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Shark of the week: Blacktip reef shark — 16. October 2017

Shark of the week: Blacktip reef shark

The Blacktip reef shark (Carcharhinus melanopterus – derived from melas = black and pteron = wing or fin (see Pterosaurs)) is another requiem shark living in coral reefs. It inhabits nearshore waters of the tropical and subtropical Indo-Pacific from South Africa to Hawaii (even colonized the eastern Mediterranean Sea by way of the man-made Suez Canal) and prefers more shallow waters than its colleagues grey reef shark and whitetip reef shark. Their nursery grounds (females are viviparous and give birth to living young after feeding them from yolk and via placental connection) can be so shallow that the pups have to swim with their dorsal fins above the water looking “like a flotilla of tiny sailboats” (quote from here).

An expanse of clear water and white sand, and several sharks swimming with their black-tipped dorsal fins protruding above the water
Carcharhinus melanopterus by Leon Brocard from London, UK – PIMG_2915, CC BY 2.0, Link

Not only pups prefer to stay in groups for protection, but also adult Blacktip reef sharks form stabile groups for social purposes and hunting. Their prey are fishes, crustaceans, molluscs and squids, but also terrestrial and sea snakes and even rats and birds. They have got large eyes with a reflective tapetum lucidum like a cat, indicating that they can excellent hunt at night.

Due to their extremely small home ranges and strong site fidelity, Blacktip reef sharks are susceptible to habitat degradation and fishing pressure (only as bycatch by commercial fisheries, but targeted by artisanal fishermen for their meat, liver oil and fins, and also by recreational fishermen and for aquarium trade). Their small litter sizes (only 2 to 5 pups) and long gestation periods (up to 16 month) are no help, either. They are considered “near threatened” by the IUCN. They are normally timid and despite their size (typically up to 5.2 ft -1.6 m-) no danger to humans, but sometimes bite the legs or feet of waders encroaching into their space or spear fishers for their catch.

Sources: here, here and here

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Shark oft the week: Silvertip shark — 2. October 2017

Shark oft the week: Silvertip shark

The Silvertip shark (Carcharhinus albimarginatus), despite being easily recognized due to its eponymous white margins on all fins (even pectoral fins, unlike all other “whitetip” shark species), it often confused with its smaller brother Grey reef shark. Both are requiem sharks and can be found on or near coral reefs in the tropical Indian and Pacific Oceans. They are known to form mixed-species aggregations and both perform (slightly different) ritual threat displays to chase away threats (like other sharks or divers) and protect their personal space. The mock attack of an up to 10 ft -3 m- long Silvertip shark (as described here) remind me of male gorillas. While it seems aggressive, I consider this as defensive behavior. Remarkable that they rather flee or chase away instead of attack, in which case they would, more often than not, prevail due to their size.

0979 aquaimages.jpg
Silvertip shark, Image taken by Clark Anderson/Aquaimages – Originaly uploaded to the english wikipedia, Image:0979 aquaimages.jpg, CC BY-SA 2.5, Link

Like all other requiem sharks, Silvertip sharks are viviparous. Females bear every other year litters of around five young after a gestation period of about one year. They grow slowly and mature at around 6.6 ft -2 m- with 20 years.

Silvertip sharks are caught in direct shark fisheries, as bycatch and by illegal practices (IUU) mostly for their fins and meat, but also for cartilage, liver, teeth, jaws and skin. This resulted in areas like Scott Reef off northern Australia, where they are now extinct because of Indonesian fishers. Hence the IUCN considers them as “Vulnerable“. However, there are no species-specific management measures in place.

Despite their inquisitive and aggressive behavior (see above) and being able to take on large prey (due to their dentition, size and power), Silvertip sharks are not known to attack humans. The ISAF lists only 4 provoked, none fatal attacks under this species.

Sources: here and here

Shark of the week: Bignose shark — 18. September 2017

Shark of the week: Bignose shark

There are sharks like last weeks Smoothtooth blacktip shark that are very rare. It is understandable that not much data is available on these species, and the IUCN has to decide if they thus are considered threatened or “only” data deficient.

