As mentioned last week, the Broadfin shark had been categorized as Endangered by the IUCN. But this happened before its sister species, the Borneo Broadfin shark (Lamiopsis tephrodes), has been again considered as separate species due to biomolecular analyses. Discovered in 1905 (instead of 1839, where he wasn’t even alive) by zoologist Henry Fowler, it had soon been equalized with the Broadfin shark.
Reaching only 4.3 ft – 1.30 m -, the Borneo Broadfin shark lives solely off Borneo. Due to fishing pressure in this waters and 2 separate species, the IUCN should maybe revise its classification.
Another species of requiem sharks is the Broadfin shark (Lamiopsis temminckii). Living solely in shallow waters off India, China and Southeast Asia, it suffers greatly from habitat destruction, overfishing and water pollution. The IUCN considers this species as endangered.
Like almost all other requiem sharks, the Broadfin shark is viviparous. 4 to 8 embryos feed at first from yolk and later via a placental connection. After about 8 month they are born at 15 to 23 in -40 to 60 cm- length. Maximal length is 5.5 ft – 1.7 m.
Another example to prove the Plate tectonics, anyone? Well, the Smalltail shark (Carcharhinus porosus – named after really big pores behind its eyes) was considered, until 2011, to live in coastal waters on both sides of the Isthmus of Panama: in the western Atlantic Ocean from the northern Gulf of Mexico to southern Brazil as well as in the eastern Pacific from from the Gulf of California south to Peru. A momentous mistake that seems to continue until now (see IUCN Red list and homepage of the Florida Museum). In fact, its sister species Pacific smalltail shark (Carcharhinus cerdale) can be found in the eastern Pacific, and the Smalltail shark lives only in the western Atlantic part. Both had been separated around 2.8 million years ago and developed into different species (just like the Whitenose shark and the Blacknose shark a proof of evolution, as well).
This small (generally only up to 3.6 ft -1.1 m- long) requiem shark is slow growing and viviparous with two to nine young every two years after an approximately 12-month gestation period. It can be found near the bottom of coastal waters and estuaries and forms large aggregations segregated by sex.
Along the northern Brazilian coast, where some of its nursery areas are, the Smalltail shark is the most common shark, and in Trinidad the most economically important shark (under the name puppy shark). Fishing pressure as bycatch as well as targeted (for its meat, fins, cartilage and liver oil) caused a significant decline in numbers, resulting in an ICUN-classification as vulnerable in Brazil, where mostly juveniles have been caught, but only data deficient overall. Since its habitat is much more narrow as previously assumed (see above), the IUCN should revise that urgently (in my opinion).
Bren Smith wants to create thousands of decent jobs, transform how we harvest food from the oceans, and blunt the effects of climate change and marine degradation — all at the same time. His big idea: small-scale marine farms.
Bren Smith will tausende von ehrlichen Jobs schaffen, verändern wie wir Nahrung aus dem Meer holen, und die Effekte von Klimawandel und Meereszerstörung abmildern- alles zur gleichen Zeit. Seine große Idee: kleine Meeres-Farmen.
One shark species considered endangered by the IUCN was possibly nearly extinct, since it had not been recorded since 1937 and only five confirmed specimens were known. But in 2004, several specimens of the Borneo Shark (Carcharhinus borneensis) has been found again by scientists in fish markets (just like the Indonesian wobbegong), making images never seen before possible, like color images (of fresh specimens instead of preserved) or of its enlarged denticles.
The Borneo shark has a slender body with a long, pointed snout, rather large, round eyes with nictitating membranes and oblique, slit-like nostrils. Unique is a row of enlarged pores above the corners of its mouth. Like most requiem sharks it is viviparous with up to 6 embryos, provisioned by the mother through a placental connection formed from the depleted yolk sac and born alive at close to 9.4 to 11.0 in -24 to 28 cm- long. Maturing at around 22 to 23 in -55 to 58 cm- in males and 24 to 26 in -61 to 65 cm- in females, the Borneo shark is a rather small shark (the largest known specimen measures 26 in -65 cm- long).
Formerly, the Borneo shark lived in shallow coastal waters of the Indo-West Pacific off Borneo, China and possibly Java. Now it lives maybe solely around the mouth of the Mukah River in Malaysia, where, due to high fishing pressure, it is in grave danger.
You know that most sharks travel to bear their young in a special area (nursery) to protect them. Mostly it is a shallow bay or estuary, sometimes even a river, but the Pacific Angelshark (Squatina californica) does the very reverse: normally living on sandy flats or rocky reefs as shallow as 10 ft -3 m-, female Pacific Angelsharks wander as deep as 180 to 300 ft -55 to 90 m – to give birth to 1 to 13 living young (their number is independent from the size of their mother, unlike in other sharks). But that is not the only peculiarity of this species.
