.
Given the large size of most sharks, they themselves are excluded from predation by most other animals. Their eggs and young, however, are susceptible to certain types of fishes and mollusks. In addition, adult sharks are eaten by a few species, including humans, other sharks, some bony fishes such as large groupers, and a few marine mammals such as the killer whale and sperm whale.
As predators and scavengers, sharks play an important ecological role in marine communities. From their position as apex predators, they exert substantial control over the sizes of the populations of many species on lower levels of the food web. Consequently, they contribute to the stability of marine ecosystems and maintain biodiversity.
Attacks on humans
While sharks do not normally pursue humans as prey, some people are occasionally attacked by certain species. The three species responsible for most attacks are the great white shark, tiger shark (Galeocerdo cuvier), and bull shark (Carcharhinus leucas).
Equipped with wide mouths and big, serrated teeth, these powerful creatures normally feed on large prey. In addition, they are widely distributed in the seas of the world. Sometimes they have been aggressive toward humans without apparent provocation, but there have been many reports of these animals approaching divers and bathers closely without showing any aggressive tendencies.
Other species that have occasionally harmed humans are the blue shark (Prionace glauca), shortfin mako, great hammerhead shark (Sphyrna mokarran), oceanic whitetip shark (Carcharhinus longimanus), lemon shark (Negaprion brevirostris), dusky shark (Carcharhinus obscurus), gray reef shark (Carcharhinus amblyrhynchos), bronze whaler shark (Carcharhinus brachyurus), blacktip reef shark (Carcharhinus melanopterus), and blacktip shark (Carcharhinus limbatus).
The attacks may occur in water that is shallow or deep, warm or cold, but most have been recorded in areas favorable for recreational swimming, particularly in Florida. Not only is Florida's coastline popular with swimmers and surfers, but many potentially dangerous sharks inhabit those waters.
What motivates a shark to bite a human? It appears that in most cases, the animal mistakenly identifies the person as a marine mammal or other type of prey. After taking a bite, it discovers its mistake and releases the person without inflicting additional injury. Alternatively, the shark may be using a defensive tactic, to protect itself from what it may perceive to be a potential threat. Repeated attacks on a human victim are rare. Whatever the case, it is important to take certain precautions to minimize the risk of being bitten by a shark [see "Minimizing the Risk of Shark Attacks" on p. 152].
Depletion of sharks
For centuries, fishermen around the world relied on traditional methods to catch sharks (as well as other types of fish), and shark populations remained stable. Recently, however, several factors have led to serious depletion of shark populations.
One problem is that large-scale fishing methods have substantially reduced stocks of bony fishes, and commercial fishermen have compensated by vastly increasing the capture of sharks. Today, most shark species are being overfished in many seas of the world. Many species yield marketable products--namely, their meat, cartilage, skin, and oil. Shark meat is consumed in almost all Atlantic and Mediterranean countries, where it is traded fresh, chilled, frozen, and dried.
Moreover, an estimated 50 percent of the sharks captured worldwide are part of what is called the bycatch--that is, the fish caught accidentally, while attempting to catch other species. A major factor contributing to this problem is the use of gear known as pelagic longlines, generally employed to catch swordfish and tuna. This gear consists of single-stranded fishing lines, each stretching anywhere from 10 to 40 miles, carrying an average of 1,500 baited hooks. In some areas, the number of sharks caught accidentally in longlines reaches 90 percent of total captures. Species such as the blue shark and shortfin mako are especially vulnerable to this method.
Another problem is that large numbers of sharks are killed and wasted through the practice of finning them at sea. Shark fins are often used in Asian soups, and their market value is much higher than that of other shark products. Consequently, some fishermen remove the fins and discard the rest of the shark's body into the sea. This practice has recently been banned by Canada, Brazil, the United States, and the European Union.
Several shark species are caught by recreational anglers, many of whom consider even young sharks to be "large fishes." The population of blue sharks in the Adriatic Sea has been devastated by years of unmonitored, unregulated sportfishing in their nursery area. Thresher sharks (Alopias species) have also been greatly affected by sportfishing.
Furthermore, shark populations are being adversely affected by human activities that are less direct but just as harmful. One such factor is overfishing of their prey, such as tunas, mackerels, pilchards, rays, squids, and crustaceans. Other indirect factors include environmental pollution and habitat destruction. If certain toxic chemicals are ingested by animals, they accumulate in each individual and are passed up the food web, from prey to predator. Consequently, apex predators such as sharks are at higher risk of receiving concentrated toxins from their prey.
Sharks are much more vulnerable to overexploitation than bony fishes, for several reasons. Their growth rate is slow, their sexual maturation and gestation periods are long, and they produce small litters of young. The exploitation of pups in a nursery area can be particularly devastating for their population. By contrast, bony fishes lay numerous eggs, and they grow and reproduce much faster. As a result, shark populations recover from overexploitation far more slowly.
The problem of overfishing of sharks is difficult to quantify. Annual landings of cartilaginous fishes, as reported to the UN Food and Agriculture Organization, amount to around 820,000 metric tons. The actual total, however, is surely much higher because large quantities of catch are not recorded. Every year, thousands of dead sharks are thrown back into the sea for lack of demand in many countries.
The piked dogfish is the leading commercial shark taken in the world. In the Atlantic Ocean, species such as the porbeagle (Lamna nasus), shortfin mako, piked dogfish, smooth hounds (Mustelus species), and requiem sharks (family Carcharhinidae) are heavily exploited and their stocks have dramatically declined. Species that have become sporadic or rare in the Mediterranean Sea include the great white shark, shortfin mako, porbeagle, sandtiger (Carcharias taurus), smooth hammerhead (Sphyrna zygaena), sandbar shark (Carcharhinus plumbeus), bramble shark (Echinorhinus brucus), and angular roughshark (Oxynotus centrina).
The decline of shark populations warrants an urgent investigation into the status of various species. To take effective conservation and management measures, further research needs to be performed to determine specifics about their life cycles, feeding habits, distribution, and exploitation from various seas. The information obtained, coupled with long-term analyses of their relative abundance, would offer insights into restoring their populations. Of course, the protection of sharks also requires protecting their prey and managing other fisheries in which sharks constitute a significant bycatch.
The absence of research and management efforts in many countries is leading to the extinction of several species of sharks. In the Mediterranean area, no country keeps accurate records of each species captured; and in the entire Atlantic-Mediterranean region, only a few nations (particularly the United States and Canada) have developed specific shark-management programs. In some countries, certain species--such as the basking shark and great white shark--are listed as protected. That, however, is not enough; the list of endangered species is far from complete. Moreover, fishermen often breach regulations and capture even the handful of protected species.
Researchers in the Mediterranean area are fully aware of this situation, but European governments seem to lack interest in shark conservation. Shark research is almost entirely neglected in favor of studying bony fishes, such as tuna and swordfish, which are commercially important.
The threat to shark populations is part of an immense problem confronting world fisheries. Most seas have been fished to the limits of their productivity. Advances in fishing technologies, along with rising demands by a growing human population, have led to heightened efforts to catch fish, mollusks, and crustaceans. As a result, the stability of marine ecosystems is in serious danger. It is incumbent on us to devise appropriate strategies to protect and restore the populations of these sea creatures.
On the Internet
NOAA Fisheries Shark Web Site
ReefQuest Centre for Shark Research
Sharks: Florida Museum of Natural History
Alessandro De Maddalena is curator of the Italian Great White Shark Data Bank and a founding member of the Mediterranean Shark Research Group.
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|Author:||Maddalena, Alessandro De|
|Publication:||World and I|
|Date:||Jan 1, 2004|
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