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Sharks

    The following is an educational contribution by Moss Landing Marine Labs Biologist and Graduate Student (Currently working on shark studies in Mexico) Wade Smith


I've always had a great interest in sharks. As a scuba diving instructor, I've tried to dispel the general public belief that sharks are killing machines. My first shark sighting while snorkeling was a fascinating experience. Since, I have developed a greater curiosity and interest for those beautiful creatures. Most books on sharks are outdated by the time they go to press. For the latest information, I asked my friend Wade Smith a Marine Biologist studying sharks to contribute to our educational goal by writing this section on sharks. Wade and I are working together to provide you the most accurate and interesting facts about sharks.    J-Philippe


Table of Contents

Introduction

Shark Biology

Senses

Jaws and Teeth

Reproduction

Interaction with Humans

Attack Location

Attack Prevention

Dangerous Species

The Most Dangerous

Threat from Humans

Profile of the Author

References and Recommended Reading


Introduction

Sharks have long evoked more fear than any other creature. Snakes, crocodiles, and tigers are highly effective predators and dangerous to humans, but they don’t seem to be as universally feared. Sharks move efficiently and swiftly through an environment where humans may struggle or feel awkward. The power of the ocean and the mystery of what cannot be seen beneath its' waves adds to the fear of sharks. In 1865, Thoreau noted, "I have no doubt that one shark in a dozen years is enough to keep up the reputation of a beach a hundred miles long." The reputations and mysteries surrounding sharks far exceeds our understanding of them.

Tales of interactions with sharks are often exaggerated or sensationalized. Some 'experts' present their involvement with sharks as heroic efforts or seek to primarily record images of sharks involved in baited, bloody feeding encounters. Public attention is usually drawn to a minority of large, active species. The common image of the large torpedo-shaped shark constantly feeding and moving is misleading. Sharks are extremely diverse in form, feeding habits, range of movement, and use of habitat. Some occupy the deep sea and polar regions and others move throughout entire ocean basins during the course of their lives. Of the more than 360 species of sharks currently recognized the majority of these grow to less than one meter (3.2 feet) at their maximum length.

Shark Biology

Sharks are often thought of as highly primitive creatures that have remained unchanged since their first appearance in the oceans. Although sharks have been around for approximately 400 million years, they have undergone many adaptations and specializations during this time. One of the oldest living families (Heterodontidae), the horn sharks, extends back to the Jurassic, some 160 million years ago. They are believed to have remained relatively unchanged since the time of dinosaurs and the first birds. However, many of the present day sharks, such as the mako or white, are believed to have developed only 50 million years ago. This sounds like a long time, but it means that they date back to the origin of some of the fish considered to be the most modern or derived.

The 360 or so species of sharks represent only a small fraction of the approximately 20,000 living fishes. Sharks and rays are different from most of their other fish relatives by having a skeleton that is entirely made up of cartilage (including their "backbone"). These skeletons are much less dense than bone and are believed to aid in buoyancy and swimming efficiency. Bony fishes have gills that are protected and covered while those of sharks are exposed. Sharks also possess many unique adaptations to their environments, some of which are discussed below:

Senses (coming soon!)

Jaws and Teeth (coming soon!)

Reproduction (coming soon!)

Interaction with Humans

Attitudes and impressions about sharks are slowly beginning to change. As more people move to coastal area, take part in water sports, or learn about marine environments, positive shark-human interactions have increased over the years. Diving and snorkeling trips provide the opportunity for close observation of sharks in their own habitat. Although we may be unaware, we consistently share the water with many types of sharks. However, each year only an approximately 50-75 shark attacks are reported around the world, resulting in 5-10 deaths. In areas of heavy ocean recreation, such as Florida, attacks are more common. Encounters range from a brush or bump to serious bites. The fear of these attacks is much greater than the actual risk. Injuries from other animals result in many more deaths each year. For example, over 50 fatal snakebites occur annually in Brazil alone. Beach injuries or fatalities from drowning, heart attack, and jelly fish or stingray wounds are far more common than bites or abrasions inflicted by sharks.

