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SHARKS
WHAT IS A SHARK?
Sharks are fish, but they possess several unique characteristics that distinguish them from most other fishes. These characteristics are:
- Sharks have a SKELETON MADE OF CARTILAGE - most fish have bony skeletons;
- Sharks have GILL SLITS that open directly to their watery environment - other fish have plates (or operculum) that cover and protect their gills;
- Sharks do not have a swim bladder like most fishes. Instead they have large OILY LIVERS with 2 LOBES that help them stay buoyant;
- Sharks reproduce through INTERNAL FERTILIZATION, similar to mammals. Most other fish reproduce through external fertilization;
- Sharks do not have obvious scales like many other fish. Instead they have protective DENTICLES which help them move quickly through the water by minimizing drag & friction.
DID YOU KNOW?
Sharks and their relatives, skates and rays, are collectively referred to as elasmobranchs? The term elasmobranch refers to
fish with a cartilaginous, non-bony skeleton.
There are more than 1000 known species of elasmobranchs? This number is constantly being updated as we learn more about our
oceans.
The swimsuits of Olympic athletes are designed to mimic the skin of sharks? This is because shark skin (denticles) is
designed to minimize the drag of the shark's body as it moves through the water.
Shark meat is often used in fish 'n chips? It is sold as "flake" at many seafood markets and fish & chip
stands.
The two biggest shark species eat the smallest food in the ocean? Whale sharks and basking sharks feed on tiny plants and
animals, known as plankton.
It is estimated that over 1 million sharks are killed by humans each year. In contrast, about 10 humans are killed by sharks
each year.
Some sharks live almost as long as humans? Estimates show some species can live longer than 100 years.
Sharks have 7 very advanced senses? Many sharks have better night vision than cats and can detect electrical fields.
The world's largest fish is the whale shark? This fish grows to a known maximum of 50 feet in length and weighs several
tonnes. In contrast, the world's smallest shark is the spined pygmy shark which grows to a known maximum length of
approximately 25 cm.
The mako shark is the fastest shark? It has been clocked at speeds up to 60 mph.
COMMON SHARKS OF THE DYER ISLAND MARINE ECOSYSTEM
Soupfin shark (Galeorhinus galeus)
Status: Classified as Vulnerable (VU) on the IUCN Red List 2006.
Description: The soupfin shark is a medium-size, slender shark with a long snout. It has a large mouth with sharp,
triangular teeth, typical of predatory sharks. The large almond-shaped eyes are located in front of pronounced spiracles:
openings which enable water to be pumped through the gills whilst the shark is resting. The colour varies between bluish
and dusky grey on top, and blends to white underneath. It has two dorsal fins; the second, situated over the anal fin, is
much smaller than the first. Juveniles less than 61 centimetres in length have black tips on their dorsal and caudal fins
and a white edge on the pectoral fins.
Range: The soupfin shark is cosmopolitan in distribution, being found in most temperate ocean waters except for the
northwest Pacific and northwest Atlantic.
Habitat: The soupfin shark inhabits cold to warm temperate waters. It is found well offshore, in shallow bays, or at
the surf zone, at depths of 2 to 500 meters. It often occurs near the bottom and is referred to as a demersal shark, but can
also be found in mid-water or near the surface when feeding.
Biology: Soupfin sharks are strong swimmers and opportunistic predators which feeds on schools of fish such as cod,
herring, sardines and whiting. Although it feeds primarily on bony fish, it also consumes bottom-dwelling animals such as
crustaceans and molluscs. Soupfins are prey for larger sharks such as the great white shark (Carcharodon carcharias).
Soupfin sharks occur in small schools that migrate long distances in the higher latitudes of their range where they move
towards the equator in winter, and poleward in the summer. They are known to segregate by sex and by age, making
them especially vulnerable to fishing. Soupfin sharks are ovoviviparous: embryos develop within eggs that remain inside
the mother's body until they hatch. No placenta is formed, and instead the embryo depends on its own egg yolk for nourishment.
Gestation is thought to last for about 12 months, and females move inshore to coastal nursery areas in the late summer to
give birth. Between 6 and 52 pups are born in a litter, and pups measure about 40cm in length. They are believed to have a
life expectancy of up to 60 years.