Pacific Angelsharks are up to 5 ft – 1.5 m – long sharks living in coastal Pacific waters from Alaska to Chile in a number of genetically discrete subpopulations (or possible even different species like the Chilean Angel Shark (Squatina armata) off Chile). Like all other angel sharks, they have a flattened body with greatly enlarged pectoral and pelvic fins and are ovoviviparous (young hatch inside the womb). The embryo has at first an external yolk sac, but that begins to shrink as the yolk is transferred to an internal yolk sac. The embryo feeds from this yolk until it is fully resorbed (if the pup is born prematurely, it does not feed until everything is gone). Born after a 10 month gestation at 9 in – 23 cm- length, Pacific Angels mature at 35 to 39 in – 90 to 100 cm – (both sexes, unlike in other sharks). At what age is unknown, since, unlike in other sharks, their vertebral growth rings (analog to annual growth rings in trees) don’t indicate age but only size (and therefore feeding success). That makes age verification difficult. But tagging and recapturing indicates a relatively slow growth (adults around 0.79 in -2 cm- per year) with maturity occurring relatively late in life, and therefore a moderate fecundity.
As ambush predators, Pacific Angel Sharks bury themselves in the sand near rocky reefs, facing up-slope to better see the silhouette of prey against the sunlight, and lie quietly on the bottom. They appear sluggish, but high-speed videography has revealed that their “predatory strikes are sudden and dramatic: in about a tenth of a second, the front half of the shark’s body snaps upward about 90 degrees from the bottom, the bear trap-like jaws protrude a remarkable distance from the head, and snap shut with audible authority”. During the strike, the eyes roll backward into the head for protection. After a strike they bury themselves again. But, since prey animals often learn quickly where local predators tend to lie-in-wait, ten days later they move under cover of darkness to a new site up to 4.5 mi – 7.3 km – away.
To detect prey, Pacific Angel Shark nearly entirely depend on vision. Experiments showed that they caught fish models every time on vision alone, without scent, electrical or vibratory cues. Even at night, they detect prey indirectly by the faint greenish sparkle of bioluminescent plankton agitated in its wake (their retinal pigments suggest that this species’ peak visual sensitivity occurs at wavelengths almost identical to that produced by local bioluminescent plankton).
A fascinating shark, isn’t it? But humans nearly wiped out this species. Why? Due to fear for their lives? Hardly, since no human has been attacked by a Pacific Angelshark without provocation – they do snap when provoked, but even then don’t leave much damage. No, only for profit. The promotion campaign of a seafood processor in California named Michael Wagner in 1976 and later changed the former “junk fish” almost single-handedly to a tasty seafood, resulting in an annual caught of up to 1.2 million pounds -544,311 kilograms- of Pacific Angelsharks (in 1985) and making this species the number one shark fished off California. The eradication of this species was averted, as in the 90th the central Californian halibut and angel shark fishery (caught with the same gillnets with medium-sized mesh) was closed completely. Pacific angelshark numbers off California appear to be increasing, resulting in an assessment of Least Concern in the United States. Globally however, it is considered as Near Threatened, because the largely unregulated Mexican fisheries took over. This species is now absent from regions in Baja California Sur where it was historically found.
Small sharks often fall prey to bigger sharks and usually survive by getting more pups. But the Hardnose shark (Carcharhinus macloti) has a low reproduction rate: only one or two pups are born after a twelve-month gestation period every two years, just like in apex predators.
Von Hamid Badar Osmany – FishBase, CC BY 3.0, Link
The Hardnose shark is a small species of requiem sharks living in shallow coastal waters of the Indo-West Pacific from Kenya through southern Asia to southern Japan and northern Australia. It is grey or bronze above and white below and was named hardnose because of the heavily calcified cartilages in its snout, unlike in other species of the genus Carcharhinus.
Female Hardnose sharks are oviviparous: one or two embryos hatch inside the uterus and are fed by yolk and later through a placental connection. Born alive at a relatively large size (18 in -45 cm-), they mature at 28 to 30 in -70 to 75 cm- and reach a maximum of only 3.6 ft -110 cm. Their skin is covered by overlapping, oval-shaped dermal denticles.
Hardnose sharks form large, sex-segregated groups and are homebodies (tagging data has shown that 30% of re-caught individuals having moved less than 30 mi -50 km- from their initial tagging location). Due to a high fishing pressure by artisanal and commercial fisheries and their low reproduction rate they are considered as Near Threatened worldwide (although in Australian waters as Least Concern).
Despite being the most common shark in reefs of the Caribbean Sea, the Caribbean reef shark (Carcharhinus perezii) is one of the least-studied large requiem sharks. It looks a lot like its sister species Dusky shark, but lives solely on or near coral reefs in the tropical western Atlantic Ocean from Florida to Brazil.
Caribbean reef sharks are nocturnal and can be found resting in caves or on the sea floor by day (once famous in Mexico as “sleeping sharks”). They mature at 5 to 5.5 ft -1.5 to 1.7 m- (males) or around 6.5 ft -2.0 m- (females) and can reach a length of up to 9.7 ft -3 m, making them one of the largest apex predators in the reef ecosystem. Females are ovoviviparous and give birth to 3 to 6 24 to 30 inches -61 to 76 cm- long living pups every 2 years.
