Posted by: Jimely Flores | February 28, 2011

Effects of Volcanic ashes in the blue swimming crabs of Sorsogon Bay, Philippines

Effects of Volcanic ashes in the blue swimming crabs of Sorsogon Bay, Philippines

Author: Jimely O. Flores

28 February 2011

Blue swimming crab are scientifically known as Portunus pelagicus or locally known in the languages of the Philippines as kasag, alimasag, masag, kappi, etc.

Continuous volcanic ash fall in Sorsogon Bay which is one of the fishing grounds for the blue swimming crabs is relatively unknown at present as there are no studies measuring the impacts. However, assumptions could be made basing on the biology of the organisms, the condition of the habitat and the oceanographic features and processes in that particular body of water, and the characteristics of the volcanic ashes.

Volcanic ashes are inherently acidic and when dissolved in waters result into the decreased of ph of the water concerns. It also increases turbidity as well as addition of other chemical components which may or may not affect the organisms.

Sorsogon Bay is the smallest body of water immediately within the vicinity of the Bulusan Volcano spewing ashes at the present. It is a shallow bay and relatively enclosed with an area of about 22,000 hectares, with its narrow channel opening into the inland seas near the Islands of Ticao and Burias Islands. Continuous high volume of volcanic ashes falling into the Bay may temporarily affect the physical and chemical water characteristic especially in the coastal areas especially at slack waters, rendering the waters more acidic and turbid.

The blue swimming crabs life cycle starts from being an egg attached in the mother’s abdomen. Most of the mother crabs are benthic preferring sandy bottoms in deeper marine waters. When the eggs are released from the females they became part of the organisms drifting with the surface and sub-surface currents (planktons) while undergoing different metamorphic stages for as long as 15 days until they grow and become crab instars and are ready to start a benthic life (living near or at the bottom of the sea).  Being planktons, the survival and developments of the eggs and larvae of the blue swimming crabs, may be affected by the lowered pH, turbidity and the other chemicals (from the volcanic ashes) dissolving into the waters. This is especially true as the organisms are calcium carbonate builders (their shell).

When the larvae have metamorphosed into crab instars and start living as part of the benthic ecosystems, they are found to be quite abundant in seagrass and seaweeds areas maybe because of the abundant protection and food these kinds of habitats offer.  However, these habitats are also prone to the higher incidence of volcanic ash accumulation as compared to the open waters where circulation maybe faster and constant. This stage of the crab lifecycle may also be said to be relatively vulnerable to the volcanic ash falls.

While the blue swimming crabs grow and mature, the blue swimming crabs undergo passive migration into the deeper waters and thus lesser prone to the impact of volcanic ashes.

In general, there is a probability that the blue swimming crabs may be directly negatively impacted by the ash falls of the Bulusan Volcano but the impact might not be immediately felt. If the impact had significantly reduced the development and survival of the affected blue swimming crabs’ eggs, larvae and juveniles, then it could be expected that the next batch of individuals expected to enter the fisheries in the future harvest seasons may be reduced.

There could also be indirect impacts of the volcanic ash falls by affecting negatively the development and survival of the other organisms and the habitat, the blue swimming crabs may in the long term be also affected.


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