Publié le August 31, 2020

Sharks around the world

Like all predators, sharks form an essential link in biodiversity

 

Masters of the open sea, about 500 different species of sharks populate all the seas across the world. From a very young age, you were taught to be afraid of this supposed “man-eater”, but this is a long way from reality!

This predator plays a fundamental role in the balance of ocean life.

As adults, sharks have virtually no predators in the ocean. Today, the greatest threat comes from humans and fishing. It is more urgent than ever to protect them because their disappearance would create an indisputable ecological disaster and an irreversible imbalance within the oceans!

What is a shark?

Sharks are very unique animals!

Their ancestors appeared in the ocean a very long time ago: this lord of the seas already existed 430 million years ago. Their body is a model of adaptation to their environment and their function as predators.

Sharks are cartilaginous fish like rays and chimaeras. They can be distinguished from bony fish by their internal skeleton made up of cartilage and by their gill slits. They have between 5 and 7 gill slits on each side of the head depending on the species, as opposed to only 1 for bony fish.

Some species of sharks have a smooth torpedo-shaped body that makes them very efficient swimmers. This hydrodynamic profile combined with their powerful muscles enables them to move very fast: some sharks, such as the mako shark (Isurus oxyrinchus), can reach top speeds of around 80 kph.

Like humans, sharks have five senses: hearing, smell, taste, sight and touch. Their sensory organs are very efficient. In fact, most sharks can detect the smell of blood several kilometres away and perceive sounds up to two kilometres away.

In addition to these 5 perfected senses, sharks have a 6th sense: electroreception. Using sensitive organs called Ampullae of Lorenzini, they perceive the electromagnetic radiation emitted by all living beings, enabling to detect their prey more efficiently. They also have an organ that can detect pressure waves, which is called the lateral line. They can use this organ to detect tiny movements in the water, such as movements caused by a passing fish.

Their teeth continuously regenerate when they are damaged and they have several hundred of them! Some sharks lose more than 30,000 teeth throughout their lifetime.

They have a strong and very mobile jaw that enables them to open their mouth wide to capture even very large prey! The pressure exerted by the jaws of some sharks is impressive: it can reach up to 60 kg per tooth in the white shark (Carcharodon carcharias).

Their fins act as a rudder and stabilizer. The caudal fin is used for propulsion and enables them to abruptly change direction.

Finally, most sharks never stop swimming for 2 reasons: they need to keep water flowing through their gills to breathe and also because they do not have an air bladder. This air bladder is what other fish use to float.

The nurse shark (Ginglymostoma cirratum) and the western wobbegong or carpet shark (Orectolobus hutchinsi) are exceptions: they spend most of their time on the seabed. Carpet sharks are poor swimmers, so they have adapted their behaviour and use camouflage techniques to surprise their prey.

Shark diversity throughout the world

There are about 500 species, living in all the seas around the world, apart from the Antarctic Ocean. They can be encountered from the surface to the deep ocean floors. Some species are said to be benthic (living near the seabed), whereas others are said to be pelagic (living in open water). They live in the open sea or near the coast, for example, in lagoons.

Sharks come in different shapes and sizes: the hammerhead shark can be recognized by its T-shaped head; the carpet shark has a very flattened spotted body; the tiger shark has a massive body and owes its name to the vertical stripes on its back; and the epaulette or walking shark has a long, slender body and fins that it uses to « walk” on the seabed.

 

Most sharks are about two metres long. The smallest shark in the world is the dwarf lantern shark (Etmopterus perryi), which does not exceed 20 centimetres in length; the largest is the whale shark (Rhincodon typus), which can grow up to 18 metres long.

Most sharks are predators. They are carnivorous and feed on fish, shellfish and marine mammals. Only 5% of sharks feed on plankton. This is the case for the 2 largest sharks in the world, the whale shark and the basking shark (Cetorhinus maximus).

When it comes to breeding, the species can take time to reproduce, which exacerbates the overfishing phenomenon. Sharks reach sexual maturity late in their life and have a small number of individuals per litter. Gestation is rather long, from 7 months to 3 years (for the frilled shark – Chlamydoselachus anguineus).

