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Whale shark
Rhiniodon typus
The whale shark is the giant, slow swimming king
of the fishes, slowly passing through the tropical waters of the oceans.
It is the world’s largest living fish with a length of more
than 12 meters (largest accurately measured specimen), but may reach
sizes up to as much as 18 m in length, although these cases are not
measured accurately and therefore total length may have been overestimated.
However, most specimens encountered seem to be between 4 and 10 m.
Apart from its huge size, the whale shark is externally characterized
by a broad, flattened head, a very large and nearly terminal mouth,
and very large gill slits. On its upper flanks three prominent longitudinal
ridges are stretched and in the rear end a large half-moon caudal
fin is situated.
Whale shark, Rhiniodon typus (Photo B L Marshall).
Information about size at sexual maturity and how old whale sharks
can get is sparse. Some scientists speculated that whale sharks
do not reach sexual maturity until they are over 30 years of age,
and that they may have a life span of over 100 years. These statements
are supported by the fact that many large animals (8-9 m in length)
captured are found to be sexually immature, in the case of females,
having immature ovaries. These evidences suggest that sexual maturity
in both sexes may not occur until the sharks are old and have grown
to considerable length, possibly over 9 m. The estimations that
have been done on the age of whale sharks are all very rough and
should be treated with care since very little is known about growth
rates and aging in this species.
The whale shark has a global distribution, occurring in all tropical
and warm temperate seas apart from the Mediterranean. The distribution
seems to be linked to a number of different environmental variables,
such as temperature, salinity, wind and current patterns, etc. Whale
sharks are for instance predominately found in areas with surface
water temperatures from 18 and 30° C, but they appear to prefer
locations with surface water temperatures between 21-25° C,
where cool nutrient-rich upwellings are mixed with warm surface
water. These preferred setting probably correspond to the optimal
conditions for plankton production, the food of the whale shark.
It is seen both offshore and in coastal waters and sometimes the
species enters lagoons or coral atolls. In areas with freshwater
outlets there have been reports of whale sharks entering shallow
water areas near estuaries and river mouths, sometimes during seasonal
shrimp blooms. Generally, whale sharks are encountered singly but
sometimes large aggregations of sharks are observed. In some areas
during specific times more than 100 animals have been seen together.
The whale shark is one of three very large filter feeding shark
species that primarily rely on planktonic prey for food. The two
other sharks are the basking shark and the megamouth shark. The
whale shark does not, like the basking shark, depend on forward
motion to operate its filtration mechanism, but can instead use
a more active sucking feeding technique. This method enables it
to capture larger, faster moving prey as well as zooplankton aggregation,
and at the same time filter a far smaller volume of water making
the feeding process more effective. The food usually includes small
crustaceans such as krill and crab larvae, phytoplankton (small
algae and bacteria), seaweed, small schooling fishes, and occasionally
small tunas and squid.
Whale shark, Rhiniodon typus (Photo D Burton).
The whale shark is ovoviviparous, witch means that the egg cases
are hatched in the uterus and that the mother then give birth to
live young. One of few pregnant females examined was a specimen
caught outside the eastern coast of Taiwan. It contained 300 embryos,
from 42 to 63 cm in length.
Like many other shark species, the whale shark has a number of
different biological characteristics that make it particularly susceptible
to exploitation. The large size, the slow growth, and the late maturation
are all features that make the species vulnerable and slow to recover
from any overexploitation. At the present the commercial fisheries
for whale sharks are limited, but may expand in a near future from
an increased demand for food products.
In the past, the whale shark has been of little interest to man. This
has resulted in a near lack of scientific research on this species,
and today still very little is known about ecology and behavior of
the whale shark. On a number of different places around the world,
where occurrences of whale sharks appear to be predictable, they are
being targeted increasingly by commercial tourist operations. A key
factor in sustainable management of this industry is a clear understanding
of the population dynamics of the animal. At the present, this knowledge
is missing and it is impossible to say if there will be any long-term
impacts.
Whale shark (Rhiniodon typus) swimming
around a plattform (Photo B L Marshall).
Ref: Colman JG (1997) A review of the
biology and ecology of the whale shark. Journal of Fish Biology 51:1219-1234
-- CITES (2003) Proposal to include the whale shark (Rhincodon typus)
in Appendix II of the Convention on International Trade in Endangered
Species of Wild Fauna and Flora (CITES). Prop. 12.35. International
Trade in Endangered Species of Wild Fauna and Flora (CITES) -- Compagno
LJV (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, Vol.
4, Part 1) 249 p.
 Further reading
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