Sharks seem to constantly be in the news. Shark attacks, shark encounters, even just shark sightings often end up on the front page of the newspaper and the lead story on the TV evening news. In some places, shark attacks have even led to calls for the controversial practice of shark culling (organized shark hunts) to reduce populations. There is little evidence that shark culling is effective in reducing shark attacks, and a majority of both ocean conservationists and the general public opposes this practice. In early 2017 champion surfer Kelly Slater and other pro surfers generated a swarm of controversy when they called for "daily culling" of bull sharks in the waters around Reunion Island in the Indian Ocean to address the problem of fatal shark attacks there. In response to the call for shark culling at Reunion Island, Dr. Ken Collins, senior research fellow in ocean and earth science at the University of Southampton has suggested the consideration of alternative measures, including underwater pylons that create a shark-repelling magnetic field. Other shark management techniques have been implemented successfully in Brazil and elsewhere. In 2015 Australia held an international "shark summit" where more than 70 shark experts and scientists gathered at Taronga Zoo in Sydney to discuss a range of shark deterrent and detection methods. The technologies discussed ranged from dead shark scent in a can and electrical and magnetic barriers to camouflage wetsuits.
But humans are taking a substantially greater toll on sharks than sharks have ever taken on humans. In fact, sharks are in trouble – victims of irresponsible and unsustainable fishing practices. The introduction of modern fishing techniques (and the practice of shark finning) has spelled disaster for these ancient creatures. Overfishing has caused a massive decline in shark numbers – a huge jolt to ocean ecosystems in little over 50 years. It is estimated that 100 million sharks are killed by people every year. The average number of fatalities worldwide per year between 2001 and 2006 from unprovoked shark attacks was 4.3.
And the decline of sharks spells trouble for the ocean. Removing these key predators from the food chain has serious consequences for marine ecosystems, which in turn has repercussions for people everywhere.
Modern forms of sharks and rays first emerged over 150 million years ago. The earliest known sharks date back to more than 420 million years ago, predating the dinosaurs by 200 million years. There are approximately 400 known species of sharks living in our oceans today. Well-known species such as the great white shark, tiger shark, blue shark, mako shark, and the hammerhead shark are apex predators—organisms at the top of their underwater food chain, and they regulate the populations of species below them. Research has shown that massive depletion of sharks has cascading effects throughout the ocean’s ecosystems.
Sharks belong to a family of fish that have skeletons made of cartilage, a tissue more flexible and lighter than bone. They breathe through a series of five to seven gill slits located on either side of their bodies. All sharks have multiple rows of teeth, and while they lose teeth on a regular basis, new teeth continue to grow in and replace those they lose.
Shark ‘skin’ is made up of a series of scales that act as an outer skeleton for easy movement and for saving energy in the water. The upper side of a shark is generally dark to blend in with the water from above and their undersides are white or lighter colored to blend in with the lighter surface of the sea from below. This helps to camouflage them from predators and prey.
Sharks range in size from the small dwarf lanternshark, a deep sea species of only about 6 inches in length, to the whale shark, the largest fish in the world, which can reach 50 to 60 feet in length. Sharks are found in all seas and are common to depths of 6,600 feet. They generally do not live in freshwater although there are a few known exceptions, such as the bull shark and the river shark, which can survive and be found in both seawater and freshwater.
Present-day sharks fall into just 8 major groups, called orders. These 8 orders contain about 400 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 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.
Most species of shark eat things like fish, crustaceans, mollusks, plankton, krill, marine mammals and other sharks. Sharks also have a very acute sense of smell that allows them to detect blood in the water from miles away.
Most sharks are especially active in the evening and night when they hunt. Some sharks migrate over great distances to feed and breed. This can take them over entire ocean basins. While some shark species are solitary, others display social behavior at various levels. Hammerhead sharks, for instance, school during mating season around seamounts and islands.
Some shark species, like the great white shark, attack and surprise their prey, usually seals and sea lions, from below. Species that dwell on the ocean floor have developed the ability to bottom-feed. Others attack schooling fish in a feeding frenzy, while large sharks like the whale and basking sharks filter feed by swimming through the ocean with their mouths open wide, filtering large quantities of plankton and krill.
Threats to Sharks
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 above a certain size 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.
