Blue shark
| Blue shark | ||||||||||||
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Blue shark ( Prionace glauca ) |
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| Prionace glauca | ||||||||||||
| ( Linnaeus , 1758) |
The blue shark ( Prionace glauca ) belongs within the sharks (Selachii) to the family of the requiem sharks (Carcharhinidae) and represents the only kind of the genus blue sharks ( Prionace ). In addition to the whitetip deep sea shark ( Carcharhinus longimanus ) and the silk shark ( Carcharhinus falciformis ) belongs he is one of the three most common deep sea shark species.
features
The blue shark usually reaches a body length of around 3.40 meters, but can also be significantly larger in individual cases. The longest blue shark measured so far was 3.83 meters long and unconfirmed reports even speak of animals over 4.5 meters in length. The maximum known body weight of a blue shark was 205.9 kg. The back of the animals as well as the top of the fins are bright dark blue and thus eponymous for the species. The belly and the undersides of the fins are white and the animals are black on the fin tips. Compared to other representatives of the requiem sharks, the first dorsal fin is shifted significantly further back and the pectoral fins are long and sickle-shaped. The caudal fin is medium-sized compared to other sharks and there are small keels on the base of the tail.
The shark's snout is relatively long - the length is greater than the width - and curved upwards towards the tip. As with many other sharks, the very large eyes are protected by a nictitating membrane . The teeth of the blue shark, which has a species-specific set of teeth , are characteristic:
- 14 - 0 or 1 - 14/13 to 15 - 1 to 4 - 13 to 15
This means that the animals usually have one central tooth in the upper jaw and then up to 14 additional teeth on each side, in the lower jaw one to four central teeth and 13 to 15 posterior teeth. The triangular teeth of the upper jaw are characteristic sawn and have concave curved cutting edges, the tooth bases overlap each other. There are also triangular, almost symmetrical, teeth in the lower jaw, but the sawing is much finer.
Habitat and Distribution
The blue shark mostly lives further from the coast or oceanic waters off the coast shelf and dives to a depth of around 350 meters. It occurs as a cosmopolitan in all tropical to temperate marine areas, and it is probably the most widespread species of all cartilaginous fish.
It is very common in the North Atlantic from the south coast of Great Britain to Senegal and around the Azores , the Canary Islands , Cape Verde and Madeira . It also colonizes the Mediterranean Sea and occurs as a summer visitor in the North Sea as well as the Skagerrak and the waters off Norway . In the Northwest Atlantic, it is found above all far from the coast of Florida and the Antilles ; here he is a summer visitor in the Gulf of Maine , off Newfoundland and Nova Scotia . In the South Atlantic the shark occurs regularly as an offshore species off South Africa and off the South American coast ( Argentina , Brazil , Uruguay ). In the Indian Ocean , too , it is an offshore species that is mainly found between the 35th and 13th parallel degrees south ; it is rare here near the coast. In the Pacific it can be found in all warm sea areas.
In parts of its range, there are seasonal migrations to areas that warm up in summer. These occur partly due to the migration of schools of fish such as herring , mackerel or tuna , especially in the northern Atlantic and northern Pacific. The longest migration route that could be determined so far came from an animal that was marked off the coast of New Zealand and caught again around 12,000 kilometers away, off the coast of Chile . The animals obviously prefer water temperatures in the surface area between 7 and 15 ° C, more rarely temperatures up to 21 ° C. In the tropical area, the animals are found in deeper and therefore cooler water layers, which means that they are spotted less often.
Diet and Enemies
As a large deep sea shark, the blue shark eats almost everything it can capture. The main part of its food is made up of fish of all sizes, mainly schooling fish such as herring, mackerel, sardines or tuna. He also hunts cephalopods and smaller shark species. He eats fish leftovers thrown into the sea by ships as well as indigestible garbage that can regularly be found in his stomach. It is regarded as a pest by deep-sea fishermen and whalers, along with other species, as it attacks animals caught in nets as well as in suspension lines.
The enemies of blue sharks, in addition to humans, are mainly larger sharks such as the great white shark ( Carcharodon carcharias ) or the mako shark ( Isurus oxyrinchus ), and the California sea lion ( Zalophus californianus ) is a frequent hunter , especially for smaller blue sharks . Parasites of the blue shark are primarily parasitic crustaceans that attack various sharks and other large fish in a relatively unspecific manner. Frequent parasites are the copepods Pandarus satyrus on the pectoral fins, Kroeyerina elongata in the shark's nose, Echthrogaleus coleoptratus on the surface of the body and Kroyeria carchariaeglauci and Phyllothyreus cornutus in the gills. The rate of parasite infestation can be up to 3,000 animals on a shark and result in a change in the gill structure.