Threats: Soupfin sharks have been commercially exploited since the 1930s in most parts of its range where its flesh
is consumed by humans and its fins are used in shark fin soup. Large quantities of vitamin A can be extracted from the oil
in the liver, and the skin is made into leather products. Large scale commercial fisheries targeting soupfins continue in
many regions, including Uruguay, Argentina, California, southern Australia, and South Africa. Its life-history and biology
make this species particularly vulnerable to overexploitation and fisheries for these sharks in both California and Australia
have collapsed. Currently, the Australian population has recovered and the fishery remains well-managed. They are also
commonly caught in recreational fisheries. Soupfin sharks may also be threatened by the degradation of inshore nursery areas,
as these habitats are particularly vulnerable to human activities. The installation of high-voltage cables under the sea bed
can induce magnetic and electrical fields across their migration lanes, which might disturb their migration, feeding and
reproduction.
Conservation: There are several measures in place in Australia and New Zealand to regulate soupfin shark fisheries, such
as limits on the fishing gear used, closed seasons for nursery areas, and limits on the number that recreational fishermen
can catch. South Africa has a limit on recreational catches, but there are no other regulations to protect this vulnerable species.
Further information:
1. IUCN Red List (June 2007)
2. Compagno, L.J.V. (1984) FAO Species Catalogue. Vol 4: Sharks of the World. An annotated and illustrated catalogue of
shark species known to date. Part 2. Carcharhiniformes. Food and Agricultural Organization of the United Nations, Rome.
shark species known to date. Part 2. Carcharhiniformes. Food and Agricultural Organization of the United Nations, Rome.
3. FishBase (April, 2008)
4. Florida Museum of Natural History Ichthyology Department (April, 2008)
5. Tope Shark Biological Profile,
Ichthyology Department, Florida Museum of Natural History (August 2007)
6. McCord, M.E. (2005). Aspects of the ecology and management of the soupfin shark (Galeorhinus galeus). MSc thesis. Rhodes
University, Grahamstown, South Africa.
7. Shark Specialist Group.
(2007) Background paper on the conservation status of migratory sharks and possible options for international
cooperation under the Convention on Migratory Species. CMS Secretariat, Bonn, Germany. Available at:
Great white shark (Carcharodon carcharius)
Also known as: Grootwithaai
Status:
Great white sharks are classified as Vulnerable to extinction (VU) by the IUCN Red List. They are listed
in Appendix 2 of the CITES species list, making it illegal to trade or distribute products from great white's. South Africa
was the first country to protect great white sharks in 1991 and they are now protected internationally throughout most of
their range.
Description:
Great white sharks are streamlined for efficient movement through the water. They possess a pointed
snout, two large, sickle-shaped pectoral fins and a large triangular first dorsal fin. The mouth is armed with an array of
sharply pointed, serrated teeth. In the lower jaw, there are 24 more slender, pointed teeth per row. Teeth break and fall
out regularly, and are replaced constantly throughout the shark's life. The generic name for this species is derived from
the Greek word carcharos for ragged and odon for tooth. These sharks are grey or bronze on the upper surface of the body
and are white underneath. They have an acute sense of smell and are able to sense electric fields through sensors in the
snout. Great Whites have been recorded to reach up to 6.1m in length, and weigh up to 3.4 tons. The body of the great white
is covered from head to tail with dermal denticles that protect the shark from injuries, and minimize the friction when
swimming thereby allowing the shark to swim faster. Great Whites can swim almost 50km an hour in short bursts. When crossing
the ocean, a minimum speed of 4.7 km per hour is usually maintained. In the upper jaw, there are 26 broad triangular serrated
teeth in each row.
Range:
Great white sharks are found throughout the world's oceans in temperate and sometimes warm waters. They
occasionally occur in cold environments such as the Northwest Atlantic Ocean. Recent scientific research using satellite tags
found that adults can undertake long return migrations across entire ocean basins and back. Juveniles appear to stay closer
to the shore, but can also undertake long-distance coastal migrations. Great whites are found throughout southern Africa's
oceans with the highest concentration being found in the temperate waters of the Western Cape, South Africa. They are
particularly abundant and common around Cape fur seal colonies.
