Ascension : Conservation Weekly - Ascension's Threatened Sharks
Submitted by The Islander (Conservation Office) 06.03.2008 (Article Archived on 20.03.2008)

In a couple weeks the Ascension Post Office will release a beautiful set of stamps featuring Ascension’s shark species. In anticipation, the Conservation Department would like to share some information with you to help you appreciate these unique fish:

Ascension’s Threatened Sharks (originally submitted 25 Feb 08)

In a couple weeks the Ascension Post Office will release a beautiful set of stamps featuring Ascension’s shark species.  In anticipation, the Conservation Department would like to share some information with you to help you appreciate these unique fish:

The World Conservation Union (IUCN) has adopted a classification scheme or “Red List” to categorise wildlife that are at risk of extinction. The basic categories for risk range from lower risk to extinct:

LOWER RISK: least concern -> near threatened -> conservation dependent

THREATENED: threatened -> vulnerable -> endangered -> critically endangered

EXTINCT: extinct in the wild -> extinct

Galapagos shark

A near-threatened species can be considered threatened with extinction in the near future.  A threatened species is vulnerable to extinction in the near future.  A vulnerable species is likely to become endangered unless the circumstances threatening its reproduction and survival improve, and an endangered species is at risk of extinction.  Nearly all of the shark species found near Ascension are threatened or at risk under the Red List system.  This is troubling, as sharks play a very important role as predators in our marine ecosystem. For this reason, we recommend that fishermen release and do not land sharks, especially those species listed below.

The most common species of shark found on Ascension is the Galapagos shark, Carcharhinus galapagensis. This shark has a maximum size of 370 cm/12.1ft and is distributed around the world in warm tropical waters in depths of 16 to 200 ft. They feed on fish and octopus living on the bottom of the sea floor and will sometimes even feed on the young of their own species. They are classified as near threatened.

The bluntnose six-gill shark, Hexanchus griseus, usually inhabits depths greater than 300 ft, and has been found at depths greater than 6,000 feet.  These primitive sharks migrate from the depths towards the surface during the night and return to the deep by dawn.  This species gets its name from its sixth gill slit as most shark species have 5 gill slits. They feed mostly on crustaceans, sea lampreys, hake and mollusks.  They are poisonous to eat and reach a maximum size of 482 cm/15.8 ft.  Six-gill sharks are a long-lived species; males do not reach sexual maturity until 14 years of age and females don’t mature until 18 years of age. The gestation period for this shark is believed to be 2 years.  These factors make put this species at risk and contribute to its near threatened status.

Scalloped hammerhead shark

The shortfin mako shark, Isurus oxyrinchus, is the world’s fastest shark and can leap more than 20 feet out of the water.  They are a warm-blooded shark and a warm water species occuring in depths of 0-490 ft worldwide. Shortfin makos can reach lengths of 400cm/13.1ft and feed on bony fishes like mackerels, tunas and swordfish but can also eat sea turtles, porpoises and other sharks.  They are listed as vulnerable on the IUCN Red List.

Whale shark

The scalloped hammerhead shark, Sphyrna lewini, is found in warm temperature and tropical waters to depths of 900 ft. These unusual looking sharks have a maximum length of 430cm/14.1ft and feed on fish like mackerel and herring in additional to squid and octopus. They can form large schools and are listed as endangered.

Ascension’s largest shark (and the largest fish in the world) is the whale shark, Rhincodon typus. This gentle giant can reach 2000cm/65.6ft in length, and feeds on plankton.  They are distributed in warm temperature and tropical seas around the world.  They don’t reach sexual maturity until 30 years of age, and it is estimated that they can live to be 100.  The whale shark is listed as vulnerable on the IUCN Red List.

The bigeye thresher shark, Alopias superciliosus, is found in tropical waters around the world.  Bigeye thresher sharks can be found at depths of 500m and can reach 5m/16.4ft in length.  They’re identified by their large eyes and long tail (or upper caudal fin lobe).  They feed on pelagic and bottom-dwelling fish and squid.  These sharks use their long tails to slap or stun their prey before feeding.  The bigeye thresher shark is listed as vulnerable on the IUCN Red List.

REFERENCES:

http://www.wikipedia.org, http://www.fishbase.org, http://www.flmnh.ufl.edu/fish/organizations/ssg/redlistupdate2007.pdf, Images from FishBase and Robbie Cada

REMINDERS:

The Conservation Department has FREE collars for all cats on the island; you can pick one up in the Conservation Centre, x6359.  Please make sure that you cat is wearing a collar.