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The whale
shark, Rhincodon typus, is the largest shark, yet
like the largest marine mammal it feeds on zooplankton,
the smallest animals in the sea. The whale shark is one of
three filter-feeding sharks, the other two being the
Basking shark, Cetorhinus maximus, and the
Megamouth, Megachasma pelagios.
Whale sharks are opportunistic feeders foraging for rich
and often patchy food sources. Two feeding modes have been
observed, namely
ram filter-feeding for diffuse plankton blooms, as
employed by the other two filter-feeding sharks, and
suction filter-feeding which is unique to the whale shark.
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Whale
sharks are opportunistic feeders foraging for rich
and often patchy food sources. Two feeding modes
have been observed, namely ram filter-feeding for
diffuse plankton blooms, as employed by the other
two filter-feeding sharks, and suction
filter-feeding which is unique to the whale shark.
This may have evolved to allow whale sharks to
feed efficiently on dense plankton blooms.
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whale shark is able to actively filter-feed in
areas of dense plankton to make the most of food
availability.
Photo Bob Wharton |
While
ram filter-feeding, the shark swims forward with
its mouths open passively scooping everything in
its path. Periodically, the mouth is closed, water
is ejected through the gills and the prey filtered
out and swallowed. |
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Swimming
with its mouth open the whale shark passively
filters out zooplankton form the water column. Photo
James D. Watt
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Suction
filter-feeding, on the other hand, is an active feeding
mode whereby the whale shark remains stationary in a
vertical or horizontal position and opens its mouth
forcefully, sucking in its prey. In areas of low
zooplankton densities this feeding method is likely to be
less efficient as a smaller volume of water is filtered.
It is therefore not surprising to see whale
sharks actively feeding in dense plankton patches and
passively feeding in areas of low prey densities.
The whale sharks diet is not limited to plankton. Stomach
content analysis has shown that they also feed on small
schooling fishes such as sardines, anchovies, mackerel and
occasionally larger prey such as small tuna, albacore and
squid. As for their planktonic prey, these range from
small crustaceans such as krill, copepods and crab larvae
to jellyfish as well as fish spawn. In fact whale
sharks can feed on particles as small as 0.75 to 0.78 mm
in diameter. Scientists have also found plant matter
(phytoplankton and macro-algae) in the stomachs of whale
sharks, but these are believed to have been swallowed
accidentally during feeding activities.
Whale sharks are though to be dependent on localised
productivity events. In March to May each year,
whale sharks aggregate at Ningaloo Reef in western
Australia to feed on dense zooplankton (krill) blooms
associated with peak coral spawning. In Belize, similar
aggregations occur near Gladden Spit (Belize Barrier
Reef), where they feed on fish spawn released during April
and May each year. From December to March whale sharks
migrate to the waters around Christmas island to feed on
the eggs of the red crab.
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Whale
shark feed on a wide range of planktonic and
nektonic (free swimming) prey. (from left to
right: crab larvae, chaetognath, copepod,
jellyfish).
Photo Dave Conway
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Whale
shark feeding aggregations also occur in the waters around Seychelles during
June to August and October to December. As yet it is not clear what the whale
sharks are targeting in Seychelles waters. MCSS has been sampling plankton on a
weekly basis at one of the sites frequented by whale sharks in Seychelles to
better understand what the sharks are feeding on when they come to our waters. Preliminary
plankton analysis by staff of the Marine Unit (Ministry
of Environment) indicated that the highest number of sharks appear to be found
when there are significant numbers of krill (Lucifer) and/or arrow worms
(Chaetognaths) present.
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