Review of the Biodiversity of Southern Mindanao Marine Waters
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Philipp. Scient. 45: 1–61 © 2008, University of San Carlos REVIEW ARTICLE Review of the Biodiversity of Southern Philippine Seas A.C. Alcala1, J.A. Ingles2 and A.A. Bucol1 1Silliman University-Angelo King Center for Research and Environmental Management, Silliman University, Dumaguete City 6200, Philippines 2World Wide Fund for Nature (WWF), JBD Plaza, No. 65 Mindanao Avenue, Bagong Pag-asa, Quezon City 1105, Philippines ABSTRACT The marine biodiversity of the four seas in southern Philippines, namely, the Moro Gulf, the Davao Gulf, the Sarangani Bay, and the Sulawesi Sea, was reviewed. The biodiversity in the shallow marine waters is similar to those in other parts of the country. The biodiversity is also in a similar conservation status, that is, in various degrees of depletion. But it is possible that there are marine species that are unique to these seas. This is especially true of the Sulawesi Sea, which is characterized by great variation in terms of habitat and depth. The importance of the resources in the four seas lies in the contributions of some species, such as tuna fishes, to the livelihoods of coastal populations in the area. INTRODUCTION Some research and development projects on marine biodiversity and fisheries have been conducted in the marine waters of southern Philippines. The Department of Environment and Natural Resources (DENR) 2006 list of projects includes 11 coastal and marine resource management projects dealing partly with marine biodiversity in the shallow seas of southern Mindanao. Among the institutions and agencies involved in marine biodiversity studies in the area are the Southern Philippines Agribusiness, Marine and Aquatic School of Technology 2 The Philippine Scientist, Volume 45 (2008) (SPAMAST) in Malita, Davao del Sur, the Davao del Norte State College (DNSC) in Panabo City, Davao del Norte, the University of the Philippines in the Visayas (Miagao, Iloilo), and the Mindanao State University (Naawan campus). The development projects of the Department of Environment and Natural Resources and the Bureau of Fisheries and Aquatic Resources, usually in partnership with international funding institutions, typically included research on fisheries and biodiversity. The research findings of these aforementioned institutions have been brought together in the present paper in an attempt to construct a biodiversity profile of the whole area that can serve various purposes such as establishment of marine protected areas. In the course of this review, some gaps and deficiencies in our current knowledge on the biodiversity of the area have been identified and discussed. The socioeconomic importance of the southern Philippine seas lies in the fact that more than 10 million people now live in the four regions (Western Mindanao, Southern Mindanao, Central Mindanao, and ARMM) in the vicinity of the four seas, namely, the Davao Gulf, the Moro Gulf (including Illana Bay), the Sarangani Bay (including Sarangani Islands), and the Sulawesi Sea. This population figure comes from the National Statistics Office in 2002, and a substantial proportion is probably directly and economically dependent on marine and coastal resources of these seas. The four seas are known for their economic importance (see Fisheries Statistics 2003–2005). Davao Gulf is a well- known international shipping center. Samal Island in Davao Gulf is known for pearl culture and tourism. Moro Gulf is well known for its fisheries. Sarangani Bay, the home of the skipjack tuna, has become a center for aquaculture for milkfish and top predatory fish species. The Sulawesi Sea serves as a highway for movements of the tuna species and other fish species, marine turtles, and cetaceans, among others. Brief Description of the Study Seas This review is focused on four seas: the Moro Gulf and the Illana Bay as part of the Gulf (enclosed by the Zamboanga Peninsula, the Lanao provinces, Maguindanao province and Sultan Kudarat province), the Davao Gulf (enclosed by the Davao provinces and Compostela Valley province), and Sarangani Bay (enclosed by Sarangani and South Cotabato provinces). The Sarangani Strait separates the Sarangani and Balut Islands at the southern tip of Sarangani and Davao del Sur Alcala et al.: Biodiversity of Southern Philippine Seas 3 provinces from the Mindanao mainland. These three southern Philippine seas are continuous with the Sulawesi Sea, the fourth sea covered in this review. They have a diverse marine environment because of the variable depths and the presence of shallow marine habitats that include coral reefs, seagrass beds and mangroves. Different shallow marine ecosystems dominate in some parts of the area according to information provided by the DENR. Zamboanga provinces, Basilan, Davao Oriental and Cotabato City are still relatively rich in mangroves. Davao del Sur and Zamboanga del Sur lead in seagrass beds. Sarangani, Davao del Sur