Observations on dugongs at Calauit Island, Busuanga, Palawan, Phillipines

Abstract

A simultaneous monitoring system that used a team of local observers to count dugongs around Calauit Island, Busuanga, Palawan, Philippines, from eight vantage points was employed from March 1989 to May 1990 to identify important dugong habitats around the island. On average, five dugongs were seen per survey day. More dugongs were observed during March and July 1989 than in other months. These periods coincided with the bimodal peaking of seagrass biomass in the region and the lull between monsoons for that year. The site where dugongs were most often sighted was about 1.5 m deep, had the largest area of seagrass on the island (approximately 50 ha) and supported a seagrass (above-ground) biomass averaging 1060 g m-2 (wet weight). The dugongs were shy, making observations difficult. They were observed to graze at the study sites once per day, usually starting in the late afternoon or at night. Animals usually stayed on the seaward side of the spur-and-groove sections of the reefs. Short seagrasses such as Halophila ovalis, Halodule uninervis, Cymodocea rotundata, C. serrulata, Syringodium isoetifolium and Thalassia hemprichii showed more evidence of being grazed than the taller Enhalus acoroides. By direct observations of the area grazed by a single dugong, an average consumption rate of 30.5 kg (wet weight) of seagrass (above ground only) per day was estimated. The methodology developed in this study may be used to identify reserve areas for the protection of dugong habitats in the Philippines. The dugong populations of Palawan require further attention and implementation of conservation measures.