Habitat:
Great white sharks prefer coastal and offshore waters of the continental and insular shelves and offshore continental
islands, but recent evidence suggests that adults are probably pelagic (living in the open ocean) for much of the year.
They readily occur in oceanic waters from the surface to depths of 980 metres and possibly more.
Biology:
Males reach sexually maturity between 3.5 and 4.1 m, and females at 4 and 5 m. The female gives birth to live young between
1.1 and 1.65 m in length after. They have a gestation period of between 14-18 months. A female has two to ten embryos that
develop in her uterus, each nourished by a yolk sac. Once this is finished, the embryos will feed on unfertilized eggs.
Great whites do not have the ability to ventilate its gills by pumping water through them while immobile. They therefore
need to swim continuously with their mouth open in order to breathe. As they swim, water flows into the mouth, passes
through the gills from where the oxygen is absorbed before the water it exits through five gill slits on either side of the
head. Like other mackerel sharks (mako sharks and porbeagle sharks), the great white has a countercurrent blood flow system
that enables it to keep its vital organs up to 14°C warmer than the surrounding water, enabling it to survive and hunt
effectively in the cold Atlantic waters. This is why they are often referred to as warm-blooded animals like mammals. Great
Whites eat Cape fur seals, other sharks, rays, fish, dolphins and whales.
Threats:
Due to their migratory nature, the population size of great white sharks is difficult to assess. However, scientists believe
that their numbers have declined in several areas by up to 90 percent over the last 40 to 100 years. Great white sharks are
threatened by several commercial fisheries, including longline, trawl and purse seine fisheries. They are sometimes caught
in shark nets used in Kwa-Zulu Natal, South Africa and can also become entangled in the gear of offshore aquaculture
facilities. One of the biggest threats still facing these sharks is their systematic killing by trophy hunters and poachers
for their jaws, teeth and fins. A great white shark jaw can fetch up to $100,000USD on the black market. Other factors
affecting this species include their presence near human settlements and resultant habitat degradation, depletion of prey
species, and negative public and media perceptions leading.
Some controversial questions…
Are great white shark bites the result of 'mistaken identities'?
This question is difficult to quantify scientifically, however researchers believe that great white sharks are extremely
inquisitive animals. Recent research using Crittercams shows these sharks approach pieces of floating kelp, anchor lines,
plastic bags and other items out of seeming curiosity. They sometimes investigate these items for several minutes before
swimming away. What is known, however, is that as more people engage in ocean-based activities, the likelihood of interaction
with great white sharks increases. It is important to note that in most "attacks" on humans, great white sharks
only inflict one bite and only about 9% of these "attacks" prove fatal.
Does chumming increase the likelihood of unprovoked attacks?
This is another difficult question to quantify scientifically. However, fishers have effectively been chumming coastal seas
for decades when they discard offal and unwanted fish, often into areas where where are high densities of both humans and
sharks. Research by several South African scientists indicated that there is no correlation between chumming and attacks,
and great white sharks are unlikely to associate these vessels with humans. Thus, it is more likely that sharks associate
boats with food, not humans with food.
Further information
1. Thomas P Peschak & Michael C Scholl. 2006. South Africa's Great White Shark. Struik. Cape Town. 96 pgs.
2. http://www.
fishbase.org/Summary/SpeciesSummary.php?id=751
3. Compagno, L.J.V.C. (2001) Sharks of the World: An annotated and
illustrated catalogue of shark species known to date - Volume 2 - Bullhead, mackerel and carpet sharks (Heterodontiformes,
Lamniformes and Orectolobiformes). In: Bonfil, R. Ed. FAO Species Catalogue for Fishery Purposes No. 1. Vol. 2. FAO, Rome.
4. Proposals for amendment of Appendices I and II
(CITES) - Thirteenth meeting of the Conference of the Parties: Bangkok (Thailand), 2-14 October 2004 (June, 2006).
5.
Bonfil, R., Meÿer, M., Scholl, M.C., Johnson, R., O'Brien, S., Oosthuizen, H., Swanson, S., Kotze, D. and Paterson, M.
(2005) Transoceanic Migration, Spatial Dynamics and Population Linkages of White Sharks. Science, 310: 100 - 103.