A taxon is Data Deficient when there is inadequate information to make a direct, or indirect, assessment of its risk of extinction based on its distribution and/or population status. A taxon in this category may be well studied, and its biology well known, but appropriate data on abundance and/or distribution are lacking. Data Deficient is therefore not a category of threat. Listing of taxa in this category indicates that more information is required and acknowledges the possibility that future research will show that threatened classification is appropriate. It is important to make positive use of whatever data are available. In many cases great care should be exercised in choosing between DD and a threatened status. (Source)

Then there is this species of requiem sharks named Bignose shark (Carcharhinus altimus), well known and widespread all around the world on the continental shelf edge in tropical and warm seas, that is considered data deficient, too. How is that possible?  The IUCN suspects that “the overall volume of catch reported to ICCAT does not represent the total removals of these sharks and the data are also very limited with respect to the size-, age- and sex- composition of the catch.”

The Bignose shark is, like all requiem sharks, viviparous. During the 10-month gestation period, 3 to 15 embryos are at first feed by yolk and later via a placental connection. They may be sired by two or more males. Born at 28 to 35 in -70 to 90 cm- long, young sharks stay at first in their nursery areas at depths of around 82 ft -25 m-, but in general the Bignose shark prefers deeper water.

Bignose shark.jpg
Carcharhinus altimus by NOAA – http://www.oceanexplorer.noaa.gov/explorations/05coralbanks/logs/oct17/media/bignoseshark_600.html, Public Domain, Link

Bignose shark are highly migratory: seasonally (traveling distances between 1,000 and 2,000 mi -1,600 and 3,200 km-), but diel vertically, too. By day swimming close to the sea floor at depths of 300 to 1,410 ft -90 to 430 m-, they move at night upwards or toward the coast.

Males reach maturity at 7.2 ft -2.2 m- and females at 7.5 ft -2.3 m-. This species possibly reaches 9.8 ft -3 m- in length. Despite their size, Bignose sharks are no danger to humans. They are taken worldwide as bycatch of gillnet, bottom trawl, and deep-set pelagic longline fisheries and used for their meat and fins or to produce liver oil, shagreen (shark skin leather), and fishmeal. Due to high fishing pressure, for instance in the northwestern Atlantic (where they are considered regionally as Near Threatened), in international waters and around the Maldives, its status is of concern and data collection and precautionary adaptive collaborative management should be a priority. Bignose sharks are considered regionally as Least Concern in Australian waters, and are protected in US waters.

Sources: here and here

Shark of the week: Speartooth shark — 4. September 2017

Shark of the week: Speartooth shark

The Speartooth shark (Glyphis glyphis), also known as Bizant river shark or Queensland river shark, is a member of the family requiem sharks. Just like the Ganges shark, this river sharks lives not only in rivers but in the sea, too. It seems that rivers in northern Australia and New Guinea are nursery grounds since newborn and juvenile sharks have been found there. They stay there several years (only shift according to tides or salinity) but up until 2015 no mature specimen had been found. Now it is known that this species mates in rivers, too, but lives also in coastal marine waters.

Speartooth shark melbourne.jpg
Speartooth shark by Bill Harrison from Wellington, New Zealand – Shark attack, CC BY 2.0, Link

Speartooth sharks are named after the shape of their teeth in the lover jaw: they seems topped with a spearhead. Females are viviparous (yolk sac develops into a placental connection) and give birth to up to 6 living young every two years. The pups are 20 to 23 in -50 to 59 cm- long and grow around 7.5 in -19 cm- per year. Mature sharks are up to 8.5 ft -2.6 m- long.

Despite their size, Speartooth sharks pose no threat to humans. They are extremely rare, but are caught incidentally by commercial, artisanal and recreational fishers as bycatch and suffer additionally from habitat degradation. Consequently, they are considered Endangered by the IUCN. Even as Critically Endangered on the 1999 Commonwealth Environment Protection and Biodiversity Conservation Act (but since Commonwealth protection does not apply until a distance of three nautical miles from the coast, which is likely outside the range of this shark, it is of no use). Sadly, there are no regulations restricting the capture of this species in Papua New Guinea, and in Australia a management plan has yet to be enacted. In two Australian National Parks they are protected somewhat from habitat alteration, if not fishing.

Sources: here, here and here

Shark of the week: Daggernose shark — 14. August 2017

Shark of the week: Daggernose shark

The genus Isogomphodon of requiem sharks only consists of one living species, but it seems that the Daggernose shark (Isogomphodon oxyrhynchus), known since 1839, soon joins the fate of its extinct sister species Isogomphodon acuarius.