 

Depending on the shark species, embryo development can be oviparous, viviparous or ovoviviparous.

  • Oviparous sharks have what is known as external fertilization. The female lays eggs that are fertilized in the water by the male gametes. Embryos grow outside the female’s body.
  • With viviparous sharks, the embryos develop inside the female’s uterus. Nutrients are passed through an umbilical cord. After a gestation period, the mother gives birth to offspring that are already formed
  • Oviviparous sharks carry eggs that incubate and hatch inside the mother’s womb. After an incubation period, the eggs hatch and the female gives birth to young that are already independent.

Sharks live in relationships with other animals. We employ the term commensalism for the pilot fish, which takes advantage of the shark’s food debris but without providing much in return. We employ the term mutualism for the remora, which also takes advantage of the shark’s food debris, but in exchange, gets rid of a good part of its parasites. Finally, we use the term parasitism for the tapeworms and copepods that live on its skin.

 

The shark is an important and essential predator

As a predator at the top of the food chains, the shark is a fundamental animal in keeping the oceans in balance.

The shark is not harmful. On the contrary, it is valuable for the balance of the ecosystems.

In the ocean food chain, animals eat each other. The shark is located at the top of this chain: it has a central role in maintaining prey populations. It also contributes to keeping the oceans in good health: the shark feeds mainly on old, slow and diseased fish that are unable to reproduce.

If sharks are overfished, it creates an imbalance in the ecosystem, which results in cascading effects. For example, if there are no sharks, then the skates and rays located in the next level down will proliferate. This proliferation will cause the stocks of certain species to decrease, such as the scallops on which skates and rays feed.

The shark is a protector of the barrier reef.

When the shark population declines, the structure of the food chain on the reefs changes.

Barrière de corail, vue aérienne

There is an increase in the number of intermediate predators that feed on species essential for coral preservation. This is the situation with herbivorous fish. These fish feed on algae that appear on the coral after the passage of cyclones, for example, and are eaten by larger fish that are in turn, prey for sharks. If there are fewer sharks, there will be more big fish and therefore, fewer herbivorous fish… As their populations decrease, the coral reefs cannot eliminate these invasive algae and hence, cannot recover.

Due to global warming, there are more cyclones and bleaching phenomena that threaten coral reefs and lead to frequent proliferation of parasitic algae. The presence of sharks is one of the best ways to maintain the right balance of the reefs.

The Great Barrier Reef is the largest coral reef in the world. It is home to nearly 400 species of coral, more than 1,500 species of fish and 4,000 species of molluscs. If it were to disappear, it would be an ecological disaster on a global scale.

Sharks are an endangered species

Some species are unfortunately renowned for their spectacular attacks on humans, and the people in the affected areas are looking for ways to protect themselves from these predators. The stakes are also economic because these attacks have an impact on tourism along the beaches. However, eradicating sharks would be dangerous for the ecological balance of the marine environment. Today, it is the shark that is under threat, as overfishing has had a major impact on many shark populations.

It is estimated that one shark is killed every 3 seconds, i.e. about 100 million sharks every year!

Exploitation of sharks

Sharks are used for everything. Their flesh is eaten, their fins enter into the composition of the famous fin soup, and their skin provides luxury leather. Oil from their liver is used in the cosmetics industry and to make vitamins. Nowadays, we know that shark products have no magical effects and that plant substitutes exist, but we continue to hunt them down.

Approximately 100 million sharks are killed every year and a third of the species of sharks and rays in the high seas are already threatened with extinction.

Three-quarters of these sharks end up in fin soup in Asia. Fishermen catch the sharks, cut off their fins and then throw them back into the sea with their bodies mutilated, but usually still alive…

Why does this practice exist?

Shark fin soup remains a traditional dish in Asia. It should be pointed out that a kilo of fins can sell for between 300 and 500 euros. There is strong demand linked to the booming Asian economy. This type of fishing offers large profits very quickly. It is seen as a very profitable practice.

Effects of global warming

Rising temperatures and acidification of the oceans are affecting shark growth and behaviour. In warmer waters, sharks need more energy to acclimatize, which makes them hungrier! But the high concentration of CO2 limits their ability to hunt because the acidity of the water affects their senses. They can no longer perform their role of super predators.

