BIOLOGY AND ECOLOGY

The whale shark, locally known in Donsol as “butanding”, is the world’s largest fish. This gentle giant is known to grow to great lengths of up to 15 t0 20 meters and can weigh up to 34 tonnes. It has very distinctive color markings of pale spots and stripes against a dark background (fading to a pale underside). These patterns appear (on the basis of short-term studies) to be unique to each individual and can be used for their re-identification. The head is broad and flattened with a large almost terminal mouth and its teeth are minute. There are prominent longitudinal ridges on the dorsal surface. This impressive but harmless species is unlikely to be misidentified.

As a highly migratory species, whale sharks are distributed worldwide in warm and temperate seas of the Atlantic, Pacific and Indian Oceans, mainly between latitudes 30o N and 35o S, but occasionally to 41 o N and 36.5o S. Food is an important factor for the growth, migration and abundance of whale sharks in time and space. Its presence is highly associated with blooms of planktonic organism and changes in water temperature.

The whale shark is one of only three known filter-feeding sharks, suction feeding by gulping in dense aggregations of zooplankton, or passive feeding when plankton density is lower. Prey items range in size from very fine zooplankton (1 mm diameter) to small fishes, squid and crustacea. Unlike other species of sharks, the whale shark can neither bite nor chew. Its thousand of teeth are so tiny that it can only eat small shrimps, fish and plankton by using its modified gill rakers as suction filter.

Whale sharks are ovoviviparous, which means that eggs are hatched and developed in the uterus. Whale sharks are slow growing, maturing sexually only after so many years and with long interval between pregnancies. Females have been found to produce as many as 300 embryos in her uterus, although it is thought that less than 10% of the young survive to adulthood. Upon giving birth, the mother sharks leave her young to fend for themselves. In effect, the species have low reproductive potential and low capacity for population increase. These make the species highly vulnerable to exploitation.

The ecological role of whale sharks is not yet fully understood but the links of these giants with other processes within an ecosystem have been established. Whale sharks are related to important ecological processes such as crab, fish and coral spawning. As possible indicators of plankton-rich patches, whale sharks may be using baitfish to locate their prey.