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Whale
sharks are the largest fish in the sea, reputedly growing
to a length of 19 metres (62 feet) and weighing some 34
tons. However, when they are pupped they are significantly
smaller, being somewhere between 58 to 64cm (22 to 25
inches) total length. What is surprising is that very
few whale sharks below 4 metres in length have ever been
recorded.
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| Embryo whale shark
and egg case. Photo from McCormick, Allan &
Young (1963) |
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Whale sharks have been shown to be ovoviviparous that is
they produce eggs containing an embryo and yolk sac, which
mature and ‘hatch’ in their uterus. This was confirmed
by the capture of a pregnant female off Taiwan in 1995
that was found to have some 300 embryos in her uterus.
Most of the embryos were contained in leathery egg cases,
but the largest in size were free of the case and ready to
be released. Several of these free-swimming neonates were
kept alive in aquaria and have provided interesting
information about whale shark neonatal growth rates (see
MCNews ISSUE 3.1). However, that said, very few whale
sharks between birth size and 4 metres have ever been
encountered. The two most publicised specimens were found
in the gut contents of other predators, one in a blue
shark caught in the Atlantic, the other was found alive
inside a blue marlin caught of Mauritius.
In
many areas where whale sharks are found the smallest
sharks are generally of 4 metres and above, and so the big
question is “where are all the small sharks?”… It
seems that the North of the Indian Ocean may hold some
answers to this question as a recent programme off
Djibouti found a number of surprisingly small sharks down
to a tiny 2.5 metres in length (see MCNews ISSUE
4.1). Now news from Zahirul Islam in Bangladesh indicates that a
very small whale shark of 110cm (43 inches) was recently
entangled in a seine type net deployed about 140
kilometres off the town of Cox’s Bazar, Bangladesh.
Unfortunately the shark was sold in the market before
photographs could be taken.
The
Indian Ocean has long been regarded as hosting one of the
largest populations of whale sharks, and the Wildlife Trust
of India, in
their campaign to protect the species, have been using the concept that pregnant female
whale sharks return to Indian waters to give birth. As
such this finding from Bangladesh and that of the small sharks found
off
Djibouti is very interesting and light may yet be
shed on the first few years of whale shark life that, to
date, have eluded science.
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