Whale Shark Research on the Belize Barrier Reef
Whale Shark Biology
Photo © Jeremy Stafford-Deitsch
|
Whale Shark Biology
Whale sharks (Rhincodon typus) range throughout the world's
tropical waters and are thought to have evolved around 200 million
years ago. With nurse and zebra sharks as some of their closest
relatives (the Orectolobiforms or carpet sharks), whale sharks
are the only species in their family (Rhincodontidae). They are the
largest of the three planktivorous sharks (feeding on microscopic
organisms floating in the sea), the other two being the Basking
shark (Cetorhinus maximus) and the Megamouth (Megachasma
pelagios) that inhabit more temperate waters.
Whale sharks are
the largest fish in the
sea, purported to reach
20m, although the most
accurate measurement
made is 14.2 m. Whale
sharks feed on a variety
of tiny animals including
zooplankton, thimble
jellyfish, baitfish and
squid. The whale shark
is ovoviviparous, where the young develop in eggcases in the
uterus and are then born live. Although sharks in general produce
very few young compared to bony fish, whale sharks have been
known to produce an estimated 300 live young in one litter. Although
no one knows how long they live or where they reproduce,
they are thought to reach sexual maturity at about 30 years of
age, or when they reach close to 30 ft/9-10m in length, and may
live beyond 100 years old. World population estimates are unknown
but they are thought by shark scientists to be very low.
Although whale sharks appear to be solitary animals, they will
congregate to feed. Whale sharks appear to target high-density
food sources such as thick "soups" of plankton, zooplankton at-tracted
to coral blooms off Ningaloo Reef, and blooms of zoo-plankton
such as copepods off of Baja, Mexico. Whale sharks are
now known to undertake large-scale migrations possibly in search
of, or targeting, these patches of food. Predictable aggregations are
rare although several have been recently identified offshore near
the Philippines, Australia, Maldives, Baja/Mexico, South Africa,
Honduras and now Belize. In Belize, research has shown that
whale sharks seasonally aggregate at Gladden Spit to feed on the
spawn of aggregating snappers. Apart from
snapper eggs, whale sharks at Gladden also feed on thimble jellyfish,
pelagic baitfish and zooplankton.
Gladden Spit's Whale Sharks
Predictable aggregations of feeding whale sharks at Gladden
Spit have been known by fishermen for decades. Gladden Spit
forms a promontory on the
barrier reef, 46 km from the coast. Research undertaken
by scientists with local fishermen, revealed that whale
sharks aggregate to feed on freshly released spawn of aggregating
dog and cubera snappers. This is the first documented
observation of cubera snappers spawning, and of whale sharks
specifically targeting fish spawn for food. Whale sharks are
able to feed by filtering large volumes of water across their gills
while moving, or by gulping water and filtering food in a stationary
horizontal or vertical position.
Research worldwide
Elucidating the behaviour of elusive whale sharks requires
a range of techniques. Conventional tags have been used to look
at site fidelity and large-scale movements in South Africa, Belize,
Honduras and the Seychelles. Acoustic tags have been
used to track whale sharks from boats in Australia and the Seychelles
yielding information on fine-scale movements in relation
to food sources.
Acoustic tags used
with passive underwater
acoustic monitors
(in Belize) and satellite-linked tags
(Australia, Belize, the
Seychelles, Baja California)
have provided
information on whale
shark site fidelity and
movements independent
of researchers and
sightings. Satellite
tags deployed to date reveal that sharks are highly migratory (e.
g. Seychelles to Thailand) even making trans-oceanic crossings
of over 12,000 km (Baja California to Tonga). They are capable
of moving in a rapid and directed manner between patches of
abundant food and across deep bodies of water indicating that
they are outstanding navigators.
Whale Shark Conservation
Due to their life history, populations of whale sharks are
highly susceptible to any fishing pressures. Yet whale sharks
are currently fished primarily in India and Asia to supply the
Asian fin and meat markets. They do not benefit from any
global protection at this time mainly because so little is known
about them. Yet, most countries are slowly realizing that whale
sharks are worth far more alive through the tourist trade, than
dead. Consequently, several countries, such as the Maldives and
the Philippines, passed laws protecting whale sharks. The impacts
of tourism on whale sharks are not yet known in Belize
but to avoid them, many local whale shark tour-guides have
undergone training to promote safe & sustainable tours. Whale
sharks were recently listed on Appendix II of the Convention
for the International Trade for Endangered Species which monitors
and regulates trade in listed species.
For more information on whale sharks, see the Whale Shark Biological Profile.
|
|
|
|