Despite their size Caribbean reef sharks are normally unaggressive toward divers, except in the presence of food – spear fishermen can get accidentally bitten (there are 4 registered unprovoked non-fatal attacks by this species), but sometimes also members of baited diving tours. Attracting sharks by feeding is a controversial by-product of ecotourism (and banned in Florida). Associate humans with food by the sharks (like by bears) seems only a problem if the species usually feed on mammals (like White sharks), but the artificial concentration of predators (like in the image above) and the intensified removal of fishes from the environment for use as shark bait (instead of using fish offal) could be a concern. Showing sharks to tourists, but also photographers and filmmakers, is more profitable than killing them – and provides a sustainable livelihood for ex-fishermen in times of overfishing. Unless, of course, their colleagues exploit this changed behavior of sharks to catch them all, not on film but on the thousands of baited hooks of longlines.
Because Caribbean reef sharks have been targeted by longline and gillnet for their meat, skin, jaws, fins and liver oil or taken as bycatch, resulting in its Near Threatened status. It is the most common shark species landed in Colombia, but protected in the U.S., Bahamas and some marine protected areas off Brazil. Illegal fishing and habitat degradation (coral bleaching) are dangers, too. Caribbean reef sharks off the coast of Florida have been found with dangerous levels of methyl mercury – higher than the FDA guidelines, anyway, the European guidelines are different and incomprehensibly (imo) permit higher levels for large predator species.
After introducing the homebody Atlantic nurse shark last week, this weeks shark is the opposite: the Dusky shark (Carcharhinus obscurus). It can be found worldwide in warm waters along the coasts (and offshore following ships, too) of America, Australia and Africa (and parts of Europe and Asia). It is nomadic and strongly migratory (even if genetic tests suggest that Indonesian and Australian Dusky sharks represent distinct populations) and wanders seasonally (between the poles in the summer and the equator in the winter) up to 2,400 mi – 3,800 km.
Despite being one of the largest members of the requiem sharks (it reaches on average 10 ft -3.2 m- and up to 14 ft -4.2 m- in length) and having a (maximal of all tested sharks) punctiform bit pressure at the tooth tip of 60 kg (just like human bites, by the way), the Dusky shark is no danger to humans (the very few unprovoked attacks attributed to this species are most likely cases of mistaken identity).
Being apex predators, Dusky sharks are one of the slowest-growing and latest-maturing sharks, not reaching adulthood until around 20 years of age. Female dusky sharks are ovoviviparous and give birth to at most one litter of 6 to 12 young every three years. They use shallow inshore habitats as nursery areas, since juvenile Dusky sharks (in contrast to their parents) do have natural predators, namely other large sharks. Off KwaZulu-Natal (South Africa), the use of shark nets to protect beaches has reduced the populations of these large predators, leading to a dramatic increase in the number of juvenile Dusky sharks (a phenomenon called “predator release”). In turn, these juvenile sharks have decimated populations of small bony fishes, causing monocultures of small Dusky Sharks.
Nevertheless, due to their very low intrinsic rate of increase (renders them among the most vulnerable of vertebrates) and fishing pressure Dusky sharks are considered Vulnerable worldwide and Endangered in the Northwest and Western Central Atlantic. They are targeted for highly valued fins, meat, liver oil and skin and have a high mortality rate when taken as bycatch.
Some shark species seems to be homebodies, like the Spottail shark (carcharhinus sorrah). Tagging studies off Northern Australia have shown that 49% of sharks were recaptured within 50 km of the tagging site. But even some of them seems to got travel fever – like the one shark that was captured 1,116 km away.
The Spottail shark is a very common species of requiem sharks and up to 5 ft 3 in -1.6 m- long. It lives in discrete populations on continental and insular shelves in the tropical Indo-Pacific from the East African coast, Madagascar and the Red Sea to India, Malaysia, China, Indonesia, the Philippines and northern Australia. Female sharks are mature at two to three years and give birth once a year to a litter of one to eight 20 in -50 cm- long pups (ovoviviparous) in shallow inshore nurseries. Spottail sharks live up to five years (males) or up to seven years (females).
Spottail sharks are targeted for their meat, fins, liver oil and fish meal, but are also “utilized bycatch”, that means they were not the target species, but nevertheless utilized (catch-as-catch-can) instead of thrown back overboard (which they could survive as non-deep-sea-species). They are among the most productive of sharks due to fast growth rates, early maturity and relatively high fecundity. Despite this, the IUCN considers this shark as being near threatened. Why?
Their nursery areas are extremely heavily fished (often with illegal mesh sizes) and also affected by habitat degradation and pollution. Spottail sharks suffer from over-fishing throughout much of their range. Only the relatively well managed northern Australia fisheries seems to be spared. But there are increasing illegal, unreported and unregulated (IUU) fishing activities in northern Australian waters, mostly by Indonesian fishers, the majority of whom are targeting shark.