6. WCMC Species Sheets (March, 2008)
7. IPOA-SHARKS (March, 2008)
Bronze whaler (Carcharhinus brachyurus)
Also known as: cocktail shark, copper shark, narrowtooth shark, New Zealand whaler
Status:
The bronze whaler shark is classified globally as Near Threatened (NT) on the IUCN Red List.
Description:
Named for its bronze or sometimes greyish-bronze upper side, this is a large, fairly slender shark with a moderately long
and slightly pointed snout. The underside is white, and most of the fins have inconspicuous darker edges and dusky to black
tips. Both the large, pointed first dorsal fin and the long pectoral fins are sickle-shaped. Often, a prominent white band
runs down the flanks. Its common name 'whaler' arose in the nineteenth century, due to their habit of congregating around
the carcasses of harpooned whales hanging along the side of whaling boats.
Range:
Bronze whalers occur in most warm temperate waters in the Indo-Pacific, Atlantic and Mediterranean. The East Asia
subpopulation has been recorded from the coastal waters off Japan, China, North and South Korea and southern Siberia. There
may be two distinct populations of bronze whaler sharks in southern Africa.
Habitat:
The bronze whaler can be found from the surfline to at least 100 meters depth.
Biology:
Despite the bronze whaler being a fairly common species, its biology is relatively poorly known due to confusion with other
species. It is a viviparous shark: embryos develop within the mother and are provided nutrition via a yolk-sac placenta.
Gestation is thought to last for about one year, after which a litter of 13 to 24 pups, measuring 59 to 70 centimetres, are
born. Age at maturity for bronze whalers is around 13 years for males and 20 years for females. Adults and juveniles appear
to segregate by age. Juveniles are present in shallow water all year round, whilst adults are found inshore only in spring
and summer. Adult males occur in subtropical regions throughout the year, whereas females and immature sharks migrate to
these regions during winter, and then return to temperate regions (and inshore) in the spring to breed. However, despite
this migration, there is very little movement between adjacent regional populations. Nursery areas tend to be large and ill
defined but include shallow banks, large shallow bays, inlets and harbours as well as the open coast. Bronze whalers can be
found singly, or in loose schools of up to one hundred individuals. They feed on bony fishes, such as sardines, mullets,
hake and soles, as well as other prey such as sawfish, squid and cuttlefish. Large numbers follow the winter sardine run off
the southern Natal coast, South Africa to feast on one of their preferred prey. This powerful and fast shark is considered
to be a dangerous species, and there have been a few provoked and unprovoked attacks on swimmers and divers.
Threats:
There is little information regarding the extent to which the bronze whaler is utilized, but it is undoubtedly caught for
food, by sports anglers and taken as bycatch throughout most of its range. As the bronze whaler is exceptionally slow to
reproduce, it is particularly vulnerable to over fishing. The East Asia subpopulation is the target of commercial fishing
in China and contributes to the catch of shark fisheries in East Asia since the 1950s. The bronze whaler may also be
threatened by degradation of their inshore nursery habitats, which are vulnerable to the effects of human development and
pollution.
Conservation:
There are species-specific conservation measures for the bronze whaler. In Australia and New Zealand, measures aimed at
other species are likely beneficial to the bronze whaler, including bans on taking school and gummy sharks from nursery
areas in Tasmania, and a ban on gill netting out to five nautical miles from shore off the northwest North Island, New
Zealand. Specific conservation actions may be required to conserve this species in South Africa.
Further information:
1. Compagno, L.J.V. (1984) FAO Species Catalogue. Vol 4: Sharks of the World. An annotated and illustrated catalogue of
shark species known to date. Part 2: Carcharhiniformes. Food and Agriculture Organization of the United Nations, Rome.
2. Ferrari, A. and Ferrari, A. (2002) Sharks. Firefly Books Ltd, Toronto, Canada.
3. Compagno, L.J.V., Fowler, S. and
Dando, M. (2005) Sharks of the World. Harper Collins, London.
4. Walter, J.P. and D.A. Ebert, 1991 Preliminary estimates
of age of the bronze whaler Carcharinus brachyurus (Chondrichthyes: Carcharinidae) from southern Africa, with a review of
some life history parameters. S. Afr. J. Mar. Sci. 10:37-44.
