Whale shark
Whale sharks (Rhincodon typus), known as gentle giants, are the largest fish in the ocean, reaching an average of 12m in length, approximately the size of a bus. They are the only species in the genus Rhincodon, but are classified under the order Orectolobiformes, which is a group that contains carpet sharks including Wobbegong sharks, Nurse shark and Zebra shark. Although they are called Whale sharks, this simply refers to their size, as they are not in any way related to whales.
Whale shark anatomy and appearance
The average size for a Whale shark is 10-12m, the largest size is not known, but it could be up to a massive 18m and weigh up to 42 tonnes! As they are filter feeders, they have very large mouths, that can be as wide as 1.5m across. In comparison to the size of their mouths, they have very small teeth but make up for that in quantity. An adult can have up to 300 rows of teeth, with 3,000 teeth in its whole mouth. Whale shark teeth are thought to serve little purpose. This shark has a broad, flattened head and have short barbels (slender, whisker like sensory organs) coming from its nostrils. There are five large gill slits on each side of the head, with special spongy tissue inside the gill slits that are used in filter feeding.
Its back and sides are grey to brown in color with distinctive white spots among pale vertical and horizontal stripes, and its underside is white. Its two dorsal fins are set rearward on its body, which ends in a large dual-lobed caudal fin (tail). Lifespans of Whale sharks average around 70 years, but they can live to over 100 years. Little is known about their life history, compared to other large sharks.
Whale shark habitat
Whale sharks are found in temperate and tropical waters along the equator line, as they prefer warmer waters. These sharks have no real barriers to movement and migrate through all the temperate and tropical waters of the world, with the exception of the Mediterranean Sea. They usually stay in deep waters, up to 1,928m deep, but sometimes swim closer to coastal areas, especially around coral reefs.
Results from tagging studies over the last 15 years show that Whale sharks are very mobile but swim very slowly. They tend to stay within 125 miles of the shore and make regular deep dives, although they do come to the surface sometimes to feed. One tagged shark swam 13,000km over 3 years. It is still a mystery as to where pupping occurs and where the young are raised. There is still plenty to learn about these magnificent creatures.
Whale shark diet
The Whale shark is one of three large filter-feeding sharks; the others are the megamouth shark (Megachasma pelagios) and the Basking shark (Cetorhinus maximus). Whale sharks swim with their mouth open, allowing seawater to enter the mouth and filter through the gill slits. The spongy tissue in the gill slits acts like a sieve, catching plankton and other small organisms while letting the water pass through and return to the sea. The shark then periodically closes its mouth to swallow any trapped prey. Prey typically includes zooplankton (small animals such as copepods, shrimp, and other invertebrates) and phytoplankton (algae and other marine plants). They can also eat small fish (sardines, mackerels, squid, small tuna, and albacore) and molluscs.
The shark is a slow swimmer, with speeds of around 5km per hour. As they have no natural predators and their method of feeding does not require speed, they have no need to swim any faster.
Whale shark population
Not much is known about the reproductive cycle of the Whale shark, but we do know it reaches sexual maturity around 30 years of age and it is ovoviviparous, meaning that the egg remains within the body and the females give birth to live young. There is evidence to suggest that Whale shark pups are not all born at once. Pregnant female sharks are seasonally found off the coast of Darwin Island, Galapagos and also St Helena Island in the mid-Atlantic, but are rarely sighted outside of these areas. The smallest Whale sharks recorded measured 55cm, which is assumed to be their approximate size at birth. Each litter contains around 16 pups, but larger litters are possible.
These gentle marine giants roam the oceans around the globe, generally alone. However, large numbers of Whale sharks often gather in areas abundant with plankton. These areas can be prime tourist spots. The distribution of Whale shark feeding areas indicates the presence of plankton and can signify the overall health of our oceans.