It should also be noted that some of the "attacks" reported each year are the result of provocation. If a diver is bitten after pulling on the tail of a shark or a fishermen is bitten by a shark that has been landed, these occurrences are included as attacks. Shark attack is a potential danger to anyone who spends time in the ocean or a few select rivers. The possibility of an attack is extremely low, however, and must be kept in perspective. An understanding of how to reduce the risks of an attack and of shark behavior may provide a greater appreciation of these animals and increased comfort when sharing their environment.

Attack Location

Shark attacks have been recorded around the world, both near shore and in the open ocean. In the United States, the majority of shark attacks are reported in Florida. The states of California and Hawaii, where ocean recreation is also extremely popular, have long histories of fatal and non-fatal encounters with sharks. Worldwide, shark attacks most commonly occur in Australia, New Zealand, Brazil, and South Africa. Many attacks in remote setting or from Third World countries are likely to go unreported.

Most attacks occur in waters near shore, typically inside of a sand bar or between sandbars where sharks may be trapped at low tide. Beaches with steep drop offs are also common attack sites. Fish tend to congregate in these areas, which attracts sharks. Seal and sea lion rookeries also attract sharks, such as the White, which feed upon the new-borns and individuals concentrated in one area. Shark attacks on humans appear to result from two basic circumstances: threat/aggression or feeding. The behavior of sharks is poorly understood, but it is believed that during periods of breeding or pupping, more aggressive actions may be displayed and attacks may be more prevalent. Sharks may also attack when a possible threat to their own lives is perceived. Although highly adapted, sharks may mistake humans for other sources of prey. Splashing bathers may create circumstances somewhat like splashing, schooling fish. When viewed from below, the image of a human on a surf or boogie board may match the silhouette of a seal, sea lion, or sea turtle. In water that is murky, the possibility of such a mistaken identity increases.

Attack Prevention

The risk of shark attack can be reduced in many ways:

  • Always stay in groups. Attacks more often occur on solitary swimmers
  • Avoid the water during darkness or twilight hours when sharks are most active and have a competitive sensory advantage
  • Do not wear shiny jewelry. It may reflect light in a manner similar to fish scales
  • Use extra caution when waters are murky or polluted
  • Avoid uneven tanning and bright colored clothing - sharks see contrast particularly well
  • Avoid swimming with an open wound or freshly speared fish
  • Avoid swimming far offshore, in channels, or over drop offs to deeper water
  • Avoid erratic movements and swim smoothly, especially if a shark has been sited

Dangerous Species

Perhaps 60 of the 360 or so species of sharks have been suspected of attacks on humans. Because these encounters are often brief, it is difficult to determine which type of shark has been involved in an attack. However, three widely distributed species have been identified as responsible for the majority of fatalities. The Oceanic whitetip, is suspected of many attacks offshore on victims of ship or plane wrecks and is included in this list.

The Most Dangerous

The sharks considered to be most dangerous are:

  • Bull shark (Carcharhinus leucas)
  • Tiger shark (Galeocerdo cuvieri)
  • White shark (Carcharodon carcharius)
  • Oceanic Whitetip (Carcharhinus longimanus)

Each of these sharks reach large sizes and consume large prey such as marine mammals and sea turtles. The Bull Sharks cruise the shallow coastal waters and therefore might come into contact with humans the most.

Threat from Humans

The news reports from around the world are common: fisheries closures, new regulations, fisheries collapses, or a fishing community struggles to find new sources of income. Since the 1940's, global marine catches have grown approximately fourfold. The use of marine resources has increased dramatically. As traditional fish stocks decline, new fisheries are established for species that were previously unexploited. As a result, catches of sharks and rays (elasmobranchs) have reached their highest level in history. The continuation of this global trend is doubtful, considering the unique biology of elasmobranchs. Most fish (such as tuna and anchovies) release thousands or millions of eggs. The spawning success of just a few females can result in the growth of the population. Many important commercial fishes mature quickly and are reproducing successfully after only a year or two. Sharks, however, exhibit life history characteristics that include slow growth, long life spans, late ages of maturity, low birth rates, and long gestation periods. Females have only a small number of offspring and may not reproduce for the first time until they are 5, 10, or 20 years old. Thus, populations can only slowly recover from high levels of loss due to fishing activities. Furthermore, as apex predators, the absolute numbers of sharks is low to begin with. Because sharks and rays tend to serve as apex predators their removal may have significant and unpredictable effects upon the intricate food webs of many marine communities. Over-exploitation of sharks and rays often leads to severe and rapid depletion of populations, endangering their resource and ecological value over broad regions.

