Whale Sharks In Belize
Populations of large pelagic migratory fish have declined steeply in the past two decades due to overexploitation. Efforts to manage or protect these species have been constrained by their cryptic nature and a paucity of knowledge of their biology and behaviour. Conservation of migratory animals requires understanding of the movements of individual animals, populations and species. Whale sharks (Rhincodon typus
), the main subject of this thesis, are large, planktivorous, highly mobile and pantropical, and their life history traits of late maturity, longevity and low fecundity make them vulnerable to overexploitation but little is known of their behaviour.
A five-year study of their behaviour in an unexploited population was undertaken on the Belize Barrier Reef between 1998 and 2003, in relation to a spatio-temporally predictable food source, in order to improve management and conservation. Whale sharks displayed strong diel, intra- and inter-seasonal fidelity to Gladden Spit, a particular site that hosts large seasonal aggregations of spawning snappers. The population of whale sharks at Gladden Spit is transient and composed primarily of juvenile males. Individuals measured a mean total length of 6.3 m ± SD 1.7 m (range: 3.0 m to 12.7 m; error of ± 0.50 m). Satellite pop-off tags revealed that the whale sharks were physiologically robust, being able to dive over 1000 m and withstand temperatures under 50C possibly for orientation or to locate abundant sources of food. Diving behaviour displayed a strong circadian and circalunar component.
After feeding on cubera and dog snapper (Lutjanus cyanopterus
and L. jocu
) spawn at Gladden, sharks dispersed throughout the Belize Barrier Reef with directed movements of over 550 km recorded to the tip of the Yucatan Peninsula and east of the Bay Islands in Honduras. Whale sharks did not appear to aggregate at any of seven other documented fish spawning aggregation sites on the Belize Barrier Reef.
The mutton snapper (L. analis
) fishery based at Gladden Spit experienced significant declines in catch per unit effort and size of fish caught between 2000 and 2002. Declines occured despite a drop in the number of fishers fishing the spawning aggregation since the inception of the fishery. Whale sharks did not appear to prey on mutton snapper spawn and were unlikely contributors to the mutton snappers' decline. In 2002, whale shark encounter tourism brought US$ 1.35 million to the Gladden Spit and Silk Cayes Marine Reserve communities, offering an economic alternative to the mutton snapper fishery. Patterns of whale shark movement and feeding behaviour indicated that the marine reserve boundaries encompassed the main spawning aggregation and whale shark feeding zones. Increased visitor and boat numbers to the marine reserve coincided with alterations in the spawning behaviour of aggregating snappers and consequently the visitation of whale sharks at Gladden Spit. Strong management directives and enforcement are needed at the marine reserve to check unregulated growth of tourism and thus minimize its impacts on the fish spawning aggregations and visiting whale sharks.
Many tagging methods are used to gather as much information
about the population size, variability and behaviour of usually
elusive whale sharks. This is a guide to each tag type:
Conventional visual tags are plastic rectangular tags that are
color-coded for the year with a large identification number
printed on it. These are attached to the shark by means of a
tether placed near the 1st dorsal fin. This allows anyone to
identify the same whale shark over the course of a season or
over several years.
Acoustic or pinger tags release a burst of "pings" that forms
a specific code identifying the animal it is attached to. The
signal is picked up by a boat-based or underwater hydrophone
whenever the shark is within 500 m range. This allows re-searchers
to track the animal and know when a particular individual
is in the area.
Satellite pop-up tags record information on depth, tempera-ture
and light levels over the course of months. After a prede-termined
amount of time the tag releases itself from the ani-mal,
floats to the surface and transmits the data it collected to
an overhead ARGOS satellite. Data are processed and sent
to researchers for further analysis.
Satellite location-only tags provide
fine-scale movement information by
transmitting a location to overhead satellites
when the shark is at the surface.
Estimate the population structure, abundance and variability of
whale sharks occurring on the Belize Barrier Reef
Determine degree of loyalty to a particular site (site fidelity) and mi-gratory
patterns of the whale shark in Belize & the greater Caribbean
Elucidate whale shark foraging behavior
Transform data into information useful for whale shark management and conservation
Methods used include visual observation with scuba and from boats,
documentation of behaviour with video and individual shark identifica-tions
with still cameras, use of tags (see box at right), tissue sampling for
DNA analysis, plankton analysis, and oceanographic profiling.
Results to date:
Whale sharks are capable of large scale movement around the Belize
and Mesoamerican Barrier Reefs. Several of the 69 sharks tagged
with conventional tags
have been resighted be-tween
years at Gladden, in
the Bay Islands, Honduras,
near Turneffe Atoll, and
near Cancun, Mexico.
Whale sharks time
their movement with sea-sonally
available food. 22
sharks tagged with acous-tic/
pinger tags show that
whale sharks return to
Gladden monthly and
yearly timed to the snapper
spawning moons. Under-water
throughout the reef re-corded
the passage of
sharks throughout the en-tire
Belize Barrier Reef and three atolls showing that they often re-turn
to Gladden in time to feed on snapper spawn.
Whale sharks can dive to great depths and withstand large changes in
pressure and temperature. 11 sharks were tagged with geolocating
satellite pop-off tags. Whale sharks have set diving records: they are
diving to over 3,300 feet to waters with temperatures less than 5 0 C.
5 satellite location-only tags deployed in March and April 2003 are
already showing fine-scale movement along the barrier reef.
Whale sharks at Gladden Spit are primarily feeding on jellyfish such
as Linuche unguiculata
and on zooplankton such as copepods.
For more information on whale shark research in belize, please click HERE