Riesenhai
Klassifikation:
Weltweite Population
Beschreibung und Lebensweise
Physical DescriptionOriginally thought to be a whale, basking sharks, like whale sharks, are large filter-feeding sharks. The basking shark is also known as "sun fish" for its habit of basking in the sun at the surface of the water, and "sail fish" for its large dorsal fin which resembles a sail.
The basking shark can be easily distinguished by its size; second only to the whale shark. The average length of adults is 6.7 - 8.8 meters (22 - 29 feet). The size at birth is estimated to be 1.5 - 1.8 meters (5 - 6 feet). The gill slits nearly encircle the head and contain large, bristle-like gill rakers for filter feeding. The basking shark has a conical snout and an enormous mouth which extends past the small eyes. There is usually a gap in the center of the upper jaw with only scattered teeth present.
Basking sharks are grayish-brown but can range from dark gray to black. The belly may be the same color, or paler. Varying amounts of light gray or white may also be seen along the mid-ventral line. The skin is textured and covered with small denticles (conical pointed projections) which are arranged with the points facing toward the tail end. This causes the skin to feel smooth if you run your hand from head to tail, but abrasive if you move from tail to head.
Natural History
Basking sharks live in Arctic and temperate waters. They are found in the Atlantic from Newfoundland to Florida, from southern Brazil to Argentina, and from Iceland and Norway to Senegal, including parts of the Mediterranean.
In the northwest Atlantic, they are in the southern part of their range in the spring and the northern portion in the summer; they disappear in fall and winter. In the northeast Atlantic, basking sharks feed in the summer off the southwest coast of the United Kingdom before disappearing from November to March.
In the Pacific, they range from Japan, China and Korea to western and southern Australia and New Zealand, and from the Gulf of Alaska to the Gulf of California. They are also found from Ecuador to Chile. Basking sharks are migratory and may move inshore or offshore depending on the season. Often seen in pairs, basking sharks have been found in groups of 100 or more and have been known to move in same-sex groups.
They swim slowly at the surface in open water near the shore, and also travel offshore and into bays and estuaries. They are usually seen swimming with their mouths open as they catch zooplankton, their main source of prey, although they are also commonly seen breaching. The gill rakers filter particulate matter from water that passes through the gill slits. Every 30 to 60 seconds they close their mouths to swallow their prey.
Basking sharks are slow growing and reach sexual maturity between 12 and 16 years of age. It seems that they are ovoviviparous (production of offspring from eggs that hatch within the female), giving birth to an unknown number of live young. A gestation period of up to 3.5 years has been suggested, although estimates of 14 months have also been mentioned. Longevity is estimated to be about 40 years.
Status der Art
StatusThe basking shark is protected by specific country legislation throughout its range. It is listed as Vulnerable by the International Union for the Conservation of Nature and Natural Resources (IUCN) and unilaterally listed on Appendix III of CITES (Convention on International Trade in Endangered Species of Wild Fauna and Flora) by the United Kingdom.
Threats to the Species
Basking sharks were traditionally killed for their liver oil (used for lighting and tanning), meat for human consumption and animal feed, and skin for leather. Post-World War II fisheries targeted the oil for lubricants, cosmetics and vitamin substitutes.
A commercial fishery for basking sharks was set up in the late 1940s off the coast of Ireland. Between 1947 and 1975, 12,360 basking sharks were killed; the fishery ceased when it became uneconomical. A basking shark fishery also existed off the coast of California for fishmeal and oil but was stopped when no longer economical. Small fisheries also existed in waters off Scotland, Portugal, France, Peru, Ecuador, China, Japan and New Zealand.
Today, small basking shark fisheries operate in Norway from April or May to August or September. China and Japan hunt basking sharks for their fins, which are used in fin soup, and for their livers, which are used in aphrodisiac preparations, health supplements and cosmetics. The basking shark’s slow growth rate, late age of sexual maturity, and relatively low reproductive rate – coupled with over-fishing – have resulted in its becoming rare in areas where it was once common. In some areas, basking sharks have been regarded as pests, becoming entangled in fishing nets.
International Trade
Yes
Autor und Quellen
SourcesCCS and Associates. 1997. Basking sharp morphology and structure. www.isle-of-man.com. The Basking Shark Society.
CITES. 2001. Basking shark. http://www.cites.org.
IUCN. 2001. The 2000 IUCN Red List of Threatened Species. http://www.redlist.org.
Knickle, C, L. Billingsley and K. DiVittorio. 2000. www.flmnh.ufl.edu. Florida Museum of Natural History.
Martin, R. 2000. ReefQuest Expeditions: Life history and behavior of lamnoid sharks. www.reefquest.com.
Mollet, H.F. 2001. Demographic analysis of the basking shark Cetorhinus maximus (Gunnerus, 1765) Demographic analysis
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