and Zamboanga del Sur lead in coral reef ecosystems. However, the present status of these ecosystems needs verification. The four seas are interconnected (Fig. 1) and have a greater proportion of waters 1,000 m and deeper, compared to other areas in the country such as the Palawan area. Figure 1. Map of the Sulawesi Sea. Isobath in meters. (Modified from: NASA Blue Marble, Bathymetry: US Defense Agency) Moro Gulf The Moro Gulf is a deep body of water with an area of about 22,000 km2. About 40% of it is deeper than 2,000 m. Its deepest part near 4 The Philippine Scientist, Volume 45 (2008) its eastern portion is about 5,000 m. The western part of the Gulf is shallow, less than 200 m deep, with extensive mangroves and coral reefs (Zamboanga del Sur and Zamboanga Sibugay provinces) (Fig. 2). Within the Gulf is the Illana Bay. Illana Bay Figure 2. Map of Moro Gulf. Isobath in meters. (Modified from: NASA Blue Marble, Bathymetry: US Defense Agency) Davao Gulf Davao Gulf is about 7,300 km2 in area (Fig. 3). It is a fairly deep body of water. The southern half is about 1,800 m deep. The deepest part is near the mouth at 2,800 m. Only a thin strip near the shoreline is shallow, with a depth of less than 100 m, where mangroves, seagrasses, and coral reefs may still exist. Inside the bay is Samal Island, a conservation and tourism island and site of giant clam (Tridacna gigas) introduction by the University of the Philippines Marine Science Institute and the Davao del Norte State College. A small, privately-owned island with coral reef and mangrove in the northern part of the gulf belonging to the province of Compostela Valley is also a conservation island. The DENR has established a marine protected area mostly of seagrass beds along the shore of Davao Gulf in this province. Alcala et al.: Biodiversity of Southern Philippine Seas 5 Figure 3. Map of Davao Gulf and Sarangani Bay. Isobath in meters. (Modified from: NASA Blue Marble, Bathymetry: US Defense Agency) Sarangani Bay and Sarangani Islands Sarangani Bay is the smallest body of seawater in the study area (ca 480 km2) and generally shallower compared to the other two seas (Fig. 4). Inside the bay, the water depth ranges from 200 m to 1,000 m. At the mouth of the bay the depth reaches 2,000 m. The bottom substratum is dominated by mud. The northern part of the bay contains coral reef. Off the southern tip of Sarangani and Davao del Sur provinces are the Sarangani Islands composed of two small islands, Balut and Sarangani, separated from the Mindanao mainland by the Sarangani Strait. In the shallow waters around these islands are coral reefs. Currents in the vicinity of Balut Island are almost constant and swift; one measurement registered at 7 knots (A. Ong, pers comm). Currents are probably due to the Mindanao Current flowing southward along the eastern coast of Mindanao. In August 2006, a fish-kill involving coral reef fishes occurred, which, according to the Bureau of Fisheries and 6 The Philippine Scientist, Volume 45 (2008) Aquatic Resources (BFAR), was caused by the high sulfur content of the water (A. Ong, pers comm). Beyond the 183 m isobath are deeper waters ranging from 200 m to 1,946 m in depth. Sarangani Islands are well known for rare shells. Although over-fished in terms of the major tuna species, the Sarangani Bay and Sarangani Strait are still relatively rich in pelagic fish species and are favorite areas for sport and recreational fishing (A. Ong, pers comm). The Sulawesi Sea: Topography and Oceanography The Sulawesi Sea is a large marine ecosystem (LME) with variable depths and varied bottom topography that partly explain its complex oceanography (Tomascik et al. 1997). The Sulawesi Sea alone is about 450,000 km2 in area. The total area of the Sulawesi Sea and the Sulu Sea plus the Visayan inland seas (= Sulu-Sulawesi Sea Ecoregion of WWF [Ingles & Trono 2004]), is about 900,000 km2. The Sulawesi Sea lies at the center of the world’s highest marine biodiversity area (DeVantier et al. 2004). The Sulawesi Sea, with which the southern Philippine seas are continuous, varies in depth, reaching 4,000 to 5,780 m in the Sarangani Strait southwest of the Sarangani Islands and 1,000 to 4,200 m towards the southeast (Fig. 4). This highlights the importance of the mesopelagic and bathypelagic realms suspected of favoring speciation of certain marine species in this tropical sea (Madin 2008). The North Equatorial Current influences the direction of surface currents in the three Philippine seas and the Sulawesi Sea (Wyrtki 1961, Villanoy 1988, Villanoy & Udarbe 1995) (Fig. 5). This current moves westward. Upon reaching the sea area between northern Samar and Catanduanes (Zheng et al. 2006), it splits into the Kuroshio or Japan Current heading north and the Mindanao Current flowing south. During the southwest monsoon months (June to November), the Mindanao Current spits into two in the Sulawesi Sea, one stream proceeding south and another stream flowing counterclockwise as the Equatorial Countercurrent.