Although rapidly growing, fisheries for sharks and rays have existed for centuries. Shark fin soup has been a Chinese delicacy and a dish of honor for more than 2000 years. Large scale fishing operations for sharks, however, have followed a pattern of boom and bust. For example, in the 1940's demand for shark liver oil (a great source of vitamin A) stimulated a rapid growth in fisheries. The soupfin shark, Galeorhinus galeus, was a common target for this market around the world. After a few short years of profitable fishing, the fishery collapsed in California because of the decline in the population of mature females. It has become evident that only a small proportion of shark and ray populations can be harvested in a sustainable way. For the soupfin shark (which can live for as long as 50 years), it has been suggested that 2-3% of the population is an acceptable level. This species remains one of the most favored in Asian markets.

Shark species preferred for human consumption vary by availability, country, region, and customs. Overall, the shortfin mako (Isurus oxyrinchus) is considered to be the highest quality and most valuable. Thresher (Alopias spp.) and porbeagle (Lamna nasus) sharks are also popular and of high value. Dogfish and smoothhound sharks are found worldwide and are commonly consumed. France is among the world's largest consumers of dogfish, where marketed as saumonette, it is sold in fish and super markets throughout the country. Shark fins tend to be most prized from hammerhead, mako, blue, dusky, and blacktip sharks. In Hong Kong, retail shark fin prices range from US $40.00 - US $564 per kg. Shark skin is frequently utilized in leather production, while jaws, teeth, internal organs, and cartilage are commonly marketed as well.

Very limited information is available on the shark fisheries for most countries. The major shark fishing nations of Japan, Taiwan, and Mexico do not keep species specific records of their catches. They instead lump them into groups such as large sharks or rays. The growing Chinese fishing fleet simply lands sharks, but does not report the weight, quantity, or species composition of catches. The few countries which have managed and monitored their shark fisheries are beginning to learn the extent to which regulation is necessary. Upon determining that many shark populations along the Atlantic seaboard of the United States had declined by more than 75% since the 1970's, a coast-wide management plan was implemented in 1993. Despite this plan, the quotas that had been established were cut in half in 1997 and further limited in 1998. It has become evident that careful, conservative management and additional research into the life history of elasmobranchs is critical to ensure the survival of shark populations as well as a sustainable fishery.

Last summer, researchers announced that a close shark relative, the barndoor skate (Raja laevis), could only be found at the southern edge of its range in the nortwest Atlantic and suggested that it was near extinction. This skate was not the target of any fishery, but was taken as bycatch throughout the region. Shark populations are reduced through the directed fishing efforts described above and indirectly in other fisheries such as tuna and billfish. Recreational fisheries for sharks have also grown in recent years. In many Florida ports, recreational fishermen have witnessed a dramatic drop in both shark abundance and size. The success of sharks is further complicated by increased coastal development which reduces the availability of necessary nursery grounds that are crucial for survival of juveniles.

Many shark species are highly migratory and cross political boundaries on a regular basis. This complicates scientific research and effective management policies. Of the 125 countries that fish or trade in shark products, only four (Australia, Canada, New Zealand, and the United States) have designed management plans for the fisheries. Awareness of the unique problems associated with shark and ray fisheries is increasing, and many governments, research agencies, and environmental organizations are beginning to cooperate to obtain biological information and enhance communication that is necessary to implement effective, sustainable management for elasmobranchs.