Reproduction and development
Like most representatives of the requiem sharks, blue sharks are viviparous . Mating is likely to be preceded by foreplay, in which the males hold the females with their teeth. Although no mating has been observed so far, this is suspected due to the significantly thicker skin of the females and due to bite marks in the females. The animals give birth to between four and 63 pups, depending on the age and size of the mother. The young sharks feed on a yolk in the form of a yolk sac in the mother's uterus and are around 50 centimeters long at birth. The wearing time is between nine and twelve months.
From a length of about 2.20 meters, the females become sexually mature, which corresponds to an age of four to five years. Nothing is known about the onset of sexual maturity in males, but it is assumed that the males will be able to reproduce from around four to five years of age. The oldest animal known to date was around 20 years old.
Taxonomy
The blue shark was first scientifically described in 1758 by Carl von Linné under the name Prionace glauca , which is still valid today . The generic name Prionace was derived from the Greek words prion for "saw" and akis for "point", the species name glauca comes from the Latin glaucus for "blue" or "blue-green". Since the description the names Squalus glaucus Linnaeus 1758, Squalus caeruleus Blainville 1826, Thalassinius rondelettii Moreau 1881, Carcharias pugae Perez Canto 1886, Carcharias gracilis Philippi 1887 and Prionace mackiei Phillips 1934 appeared as synonyms .
The genus Carcharhinus is very likely to be paraphyletic on the blue shark , so it should include it accordingly. According to a molecular biological study in 2008, the shark is the sister species of the great nosed shark C. altimus and the silk shark C. falciformis , which are grouped in the middle of the genus.
Humans and blue sharks
The blue shark is one of the most popular catch fish for deep sea anglers. Together with other species such as the great white shark , the tiger shark and the mako shark , it is one of the seven largest species that have been included in the lists of the International Game Fish Association (IGFA) and for the catch records are registered regularly.
For commercial fishing, the sharks play a role primarily as potential attackers on the fish caught in the nets. Commercial use of the blue shark is only common in Southeast Asia and Japan due to its high uric acid content . Parts of the animals are offered fresh, salted or dried. In addition, the fins are processed into shark fin soup and the skin is used for leather products.
At the same time, the blue shark is considered to be one of the potentially dangerous shark species for humans. There have been several reports of shark attacks by this species on people bathing or diving, as well as on boats. It can be assumed that the sharks considered humans too overwhelming prey. However, due to their habitat, which is not normally in the coastal areas, these attacks are extremely rare. The blue shark can serve as an example of the exaggerated portrayal of how dangerous sharks are to humans.
Threat and protection
As with other large deep-sea sharks, the hunt for the blue shark represents only a subordinate risk. Much more serious are the losses that occur when trawling or line fishing. The sharks are caught as bycatch and mostly die in the net and on a leash because they have to be in constant motion to breathe. Every year between 10 and 20 million animals die in this way.
On the Red List of Threatened Species , the blue sharks are classified as (near threatened) potentially endangered.
In 2017 blue sharks were added to the list of appendix II at the Convention on the Conservation of Migratory Species of Wild Animals (CMS).
supporting documents
- ↑ to fishbase on Fishbase.org (English)
- ^ Mine Dosay-Akbulut: The phylogenetic relationship within the genus Carcharhinus. In: Comptes Rendus Biologies. 331, 2008, p. 500, doi : 10.1016 / j.crvi.2008.04.001
literature
- Kuno Sch. Steuben , Gerhard Krefft: The Sharks of the Seven Seas - Species, way of catching and sporty catch , Paul Parey Verlag Hamburg and Berlin, 1989 ( ISBN 3-490-44314-4 )
Web links
- Blue shark on Fishbase.org (English)
- Profile of the Haistiftung hai.ch
- Profile at the Florida Museum of Natural History
- Sharkproject: Project dedicated to the fight for more knowledge about sharks and thus more understanding for the protection of animals
- Diving with blue sharks
- Prionace glauca in the endangered Red List species the IUCN 2006. Posted by: Stevens, 2000. Retrieved on 11 May, 2006.