Sharks are often killed just for their fins. The fins are then used to make shark fin soup. Shark fin soup is a status symbol in Asian countries, and is considered healthy and full of nutrients. Fishermen capture live sharks, fin them, and dump the finless animal back into the water. The resulting immobile shark soon dies from suffocation or predators. Shark fin has become a major trade within black markets all over the world. Shark finning yields are estimated at 1.44 million metric tons for 2000, and 1.41 million tons for 2010.
Shark species that are commonly finned are:
- Blacktip (Carcharhinus limbatus)
- Blue (Prionace glauca) (a species of requiem shark)
- Bull (Carcharhinus leucas)
- Hammerhead (family Sphyrnidae)
- Porbeagle (Lamna nasus) (a species of mackerel shark)
- Mako (Isurus oxyrinchus)
- Sandbar (Carcharhinus plumbeus) (a species of requiem shark)
- Thresher (family Alopiidae)
- Tiger (Galeocerdo cuvier) (a species of requiem shark)
- Great white shark (Carcharodon carcharias)
The majority of shark fisheries have little monitoring or management. The rise in demand for shark products increases pressure on fisheries. Major declines in shark stocks have been recorded—some species have been depleted by over 90% over the past 20–30 years with population declines of 70% not unusual. A study by the International Union for Conservation of Nature suggests that one quarter of all known species of sharks and rays are threatened by extinction and 25 species were classified as critically endangered.
Why are Sharks So Susceptible to Overfishing?
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.
Besides overfishing, other threats to sharks include habitat alteration, damage and loss from coastal development, pollution and the impact of fisheries on the seabed and prey species. The 2007 documentary, Sharkwater exposed how sharks are being hunted to extinction. Another film in production is Saving Jaws, which will focus on the practice of shark finning.
Although few governments enforce laws that protect sharks, there have been some noteworthy efforts.
In 1991, South Africa was the first country in the world to declare Great White sharks a legally protected species.
In 1994, California became the second jurisdiction in the world (after South Africa) to protect the white shark. California State Assembly Bill 522 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.
Intending to ban the practice of shark finning while at sea, the United States Congress passed the Shark Finning Prohibition Act in 2000. Two years later the Act saw its first legal challenge in United States v. Approximately 64,695 Pounds of Shark Fins. In 2008 a Federal Appeals Court ruled that a loophole in the law allowed non-fishing vessels to purchase shark fins from fishing vessels while on the high seas. Seeking to close the loophole, the Shark Conservation Act was passed by Congress in December 2010, and it was signed into law in January 2011.
In 2003, the European Union introduced a general shark finning ban for all vessels of all nationalities in Union waters and for all vessels flying a flag of one of its member states. This prohibition was amended in June 2013 to close remaining loopholes.
In 2009, the International Union for Conservation of Nature IUCN Red List of Endangered Species named 64 species, one-third of all oceanic shark species, as being at risk of extinction due to fishing and shark finning.
In 2010, the Convention on International Trade in Endangered Species (CITES) rejected proposals from the United States and Palau that would have required countries to strictly regulate trade in several species of scalloped hammerhead, oceanic whitetip and spiny dogfish sharks. The majority, but not the required two-thirds of voting delegates, approved the proposal. China, by far the world's largest shark market, and Japan, which battles all attempts to extend the convention to marine species, led the opposition. In March 2013, three endangered commercially valuable sharks, the hammerheads, the oceanic whitetip and porbeagle were added to Appendix 2 of CITES, bringing shark fishing and commerce of these species under licensing and regulation.
In 2010, Greenpeace International added the school shark, shortfin mako shark, mackerel shark, tiger shark and spiny dogfish to its seafood red list, a list of common supermarket fish that are often sourced from unsustainable fisheries. The advocacy group Shark Trust campaigns to limit shark fishing. Monterey Bay Aquarium's Seafood Watch directs American consumers to not eat most species of sharks.
Under the auspices of the Convention on the Conservation of Migratory Species of Wild Animals (CMS), also known as the Bonn Convention, the Memorandum of Understanding on the Conservation of Migratory Sharks was concluded and came into effect in March 2010. It was the first global instrument concluded under CMS and aims at facilitating international coordination for the protection, conservation and management of migratory sharks, through multilateral, intergovernmental discussion and scientific research.
In 2010 Hawaii became the first U.S. state to prohibit the possession, sale, trade or distribution of shark fins.
In July 2013, New York state, a major market and entry point for shark fins, banned the shark fin trade joining seven other states of the United States and the three Pacific U.S territories in providing legal protection to sharks.
