Wednesday, December 4, 2013

Why butanding sightings in Bicol are dwindling: A marine biologist's view

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by Prof. L. O. Basmayor

The whale shark, Rhiniodon typus Smith 1828, is a species that is vital to local tourism. Every year, it draws in large crowds who marvel at its massiveness. However, in recent times, sightings of the species that is known locally as the butanding have been reported to be decreasing.

Although some may believe that this decline is due to pure chance, there is a scientific reason to why fewer and fewer whale sharks are spotted in the waters of the Bicol region. This reason involves the habitat and biology of the whale shark.

The whale shark is an epipelagic oceanic and coastal, tropical and warm-temperate shark. It is often seen far offshore but come close inshore and sometimes enter lagoons of coral atolls. It is generally seen close or at the surface as single individuals or in schools of up to hundreds of sharks. It prefers areas where the surface temperature is 21 to 25°C with cold water of 17°C or less upwelling into it, and salinity of 34 to 34.5 ppt. These conditions are probably optimal for the production of plankton and small nektonic organisms which are prey of the whale shark.

Whale sharks come to Philippine seas essentially to feed on plankton. However, by themselves, our seas are not rich in plankton. Our country is located in an area where upwelling does not occur. Upwelling is a process that brings nutrients from the bottom of the sea to the surface and fertilizes the water column which consequently results to rich plankton production.

Instead, our marine waters are fertilized by outwelling. Outwelling is caused by the monsoon flood and the wind which transports the nutrients from the coastline to the sea. Additionally, the monsoon and typhoon wind and waves stir the coastal bottom sediments and fertilize the water column. Outwelling and storm turbulence are followed by plankton production which in turn attract whale sharks.

In other words, the lack of typhoons in the recent years in the region have rendered our seas quite infertile, decreased the plankton production, and made our seas a poor source of food for the whale sharks. It is for this reason that whale shark sightings have dropped in recent years.

Facts about the whale shark

What is a whale shark?

The whale shark is a huge filter-feeding species with a broad flat head and truncated snout. It has a big transverse, terminal mouth in from of its eyes, minute numerous teeth, and unique filter screens on its internal gill slits. There are prominent ridges on the sides of its body with the lowermost one expanding into a keel on each side of the caudal peduncle. The caudal fin is asymmetrical and cresentic. The dorso-lateral part of the body has a unique checkerboard pattern of light spots, horizontal and vertical stripes on a dark background. The color is dark grey, reddish, or greenish grey above, or with white or yellow spots and transverse stripes; whitish or yellowish below.

What does the whale shark eat?

The whale shark feeds on a variety of planktonic and nektonic organisms. It feeds on small crustaceans, along with sardines, anchovies, mackerels, and even small tunas and albacores, as well as squids. It feeds at or close to the surface and often assumes a vertical position with its head at or near the surface.

Is the whale shark harmful?

The whale shark is generally considered harmless. In fact, very large individuals are often examined and ridden by divers and the sharks do not act aggressively.

Where can we find the whale shark?

The whale shark is circumglobal in tropical and warm and temperate seas, oceanic and coastal. Western Atlantic: New York, Central Brazil, Gulf of Mexico and Carribean. Eastern Atlantic: Senegal, Mauritiana, Cape Verde Islands, Gulf of Guinea. Indo-West and Central Pacific: South Africa, Red Sea, Pakistan, India, Sri Lanka, Malaysia, Thailand, China, Japan, Philippines, Indonesia, Papua New Guinea, Australia, New Caledonia, Hawaiian Islands; Eastern Pacific: Southern California, Northern Chile

How large can the whale shark grow?

The maximum total length of the whale shark is uncertain, possibly 18 m, but specimens are rarely anove 12 m. Most reported are between 4 and 12 m. This is by far the world’s largest fish.
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