Effects of Offshore Oil Drilling on Philippine Reef Corals

An offshore drilling site in an area of extensive live-coral bottom off northwest Palawan Island, Philippines, was examined 15 months after well completion to determine effects of drilling on coral growth and survival. Core samples of 38 Porites lutea head corals were collected from around the drilling site and from a control reef and their histories compared using X-radiography to reveal changes in annual growth before, during and after drilling.

Analysis of P. lutea growth rates showed that when compared to their pre-drilling growth averages and to growth of corals from a nearby control reef, little suppression of head coral growth could be attributed to drilling. Diver observation, however, together with analysis of sampling transect photomosaics, revealed an estimated 70 to 90% reduction in foliose, branching, and plate-like corals in an iron-stained area that extended out from the wellheads in a 115 x 85-m ellipse. Coral cover beyond this area was comparable to that of the control reef.