Sensitive to pollution

Sharks have high levels of toxic elements such as heavy metals in their bodies. In fact, these substances are becoming increasingly concentrated throughout the food chain, and sharks are located at the very top of it.

One of the most commonly found products is a derivative of mercury, a neurotoxin that is very dangerous to humans. Bacteria are also found in high concentrations, especially in the fins. The larger and older the shark, the higher the concentrations of heavy metals and other pollutants in its body.

Sharks and humans

The relationship between humans and sharks has always been complex and oscillates between terror and fascination.

The shark suffers from a reputation of being a bloodthirsty monster and man-eater. This image haunts it through myths, rituals and thriller movies. Memories of “Jaws” and its scary music are never far away…

Are sharks aggressive towards humans?

There is no doubt that sharks are predators, but they do not prey on everything that moves! Every attack is followed by strong media coverage linked to the spectacular size of the wounds that are inflicted. However, there are less than ten fatalities per year, out of a hundred or so accidents.

Sharks cause fewer deaths than bees, mosquitoes, jellyfish… or even hippos.

Yet none of these species is more to blame than another. Furthermore, out of 500 species of shark, only 5 species are the most often involved in these incidents: the tiger shark (Galeocerdo cuvier), the white shark, the bull shark (Carcharhinus leucas), the mako shark and the oceanic whitetip shark (Carcharhinus longimanus).

 

Why do these attacks occur?

Even though they are impressive, sharks are relatively fearful and wary animals. Shark attacks on humans are rare and exceptional.

The increase in shark attacks in recent years is believed to be linked to increased human activity along the coastlines and more and more people visiting beaches. There are fewer and fewer sharks, but there are more and more people on the beaches.

We also note that our relationship with the sea has changed and that we are venturing further and further into “shark territory”. The development of tourism and water sports attracts more and more people into the water, swimmers also stay longer in the water, so encounters with sharks are increasingly common.

 

Surfers are by far the primary victims of shark attacks, followed by swimmers and divers.

Attacks may also be explained by changes in the sharks’ natural habitat.

The development of tourist areas, the destruction of coral reefs that act as natural nets, pollution and less abundant food are forcing sharks to widen their hunting radius. The natural route of sharks remains unchanged, but unlike at the beginning of the 20th century, beaches are now very busy.

 

 

What is the right behaviour to adopt in order to cohabit with sharks?

Avoid swimming at dawn, dusk, after heavy rainfalls and at night. These times correspond to times when sharks hunt, when the water is cloudy. And of course, avoid venturing into any areas where the authorities have issued a warning against bathing.

Today, several techniques exist to provide protection against shark attacks, such as gillnets, protective enclosures, air barriers or sonar buoys to detect tagged sharks.

Scientists attach transponders, markings, or tags to the dorsal fin. Depending on the technique, they can follow tagged sharks in real time on their computers or wait for information from fishermen who may catch tagged sharks.

Associations and foundations

Nausicaá is involved in a large number of research programmes, particularly for the preservation of sharks around the world.

NAUSICAÁ is involved with the MEGAPTERA association in order to gain a better understanding of whale shark populations and preserving them. This French association is dedicated to the observation, understanding and protection of marine mammals and whale sharks. The study and protection of this species involves photo-identification, marking and tagging these giants of the sea. NAUSICAÁ and MEGAPTERA have placed three tags on whale sharks during a marking campaign in December 2017 in Djibouti, Africa. These tags are used to obtain all kinds of information on this emblematic animal: lifestyle, migration, reproduction, etc.

Read about Whale shark mission 2020

NAUSICAÁ also sponsors the Malpelo Foundation that was created in 1999 by Sandra Bessudo, French-Colombian biologist and diver. Malpelo Island, located in the Pacific Ocean off the coast of Colombia, is a unique location with outstanding biodiversity. The Foundation is conducting a shark tagging campaign, through which scientists have discovered that hammerhead shark populations are declining due to fishing. Malpelo Foundation combats illegal shark fishing and finning and is dedicated to preserving this marine sanctuary, which is an exceptional refuge for species that cannot be found elsewhere. Malpelo, which is listed as “Natural Heritage of Mankind” by UNESCO, is now a Marine Protected Area and the Malpelo Foundation contributes greatly to its conservation and to monitoring the species that live there.