To learn more about shark and ray biology and conservation, please write to Shark News, to receive a newsletter of the IUCN Shark Specialist Group. These publications are extremely current, informative, and are available to any interested party for a donation. For further information, write to:

Sonja Fordham
Center for Marine Conservation
1725 DeSales Street NW
Washington, DC 20036, USA

Shark Diversity

  • Whale Shark
  • Megamouth
  • Spiny Dogfish
  • Pacific Angelshark
  • Cookiecutter Shark
  • Scalloped Hammerhead
  • Swell Shark

Profile of the Author

Wade D. Smith is a graduate student at Moss Landing Marine Laboratories in Moss Landing, California. Moss Landing Marine Laboratories is dedicated to excellence in marine and estuarine science and education. Wade is currently a member of an international, multi-institutional team investigating the status of the shark and ray fishery in the Gulf of California, Mexico. He is also conducting research on the age, growth, and reproduction of the diamond stingray, Dasyatis brevis. The diamond stingray is a large, heavily fished ray that is found from southern California to Peru, including the Galapagos and Hawaiian Islands. Photo taken by JP in Baja, Mexico

References and Recommended Reading

Bonfil, R. 1994. Overview of World Elasmobranch Fisheries. FAO Fisheries Techinical Paper 341, 119 pp.

Castro, J.I. 1983. The Sharks of North American Waters. College Station: Texas A&M University Press. 180 pp.

Clark, E. and D.R. Nelson. 1997. Young whale sharks, Rhincodon typus, feeding on a copepod bloom near La Paz, Mexico. Env. Bio. Fish. 50:63-73.

Colman, J.G. 1997. A review of the biology and ecology of the whale shark. J. Fish. Bio. 51: 1219-1234.

Compagno, L.J.V. 1984. Sharks of the World. FAO Fisheries Synopsis 125, vol. 4, parts 1 and 2. Rome: United Nations Food and Agriculture Organization. 655 pp.

Gilbert, P.W., R.F. Mathweson, and D.P. Rall (eds.). 1967. Sharks, Skates, and Rays. Johns Hopkins University Press. Baltimore, MD. 624 pp.

Gruber, S.H. (ed.). 1991. Discovering Sharks. American Littoral Society. Highlands, NJ. 122 pp.

Holden, M.J. 1974. Problems in the rational exploitation of elasmobranch populations and some suggested solutions. In: Sea Fisheries Research. F.R.H. Jones, ed. Wiley and Sons, New York. 117-137 pp.

Joung, S.-J., C.-T. Chen, E. Clark, S. Uchida, and W.Y.P. Huang. 1996. The shale shark, Rhincodon typus, is a live bearer: 300 embryos found in one 'megamamma' supreme. Env. Bio. Fish. 46: 219-223.

Klimley, A.P. and D.G. Ainley (eds.). 1996. Great White Sharks: The Biology of Carcharodon carcharias. Academic Press. San Diego, CA. 517 pp.

Lowe, C.G., B.M. Wetherbee, G.L. Crow, and A.L. Tester. 1996. Ontogenetic dietary shifts and feeding behavior of the tiger shark, Galeocerdo cuvier, in Hawaiian waters. Env. Bio. Fish. 47: 203-211.

Pratt, H.L., S.H. Gruber, and T. Taniuchi (eds.). 1990. Elasmobranchs as living resources: Advances in biology, ecology, systematics, and the status of the fisheries. NOAA Tech Rep. NMFS 90.

Snelson, F.F. Jr, T.J. Mulligan, and S.E. Williams. 1984. Food habits, occurrence and population structure of the bull shark, Carcharhinus leucas, in Florida coastal lagoons. Bull. Mar. Sci. 34(1): 71-80.

Springer, V.G. and J.P. Gold. 1989. Sharks in Question - The Smithsonian Answer Book. Smithsonian Institution Press. 187 pp.

Taylor, L. 1993. Sharks of Hawai'i: Their biology and cultural significance. University of Hawaii Press. Honolulu, HI. 125 pp.

Taylor, L. (ed.). 1997. The Nature Company Guides - Sharks & Rays. Time Life Books. Sydney, Australia. 288 pp.

 

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