Systematische Beschreibung der Plagiostomen (Plate 15) BHL6353140.jpg
Von Henle, Jacob; Müller, Johannes – http://www.biodiversitylibrary.org/pageimage/6353140, Gemeinfrei, Link

Female Daggernose sharks are viviparous (placental connection after the yolk sac of the embryo is depleted) and give birth to 2 to 8 living young every two years in nursery areas. Newborn sharks measure 15 to 17 in -38 to 43 cm- in length. Males mature at an age of 5–6 years and females with 6–7 years and reach up to 12 years and 4.6 ft -1.4 m- (males) and 20 years and 5.2 ft -1.6 m- (females).

The Daggernose shark lives solely in coastal waters (no rivers since it is intolerant of fresh water) of the Western Atlantic off Trinidad, Guyana, Suriname, French Guiana and northern Brazil. It is highly specialised (tiny eyes but elongated snout to properly use electroreception, a lot of pointed slender teeth -the most teeth of all requiem sharks- to form a fish trap) to hunt small schooling fish in muddy waters. Since humans are after the same fish, the shark gets the short end of the stick – caught as bycatch in gillnets (but also by fishery targeting sharks), it has been considered critically endangered by the IUCN since 2006 as a result. Between 1992 and 2002 it suffered a very large population decline (shown by more that 90% less catches), and moreover, mostly juvenile specimens have been found in the nets. And things may have gotten worse since then. There are simply no new data available regarding daggernose sharks, not for lack of trying but specimen, which means they are (despite being legally protected in Brazil since 2006, which is without enforcement not of much use) in reproductive collapse and on the brick of extinction.

Sources: here, here and here

shark of the week: Blacknose shark — 31. July 2017

shark of the week: Blacknose shark

The Blacknose shark (Carcharhinus acronotus) is a little (up to 4.3 ft -1.3 m- long) requiem shark. Its name refers to a black spot on the nose of juvenile sharks.

Carcharhinus acronotus noaa.jpg
Carcharhinus acronotus by NOAA – http://www.noaanews.noaa.gov/stories2009/20090724_sharks.html, Public Domain, Link

Blacknose sharks live in tropical or warm-temperate coastal waters of the western Atlantic from the southern USA, through the Gulf of Mexico and the Caribbean all the way to southern Brazil. There is segregation by size and sex: juveniles are typically found in shallow water while adults are located at greater depths. They are known to form large schools.

Blacknose sharks reach a maximum age of 10-16 years for females and 4.5-9 years for males. Both mature at 2 years and are relatively fast growing. Reproduction is viviparous with a yolk-sac placenta. After a 8-11 month gestation period, 3-6 15 to 20 in -38 to 50 cm- long pups are born annually (Gulf of Mexico) or biennially (Northwestern Atlantic) in shallow nursery areas like coastal bays or mangrove swamps, for instance Bulls Bay, South Carolina (U.S.).

Blacknose sharks are fished commercially as target and bycatch, but also as game-fish – as they are deemed decent fighters. This species is considered Near Threatened globally by the IUCN, and benefits from conservation measures (together with other small coastal sharks) in US waters. Although affected by high fishing pressure, Blacknose sharks seem to be in no danger in Brazil (since there are enough mature sharks to be found) but there aren’t sufficient data yet. For the Caribbean, too.

Sources: here, here and here

Shark of the week: Finetooth shark — 10. July 2017

Shark of the week: Finetooth shark

Another shark with a name leaving no doubt about the reason is the Finetooth shark (Carcharhinus isodon). Named after its nearly even, rather slender teeth in upper and lower jaw (isodon means „even tooth“), its name is also Eventooth Shark or Smoothtooth Shark, in German Feinzahnhai, in French Requin à Petites Dents and in Spanish Galana Dientefino or Tiburón Dentiliso (however, I don’t know why it is also named Night Shark or Tiburón de Noche).

Carcharhinus isodon in net.JPG
Von National Observer Program, NMFS/SEFSC – http://www.st.nmfs.noaa.gov/observer-home/about-nop/activities/index, Gemeinfrei, Link

The Finetooth shark inhabits extremely shallow coastal waters (no deeper than 33 ft -10 m- in the summer and 66 ft -20 m- deep in the winter) in the Northwest Atlantic, Gulf of Mexico and Southwest Atlantic (off Brazil) in distinct populations. In the past it was known to venture into rivers, though most of its paths are nowadays blocked by dams. That’s a problem, since the females move to special inshore nursery areas. There they give birth to 2 to 6 living, 19 to 25 in -48 to 64 cm- long young every 2 years after a 12 month gestation period. Like all species of the family requiem sharks, finetooth sharks are oviviparous, that means the embryo hatches in the womb and is born alive. Additionally, the depleted yolk sac develops into a placental connection to the mother after the embryos drain their yolk supply during their first 15 weeks (viviparous).

