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June 2004 has broken
all records for whale shark sightings in Seychelles with
an early peak of 131 sightings of which a total of 63 were
from the North of Mahe. In past years there have always
been a few sharks seen in June and sometimes around the
North of Mahe, but never anything of this intensity.
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Whale
shark feeding furiously at the surface just a few
hundred metres off the MCSS office on North Mahe
Photo: Valérie Houmeau, MCSS volunteer |
This June,
however an unusual shift in wind patterns for a few days
seems to have triggered an intense plankton bloom that has
attracted in a large group of whale sharks.
What is
very unusual about the current population is that to date
none of the sharks seen has been recorded here before and
a number of the sharks are in fact very large adults,
including an adult female shark of over 12 metres in
length! Over the past three years between 13% - 23% of the
sharks we have seen had been tagged in previous years. As
such this does indicate that we may be seeing a part of
the population that we do not normally find in our normal
monitoring activities. It is also interesting that the
sharks appeared so rapidly after the predatory zooplankton
appeared, indicating that they were probably fairly close
by.
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The feeding
process seen from below, in much clearer waters a
few seasons ago.
Photo: Bob Wharton |
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The exact
mechanism that the sharks use to find their prey is as yet
unknown but working on basking sharks indicates that they
may use either or both an olfactory sense, literally
smelling the plankton, or an electro-receptive mechanism
that can pick up the minute electrical discharges given
off by the muscle contractions of millions of copepod
shrimp. This may sound far fetched but a particularly
unscientific experiment in a bay that had been closed off
with a fishing net trapping two whale sharks gave some
interesting insights! Two divers wearing fish net pants
stuffed with krill jumped into the water along with a
camera team…. The camera team were ignored but the two
divers were pushed around the bay by these two very
excited whale sharks that could tell who had the food!
The only
problem is that it is really difficult getting hold of
truck loads of krill to act as a whale shark
attractant….
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