5. http://www.flmnh.ufl.edu/fish/Gallery/Descript/
narrowtoothshark/narrowtoothshark.
html
6. IUCN Red List (June 2007)
Smooth hammerhead (Sphyrna zygaena)
Also known as: common hammerhead, round-headed hammerhead
Status:
The smooth hammerhead is classified as Lower Risk/near threatened (LR/nt) on the IUCN Red List.
Description:
A member of a large and easily recognizable group of sharks, the mooth hammerhead can be distinguished from the other
hammerhead species by the single notch in the centre of its hammer-shaped head. The eyes are located at either end of the
hammer, and the particularly arched mouth is in line with the trail edge of the head. Olive-grey above and white below, the
smooth hammerhead has a tall and sickle-shaped first dorsal fin and plain pectoral fins with black tips.
Range:
The smooth hammerhead is widespread in temperate and tropical waters, including the Atlantic, Pacific and Indian Oceans and
the Mediterranean and Black Seas. It migrates northward in search of cooler waters for the summer months.
Habitat:
Preferring shallow waters, the smooth hammerhead is usually found inshore over continental shelves, or in bays and
estuaries. It may also be found well offshore in deeper waters, particularly when migrating. The smooth hammerhead has also
been seen in freshwater habitats such as the Indian River, along the coast of Florida, USA.
Biology:
The maximum known length is 5m and maximum weight is 400kg. Pups are born around approximately 0.5m. Known for their
schooling behaviour, smooth hammerheads gather in large groups as juveniles, but occur singly or in small groups as adults.
They feed primarily on bony fish, small sharks and stingrays and are known to be cannibalistic on occasion. When inshore,
their diet consists mainly of skates and stingrays. In deeper waters they may also feed on crustaceans and cephalopods.
Other large shark species may prey on juvenile smooth hammerheads, but the adults have no known natural predators. The
hammer-shaped head is thought to be a mechanism to spread out the ampullae of Lorenzini - sensory organs that detect
electric currents, chemicals in the water, and temperature changes. Males reach sexual maturity at about 2.1m and females
become sexually mature around 2.7m. Female smooth hammerhead sharks have an 11-12 month gestation period and are
ovoviviparous. The embryos are nourished by a yolk sac placenta and during the summer months, once the yolk sac has been
used, between 20 and 40 young sharks hatch. Pups measure about 0.5m at birth.
Threats:
The smooth hammerhead is rarely fished intentionally, but it is taken as bycatch in gillnet, longline and driftnet
fisheries. When caught, the liver oil will be used for the production of vitamins, its fins for shark fin soup, its hide
for leather and the carcass is often processed as fishmeal.
Conservation:
There are currently very few conservation regulations for smooth hammerhead sharks. As a bycatch species, however, targeted
conservation action may be required throughout its range.
Further information
1. Florida Museum of Natural History
2. http://www.fishbase.org/Summary/SpeciesSummary.php?id=917
Sevengill cowshark (Notorynchus cepedianus)
Also known as: sevengill shark, bluntnose sevengill shark, broad
snouted sevengill, broadnose sevengill shark, broad-snout, cowshark, ground shark, Pacific seven-gill shark, seven-gill
cowshark, sevengill shark, seven-gilled shark, spotted cow shark, spotted seven-gilled shark, and Tasmanian tiger shark.
Status:
The sevengill shark is currently listed globally as Data Deficient (DD) by the IUCN Red List. Data is lacking in most
regions, making it difficult to determine the overall conservation and population status of this species.
Description:
This shark is a large, active cowshark with a broad head, small eyes, and short, blunt snout. It gets its common name from
the seven gills on each side of the pectoral fins. This character distinguishes from most other sharks that have only five
gills. There is one dorsal fin that is located far back on the body, from over the insertion to over the free tops of the
pelvic fins. The anal fin is smaller than the dorsal fin.
Range:
The sevengill shark is found in all oceans with the exception of the North Atlantic Ocean and the Mediterranean Sea. In the
Southwest Atlantic Ocean, it is found from southern Brazil to northern Argentina and in the Southeast Atlantic Ocean it is
distributed from Namibia to South Africa. The range of the sevengill shark in the Pacific Ocean is in the western basin from
southern Japan south to New Zealand, including Australia and in the eastern region from British Colombia, Canada south to Chile.
