|
 |
|
| Basking
sharks were previously thought to hibernated
during winter when plankton abundance was very
low. This theory was based on data on several
sharks captured
from deep water at this time of year which had
shed the gill-rakers that enable them to feed on
plankton.
Photo: Alan
James |
Like
whale sharks, basking sharks appear seasonally for
only brief periods of time, however, they are found around
areas where there are substantially more resources in
terms of research and monitoring facilities and
consequently over the last 20 years a significant amount
of information into the habits of these animals has been
gathered. This research has shown that these temperate
water sharks may indeed provide a number of clues as to
possible behaviour patterns in their warm water relatives,
the whale shark.
Whale
sharks have been recorded feeding passively by swimming
through plankton-rich waters with their mouths open, a
habit termed ‘ram feeding’; however, despite its size
the whale shark’s mouth is actually quite small when
compared to that of the basking shark which, when opened,
presents a considerable orifice. As such, the basking
shark seems better adapted to feeding in waters of low
plankton densities. The whale shark on the other hand, has
also developed a suction feeding strategy whereby it gulps
in water containing dense amounts of prey organisms. This
enables it to exploit very patchy localised pockets of
high concentrations of prey organisms, such as krill,
other small crustacea and small schooling fishes. This
ability even allows them to feed on spawn from fish as in
the snapper spawning event off Belize and the Red Crab
spawning off Christmas Island.
Research on the basking shark has shown that these sharks
use natural phenomena to assist them in their quest for
sufficient food. Dr David Sims working with the University
of Aberdeen and the Marine Biological Association
Plymouth, has been able to show that basking sharks forage
selectively along the boundaries of water masses of
different temperatures. These fronts promote areas of high
secondary plankton productivity giving rise to
significantly higher concentrations of plankton than in
the surrounding waters. Dr Sims and his team have shown
that basking sharks selectively forage along these fronts,
moving from one hot-spot to another and also moved with
productive plankton patches as tidal currents moved the
patch across the study area.
These thermal fronts were also distinguished by other
indications of high productivity, such as large schools of
fishes feeding at the surface, sometimes with the sharks
and also the presence of foraging birds in the same areas.
To date there has been no published reports of whale
sharks exhibiting similar habits, however, our own studies
around Seychelles have shown that there have often been
schooling fish in the area and often whale sharks are
found with, or under, such schools. Similarly, flocks of
birds, particularly Noddy and Sooty Terns, have frequently
been seen in areas where whale sharks are found. As such,
it may well be that prey concentration by a water mass
boundary is present and affecting the feeding habits of
whale sharks in tropical waters as it does for basking
sharks in temperate waters.
More
recent studies by Dr Sims and his co-workers have also
revealed that rather than hibernate when the plankton
levels decrease in the habitats around the UK waters, the
sharks in fact migrate to more productive waters. The
recent studies have shown that the sharks can effectively
forage in waters of lower plankton concentrations than was
thought feasible. Satellite tagging of basking sharks has
shown that they move to the edge of the continental shelf,
where they forage along the shelf-edge fronts, which also
exhibited increased plankton productivity.
Studies on whale sharks off Ninglaoo had shown that they
make repeated dives from the surface waters to sea bed at
depths of 50 to 80 metres and similar behaviour was shown
by the data from the satellite tagged basking sharks. It
has been suggested that these large vertical migrations
through the water column allow the sharks a better chance
to pick up chemical scent trails of their prey organisms.
The deeper dives recorded by both species (to greater than
1000 metres) may also be an indicator that these animals
are able to exploit the deep water zoo-plankton
communities that their large mammalian competitors the
baleen whales cannot reach (diving depths reported for
these mammals ranged from only 170 to 470 metres).
|
 |
|
Like
the basking shark, whale sharks are also filter
feeders. Photo Bob Wharton |
As
such it seems that although they are separated by
thousands of miles and are found in very different oceans,
these two large plankton feeding sharks may indeed share
many attributes and researchers in both species are
becoming very interested in advances made by their
colleagues!
|