The species of shark that gets by far the most media attention is the Great White Shark. Great Whites are the largest predatory fish in the world – capable of eating marine mammals that weight several hundred pounds. The only two fishes that grow larger than Great Whites are the Whale Shark and the Basking Shark, both filter feeders that eat plankton. The Great White, on the other hand, is known to be an aggressive predator and has an extremely muscular body, capable of chasing down some of the fastest swimmers in the ocean. Reaching lengths of up to 20 feet and weights of several tons, the Great White’s body is perfectly adapted to a life of predation.
Great White Sharks are known to be highly migratory, with individuals making long migrations every year. In the eastern Pacific Ocean, Great Whites regularly migrate between Mexico and Hawaii. In other ocean basins, individuals may migrate even longer distances. Like in many highly migratory species, the very largest individuals are female. Great Whites mate via internal fertilization and give live birth to a small number of large young (over three feet/one meter). After they are born, young Great Whites are already natural predators, and they eat coastal fishes. As they grow, their preferred prey also gets larger, and the largest, mature individuals prefer to eat marine mammals, like seals and sea lions. Great Whites are known to take very deep dives, probably to feed on slow-moving fishes and squids in the cold waters of the deep sea.
While Great Whites are one of the few species known to have bitten and killed people, these events are extremely rare. Typically, when a Great White does bite a person, it only takes one exploratory bite and quickly realizes that the person is not its preferred prey. Unfortunately due to their very large size, even an exploratory bite can be fatal or extremely traumatic. People, on the other hand, capture too many Great Whites, through targeted fisheries or accidental catch in other fisheries, and scientists generally consider Great Whites to be vulnerable to extinction.
Also see Sharks - White Sharks in California
One of the most interesting and unusual species of shark is the whale shark, named for its enormous size and its behavioral similarity to some whales. Reaching a length of up to 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.
Whale sharks (and peaceful human interaction with whale sharks) were featured in MacGillivray Freeman Films’ recent movie Journey to the South Pacific.
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. Similarly, whale sharks are threatened in the southwest Pacific.
In 1998, the Philippine government banned the killing and sale of whale sharks.
The International Shark Attack File (ISAF) investigates and keeps statistics on shark attacks worldwide. On average there were eight fatalities per year worldwide in the 2011-2015 period and six deaths per annum over the preceding decade. The International Shark Attack File investigated 150 incidents of alleged shark-human interaction occurring worldwide in 2016. Upon review, 81 of these incidents represented confirmed cases of unprovoked shark attacks on humans, with four fatalities.
"Unprovoked attacks" are defined as incidents where an attack on a live human by a shark occurs in its natural habitat without human provocation of the shark. Incidents involving sharks and divers in public aquaria or research holding-pens, shark-inflicted scavenge damage to already dead humans (most often drowning victims), attacks on boats, and other incidents involving provocation by humans occurring in or out of the water are not considered unprovoked attacks. "Provoked attacks" usually occur when a human initiates physical contact with a shark, e.g. a diver bitten after grabbing a shark, attacks on spearfishers and those feeding sharks, bites occurring while unhooking or removing a shark from a fishing net, etc.
Contrary to popular belief, only a few sharks are dangerous to humans. Out of more than 470 species, only four have been involved in a significant number of fatal, unprovoked attacks on humans: the great white, oceanic whitetip, tiger, and bull sharks. These sharks are large, powerful predators, and may sometimes attack and kill people. On the other hand, several coastal locations promote swimming with sharks as a safe, fun, educational activity.
The perception of sharks as dangerous animals has been popularized by publicity given to a few isolated unprovoked attacks, such as the Jersey Shore shark attacks of 1916, and through popular fictional works about shark attacks, such as the Jaws film series. Jaws author Peter Benchley, as well as Jaws director Steven Spielberg later attempted to dispel the image of sharks as man-eating monsters.
A popular myth is that sharks are immune to disease and cancer, but this is not scientifically supported. Sharks have been known to get cancer. Both diseases and parasites affect sharks. The evidence that sharks are at least resistant to cancer and disease is mostly anecdotal and there have been few, if any, scientific or statistical studies that show sharks to have heightened immunity to disease. Other apparently false claims are that fins prevent cancer and treat osteoarthritis. No scientific proof supports these claims and at least one study has shown shark cartilage of no value in cancer treatment.
References and Additional Information
A Closer Look at Shark Conservation (NOAA)
Defenders of Wildlife - Sharks
Aquarium of the Pacific - Sharks