Habitat:
Large sevengill sharks are often found in offshore waters to depths of 570m. However, this species
also lives in deep channels in bays. Smaller individuals reside in shallow water over continental shelves including bays
and estuaries. Sevengill sharks generally swim slowly along the bottom while occasionally cruising to the surface to feed.
Biology:
The maximum known length of the sevengill shark is 3m and the maximum reported weight is 107kg. Males reach sexual maturity at
1.5m in length while females become mature at approximately 2.2m. The life expectancy of this species is about 50 years.
The sevengill shark, an opportunistic predator, feeds on most anything including sharks, rays, chimaeras, and marine
mammals such as dolphins, porpoises, and seals. It also consumes mammalian carrion including humans. It also feeds on bony
fishes including salmon, sturgeon, herring, and anchovies among others. Sevengill sharks sometimes hunt in packs, working as
a team to capture large prey such as marine mammals and other sharks. Another hunting tactic is to sneak up on prey from
behind, quickly dashing at the last moment for the capture. Sevengill sharks are ovoviviparous. Females move into shallow
bays to give birth after a 12 month gestation period. This occurs during the spring and early summer months. Each litter may
number up to 82 pups, each measuring about 16-18 inches (40-45 cm) in length. Throughout the first few years, the young sharks
remain in the shallow water nursery grounds before moving to offshore environments.
Threats:
During the 1930s and 1940s, the sevengill shark was quite common in the shark fisheries along the coast of California.
Although the commercial fishery soon collapsed, recreational shark fishing for this species became popular. However, this
fishery declined in the late 1980s and 1990s with recreational fishers targeting other species. Today the sevengill shark
is caught primarily by anglers as well as incidental bycatch in commercial fisheries targeting other species. Sevengill
sharks are increasingly targeted by demersal longline and handline fisheries in South Africa.
Further information:
- http://www.flmnh.ufl.edu/FISH/Gallery/
Descript/Sevengill/Sevengill.html
- http://www.unep-wcmc.org/isdb/Taxonomy/
tax-species-result.cfm?source=animals&genus=
Notorynchus&species=cepedianus&tabname=all
- http://www.fishbase.org/Summary/SpeciesSummary.php?
id=2531
- Compagno, L.J.V., 1984. FAO species catalogue. Vol. 4. Sharks of the world. An annotated and illustrated
catalogue of shark species known to date. Part 1 - Hexanchiformes to Lamniformes. FAO Fish. Synop. 125(4/1):1-249.
Smoothhound shark (Mustelus mustelus)
Also known as: gray mouth dog, smooth dogfish, smooth-
hound, sweet William.
Status:
The smoothhound shark is classified as Lower Risk: least concern (LR/lc) on the IUCN Red List.
Description:
Smoothhound sharks are large, plain or black-spotted and are uniformly grey or greyish-brown above and white below.
Range:
The smoothhound shark is found in most of the Eastern Atlantic, including the British Isles and France to South Africa.
They also occur in the Mediterranean, Madeira and the Canary Islands.
Habitat:
Smoothhound sharks are found on the continental shelves and uppermost slopes, from the intertidal region to at least 350m
depth. They have also been collected to depth of 624m in the eastern Ionian Sea. Sometimes occurring in mid-water, but they
prefer to swim near the bottom.
Biology:
This species feeds primarily on crustaceans, but also cephalopods and bony fishes. Reproduction is viviparous, with a yolk-
sac placenta. The maximum recorded length is 1.3m for males and 1.6m for females. Sexual maturity is reached between 70-80cm,
and females give birth to between 4 and 17 pups. Pups are approximately 39cm long at birth. Smoothhound sharks can reach 24
years, but it is suspected that they grow older.
Threats:
Smoothhound sharks are taken by shore and ski-boat anglers and are targeted for human consumption, oil, and fishmeal.
Further information:
1. http://www.fishbase.org/summary/speciessummary.php?id=4996
2. http://en.wikipedia.org/wiki/Common_smooth-hound
© 2008 Dyer Island Conservation Trust
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