Spatial and temporal abundance of Tectus niloticus in marine protected areas in Palawan, Philippines: prospects for conservation

Abstract

The large reef gastropod Tectus niloticus or 'trochus' is an important fishery resource and a significant source of revenue in its distribution range. However, the high demand for trochus shell in the production of 'mother of pearl' buttons and unsustainable harvesting have led to the collapse of its population and closure of its fishery in some countries. Conservation measures in the Philippines which focused on the restocking of hatchery produced juveniles in partly protected reefs were unsuccessful. To propose more relevant conservation measures, assessment of trochus abundance was conducted in three types of habitats of three Marine Protected Areas (MPAs) in Palawan, Philippines to document the status of its populations. In unguarded and continuously exploited MPAs in the mainland Palawan, trochus occurred at extremely low numbers. Only juveniles were encountered at the rubble and rocky habitats within the intertidal areas. Large individuals only occurred at the shallow subtidal reefs. By contrast, in effectively protected areas of the Tubbataha Reefs Natural Park (TRNP), densities were quite high with large trochus being abundant even in the rubble and rocky intertidal flats. Trends in abundance in permanent monitoring sites in TRNP indicated a sudden drop between 2006 and 2008, with slight recovering trends until 2010. Sizes of sampled trochus at TRNP in 2008-2010 were significantly larger than in 2006. No live trochus were noted in Jessie Beazley Reef, a small offshore reef which forms part of TRNP. Population recovery through natural recruits from other reefs could be very slow in this area of TRNP unless a new breeding population is introduced. The presence of recruits at constantly fished MPAs at Palawan on the mainland could lead to population recovery if these areas are effectively spared from fishing for a longer period of time. Restored biodiversity in networks of MPAs could be highly beneficial to the ecosystem, the fisheries and economic sectors.