Assessment of the short-term structural damage in the Guimaras mangroves by the M/T Solar I Oil spill


by: Resurreccion B. Sabada*, Abner P. Barnuevo, Christy S. Madas, Joseph Biñas, and Edgar Hortillosa


Mangrove forests of Guimaras were affected by M/T Solar oil spill on August 11,2006 releasing more than two million liters of Bunker C industrial fuel. Acute and lethal damage to these mangroves was assessed in the field and from aerial photographs. The study (a) measured the heights and extent of oil bands on trees, saplings, and wildings; (b) determined the percentages of oiling and mortality by plant category; and (c) determined the percentages mortality by species. Field assessment was conducted on August 19 to November 18, 2006 in 14 oiled and two unoiled sites. Mangroves surveyed in Nueva Valencia were all coated with oil except Dolores and Igdarapdap where only traces of oil were observed. The sites in Sibunag and San Lorenzo had only traces of oil painted on trunks, roots or leaves. Oil levels among trees ranged from 0.1-1.2 m in height (13.43% of the tree height coated with oil) and highest in Tandog Island and Lucmayan in Nueva Valencia. The same degree of oiling among saplings and wildings were observed among sites except that a patch of reforested area in Alegria, Sibunag was heavily coated that reached up to 0.4 m for saplings and 0.3 m for wildings. The sites within La Paz had the highest percentage of oiled trees (74.4%) and saplings (72.9%), while Cabalagnan had the highest percentage of oiled wildings (65.5%). Total area of dead mangroves accounted to only 0.932 ha distributed in Sitio Bgatnan, Lapaz (0.490 ha), Sitio Tuguisan, Lapaz (0.008 ha), and Panobolon (0.389 ha) all in Nueva Valencia. Areas with massive death of mangrove particularly trees were primarily characterized by a low hydrodynamic, tidal flushing is minimal thus leading to a delay in oil removal by natural processes. In Tandog Island, defoliated mangrove saplings and wildings were also observed, albeit very few in number and occurred sporadically. Overall species mortality was only 0.97%. Saplings had the highest with 1.60%, trees had 1.44%, and wildings had 0.47%. This low mortality among wildings is attributed by their viviparous nature that allowed them to survive in oiled sediments up to the point where food reserves stored in propagules were exhausted, where upon the plants died. Within sites, Lucmayan had 12.88% tree mortality, while Sitios Bagatnan and Tuguisan both in La Paz had 11.11% and 4.96% respectively. Saplings’ high mortality was observed in Sitio Bagatnan with 8.20%, while that of wildings was in Tandog Island  with 7.41%. Among the 29 species affected, only five species showed mortality: Avicennia marina (0.03%), Phizophora apiculate (0.16%), R. mucronata (0.26%), R. stylosa (0.48%), and Sonnerratia alba (0.04). Massive death were concentrated only in the inner part of the forest stretching towards the terrestrial margin while the outer fringe remained intact and often had one or two surviving trees along the seaward margin.

Keywords:mangroves, Guimaras, oil spill, mangrove mortality