Although having a maximum length of 6.2 ft -1.9 m- (on average males reach only 5.2 ft -1.6 m- in length and females 5.4 ft -1.7 m-), the finetooth shark is no danger to humans. However, it thrashes and snaps at anything within reach when caught. It matures at 40 in -1 m- (at 4-5 years as males and 5-6 years as females, who are growing slower in general) and has a life span of 9 years – males- or 14 years – females.

Off the coast of North America the finetooth shark is known to migrate south seasonally (namely when surface water temperatures drop below 68°F -20°C-). It hunts in large schools for small fish. It is susceptible to habitat degradation (especially it nursery areas) and overfishing (as bycatch and targeted), but is considered (despite its low reproductive rate) as Least Concern – at least in the northern hemisphere, off Brazil however there is only a badly managed fishery which already pushed other shark species near extinction.

Sources: here, here and here

 

shark of the week: Whitecheek shark — 3. July 2017

shark of the week: Whitecheek shark

If you watch the video the name of the Whitecheek shark (Carcharhinus dussumieri) seems fitting (I don’t know about the reason for its other name: Widemouth Blackspot shark).

Whitecheek sharks are, like all other requiem sharks, ovoviviparous with on average two about 15 in -38 cm- long pups annually. Both males and females mature when they are about 28 in -70 cm- long and reach a maximal length of 39 in -100 cm. Only in 2012 it was found out that sharks of the species Carcharhinus tjutjot are no juvenile Whitecheek sharks (as believed since 1982) but a different species.

Living in coastal waters of the Indo-Pacific Ocean, the Whitecheek shark is usually caught as bycatch rather than as the target species, but is nevertheless facing extinction in several Asian regions. But a relatively large and fairly robust population in northern Australia, where it comprises a stable 2-3% of trawl catch (by biomass), saves this species – thus only considered Near Threatened by the IUCN.

Sources: here and here

 

shark of the week: graceful shark — 19. June 2017

shark of the week: graceful shark

The reason for shark names are sometimes mysterious. On example is the graceful shark (Carcharhinus amblyrhynchoides) that has got a similar name in several languages. But, living in the tropical Indo-Pacific from the Gulf of Aden to northern Australia in coastal waters, this rare species of requiem sharks is not graceful but rather tubby.

Carcharhinus amblyrhynchoides phuket.JPG
Von Tassapon KRAJANGDARA – http://www.fishbase.us/photos/thumbnailssummary.php?ID=860#, CC BY 3.0, Link

With a size of up to 5.6 ft -1.7 m- the graceful sharks is potentially dangerous, but has not been implicated in any shark attacks. It is viviparous, and females bear litters of up to 9 living young (with a mean of three) after a 9- to 10-month gestation period every year. Once the developing embryos exhaust their supply of yolk, the depleted yolk sac is converted into a placental connection to the mother.

The graceful shark is regularly caught as bycatch in commercial fisheries and used for its meat, liver oil and fins. In northern Australia it made up 1.5% of the shark catch in gillnets and 0.2% on longlines. Nevertheless, it is only considered near threatened by the IUCN.

Sources: here and here

shark of the week: dwarf spotted wobby — 22. May 2017

shark of the week: dwarf spotted wobby

Last week I introduced one of the largest wobbegong species – now one of the smallest: the dwarf spotted wobbegong (Orectolobus parvimaculatus).

A Dwarf Spotted Wobbegong, Orectolobus parvimaculatus. Source: CSIRO National Fish Collection. License: CC BY Attribution

Before 2008, sharks of this species have been considered juvenile Spotted wobbegongs (Orectolobus maculatus). Now it is a separate species, and its name means both in Latin and Englisch (a little bit uninspired) nothing else than little (i.e. dwarf) Spotted wobbegong.

Like nearly all wobbegongs, the dwarf spotted wobby lives off Australia in a depth range of 30 to 443 ft -9 to 135 m. While its big brother, the Spotted wobbegong, can reach a length of 9.8 ft -3 m-, the little brother reaches only 37.1 in -94.3 cm- in length. Like all wobbegongs, it is a nocturnal, bottom dwelling ambush predator and ovoviviparous.

Due to its size, the Dwarf spotted wobbegong is not targeted directly but caught as bycatch (in gillnet- and longline fisheries and rock lobster pots), where it normally survives after being discarded.

Sources: here and here