Based on modeled population estimates, 75% of the Whale shark world population occurs in the Indo-Pacific, and 25% in the Atlantic. In the Indo-Pacific, a population reduction of 63% is estimated over the last three Whale shark generations (75 years), and in the Atlantic a population reduction of more than 30% is inferred. Combined, this gives a world population reduction of over 50% over the last 75 years.
Whale Shark Research
Wildbook for Sharks is an online photographic identification tool on WhaleShark.org, which is maintained and used by marine biologists to analyze shark sightings data. Individual sharks can be identified by using photos of skin patterns behind the gills of each shark and any scars. As of February 2016, there were 7,011 individual sharks on the Whale shark global database, however the majority of these were considered to be immature males based on sizes. This dataset is assumed to not fully represent female, small juvenile or adult sharks. Anyone can submit sighting data and photos to Wildbook to help with shark research. Traditional methods of identification and tracking involve shark tagging, which is extremely invasive and highly stressful for sharks, and can also result in injuries to the sharks. The non-invasive method of mark-recapture has so far only been successfully adopted for Whale sharks, White sharks (Carcharodon carcharias), Nurse sharks (Ginglymostoma cirratum), and with some success on Basking sharks (Cetorhinus maximus).
Whale shark Conservation
The Whale Shark has most recently been assessed as endangered for The IUCN Red List of Threatened Species in 2016. Whale sharks are highly valued on international markets. Demand for their meat, fins and liver oil remains a threat to the species, particularly by unregulated fisheries. This targeted fishing of whale sharks did not begin until 1980’s. They are also victims of by-catch, the accidental capture of non-target species in fishing gear. And whale shark tourism presents a threat to the species as it can interrupt their feeding and sharks can be injured by boat propellers.
READ: 'Slipping though the net' report by Shark Guardian
Whale Shark was listed on Appendix II of the Convention on International Trade in Endangered Species (CITES) in 2002. This requires that any exports demonstrate they are from a sustainably managed population, and this is monitored through a permit system. As no Whale shark fishing is considered sustainable, it is illegal to fish them. Unfortunately, this is still occurring in countries such as Hong Kong, China, Mozambique, Oman, Pakistan, and Tanzania.
Whale shark tourism is managed through legislation in Australia, Belize, Ecuador, Mexico, and St Helena Island, and through voluntary codes of conduct in many other locations. This includes setting minimum approach distances for boats, to minimize the risk of propeller strikes.
In areas where regular by-catch is occurring (China, India, Taiwan, and Tanzania) due to the proximity of Whale shark feeding areas to net fisheries, there are unofficial restrictions in mesh size, net length and restrictions in fishing locations. Other methods that could help reduce Whale shark injury or mortality could also include training in safe release and using bycatch reduction technologies such as deliberate weak points in nets.
Whale shark and humans
Whale sharks do not pose a danger to humans. Many individual whale sharks have been approached, examined, and even ridden by divers without showing any sign of aggression. However, this behavior is highly discouraged. There is a code of conduct as set by the CMS that include; no more than one boat per shark, swimmers keeping a minimum distance of 4m from the shark, no flash photography (Whale sharks are very sensitive to light), no feeding or touching, interactions should last no more than 30 minutes etc. Mexico have taken it even further, requiring by law, the licensing, registering, and geotagging of boats and ensuring that captains and crew have undergone training to enable them to enter Whale shark protected areas with tourists. If any of them are caught breaking these rules, they face losing their license, a $13,000 fine or even 9 years in prison.
The sharks may, out of curiosity, approach and examine people in the water, they have even been filmed trying to catch bubbles from scuba divers regulators. Whale sharks have occasionally bumped sport fishing boats, but this is most likely a reaction to the bait being dangled by the anglers above. These sharks are sometimes struck by boats as they swim at or near the surface.
Other interesting facts
One female Whale shark was recorded to have 304 pups in various stages of development (the largest recorded litter of any shark). Paternity analysis was undertaken on the litter, and they were found to be sired by a single male, which gave rise to the theory that females are able to retain sperm from a single mating session and produce a steady flow of pups over a long period of time.