Did you know?

A shark’s mood can be determined by the size of its pupils. When a shark is excited, its pupils enlarge and when it is angry, they get smaller.

Every summer, the great white sharks that live off the coast of California can all be found in the same area of the open sea nicknamed the “white shark cafe”. It is located between Mexico and Hawaii.

If you stroke a shark from tail to head, it feels like you are running your hand over a cheese grater! Its skin is covered with denticles that maximize its swimming efficiency.

The teeth, gills and body of sharks are regularly cleaned by tiny fishes that penetrate fearlessly into their mouths to remove dead or diseased skin. This process seems essential to their survival.

In Australia, sharks can be followed on Twitter: 330 sharks have been tagged to monitor their movements near public beaches and to alert surfers and swimmers on Twitter.

The great white shark only eats once every three days.

 

 

Bibliography

Books

  1. VAN GREVELYNGHE, A. DIRINGER, B. SERET. Tous les Requins du monde. 300 espèces des mers du globe. Ed. Delachaux & Niestlé, 1999. Coll. Encyclopédies du Naturaliste
  2. et S. COUPE. La Petite encyclopédie des requins. Ed. Bordas, 1993. Coll. Petite Encyclopédie des Animaux

J.D. STEVENS. Les Requins. Ed. Bordas, 1987. Coll. Encyclopédie Visuelle

  1. MACQUITTY. La peur des requins. Ed. Gallimard, 2002. Coll. Les Yeux de la Découverte

MOUTOU F. et MABIRE G. Tout sur le requin… et le reste. Ed. Le Pommier, 2016

SERET B. et SOLE J. Les requins. Les connaître pour les comprendre. Ed. Le Lombard, 2016

MADDALENA A. de. Requins de Méditerranée. Ed. Turtle Prod., 2015

CIVARD-RACINAIS A. Requins. Un monde fascinant. Ed. Belin Jeunesse, 2014

CALCAGNO R. Requins au-delà du malentendu. Ed. du Rocher, 2013

SERET B. Guide des requins, des raies et des chimères des pêches françaises. Ed. IRD, 2010

Other books on sharks 

RITTER E.K. et RODELET J.M. L’interaction homme-requin. Ed. Jean-Marc Rodelet/Sharkschool, 2016

GHISLAIN J.-M. L’invitation. Ed. Les Arènes, 2014

BUYLE F. et CIVARD-RACINAIS A. Rencontres sous-marines. Ed. Glénat, 2014

SURINA S. Comprendre et plonger avec les requins. Ed. Turtle Prod., 2015

GUILBERT N. Opération requin. Enquête à l’aquarium de Boulogne-sur-Mer. Ed. Ravet-Anceau, 2017

Useful links

Knowing sharks

World Shark Attack Database, Florida Museum of Natural History

International Union for Conservation of Nature Shark Specialist Group

South African White Shark Research Institute

Ministry of Agriculture. Guide d’identification des raies et des requins

A la recherche des chiffres, par Shelley Clarke, specialist in the shark fin trade

The European Association of Elasmobranchs L’Association Européenne des Elasmobranches – 

Some reference studies available online

Interviews

Bernard Seret, French shark specialist on France Inter

TED Conferences

Simon Berrow: How do you save a shark you know nothing about?

Shark risk and its management

Shark risk management by Western Australian authorities 

Shark management in South Africa, le KwaZulu-Natal Sharks Board

Save our Seas Foundation
Documentary by Rémy Tezier « Attaques à la Réunion: l’Enquête »

Existing protection measures

Convention on International Trade in Endangered Species

European commission

Marine Protected Areas Agency

Shark DNA identification, the future of shark trade control?

Some actors in shark protection

Shark Alliance , www.eusharkjourney.org

Shark Angels

Shark Savers

Pew Charitable Trusts, Global sharks conservation

Wildaid

Longitude 181

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