Aquatic Biodiversity of Lake Mainit, Southern Philippines
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Aquatic Biodiversity of Lake Mainit, Southern Philippines Wilfredo H. Uy1, Asuncion B. De Guzman1, 2 2 Rodrigo E. Acuña and Rey L. Roa 1College of Science and Environment 2School of Marine Fisheries and technology Mindanao State University at Naawan, 9023 Naawan, Misamis Oriental, Philippines Email: [email protected] ABSTRACT Lake Mainit is the deepest and fourth largest lake in the Philippines. The productivity and biodiversity of this oligotrophic lake have become vulnerable to various environmental and human pressures resulting from rapid economic development in the region. An inventory of fish, invertebrates and aquatic vegetation occurring within Lake Mainit and its single outlet Kalinawan river was conducted to generate a comprehensive profile and status of the lake’s biodiversity that will be useful in updating the Lake Mainit Development Plan towards the sustainable use of the lake resources. A total of 41 species of fish, 5 crustaceans, 8 mollusks and 14 species of aquatic plants were identified in the lake. The most popular fish resources are the gobies pijanga (Glossogobius giuris) and bugwan (Hypseleotris agilis) although their populations have declined through the years. Several exotic species of fish have been introduced in the lake, such as tilapia, carp and catfish. The most abundant crustaceans are two species of freshwater shrimp: the tiny shrimp Caridina mindanao locally known as isik and the freshwater prawn Macrobrachium lanceifrons locally known as ulang. The most abundant freshwater mollusks found in Lake Mainit are the bivalve Corbicula sp. locally known as bibe, and the snails Viviparia angularis (ige) and the golden apple snail Pomacea canaliculata (kuhol). Lake vegetation is dominated by submersed plants such as Hydrilla vertilicillata and Vallisneria sp., forming extensive underwater meadows. Floating plants such as the common water hyacinth Echornia crassipes are the dominant vegetation along Kalinawan river. Among the most conspicuous plants fringing Lake Mainit is the Indian or sacred lotus, Nelumbo nucifera, locally called paguse. Declining water quality, open access fisheries and exotic species introductions are among the most urgent environmental concerns that threaten the sustainability of Lake Mainit’s biodiversity. Keywords: Lake Mainit, aquatic biodiversity, vegetation, oligotrophic, LMDA INTRODUCTION Freshwater resources like lakes, rivers, streams, reservoirs, ponds, marshes and swamps are scattered over the major islands in the Philippines. These inland waters are home to a high diversity of native as well as introduced freshwater fish species (Conlu, 2 J. Environmental Science and Environment, Vol. 3 (2015) 1986). With such rich resource attributes, the freshwater habitats of the Philippines are considered economically important but threatened biodiversity systems. Lake Mainit is the fourth largest lake in the Philippines with a surface area of 17,060 hectares, a shoreline 62.1 km long, and a watershed area of 87,072 hectares (LMDA, 2005). Twenty-eight river tributaries flow into the lake from surrounding upland areas, while only one outlet river, the 29-km long Kalinawan River, drains into Butuan Bay through Jabonga, Agusan del Norte. Lake Mainit is a shared resource of the provinces of Agusan del Norte and Surigao del Norte in Northeastern Mindanao, and is bordered by eight municipalities that comprise the Lake Mainit Watershed (Fig. 1). Four of these are lakeshore municipalities, namely, Mainit and Alegria (Surigao del Norte), Kitcharao and Jabonga (Agusan del Norte), while two municipalities are located along the Kalinawan River system, namely, Santiago and Tubay (Agusan del Norte). An ecological study by Tumanda, et al. (2006) describes Lake Mainit as an oligotrophic lake with high transparency and sufficient nutrient supply to support high productivity. Lake Mainit has well-oxygenated freshwater of excellent quality to supply domestic uses and is considered one of the cleanest lakes in the Philippines. Woltereck (1941 cited by Lewis, 1973) provided the earliest depth measurements at one site of the lake at 167m. Bathymetric measurements obtained by Tumanda, et al. (2006) confirmed that Lake Mainit is the deepest lake in the country, with a mean depth of 128 m and a maximum depth of 223m (Fig. 2). Mainit Alegria Kitcharao Jabonga LAKE MAINIT LAKE MAINIT Total area = 170 km2 Total coastline = 62 kkm Kalinawan river Santiago River tributaries = 28 Tubay Figure 1 Map showing the location of Lake Mainit in northeastern Mindanao. Very limited information is available on the aquatic biodiversity of Lake Mainit. Pauly, et al. (1990) reported 37 species of finfish occurring in the lake compiled from various sources spanning six decades (1930s-1980s) (Manacop, 1937; Herre, 1953; Gracia, et al. 1962; Gracia, 1981; Gracia and Magsumbol, 1985 all cited in Pauly et al. Uy et al.: Aquatic biodiversity of Lake Mainit…. 3 1990). Diversity of molluscan fauna in the lake was studied by Abrea (2003) but no information on the lake’s flora is available from literature. The motivation behind this study is to obtain an inventory of fish, invertebrates, and aquatic plants in Lake Mainit and Kalinawan River. This study is part of a comprehensive research program jointly funded by the Philippine Council for Aquatic and Marine Research and Development of the Department of Science and Technology (PCAMRD-DOST), the IFAD-Northern Mindanao Community Initiatives in Resource Management Project (NMCIREMP) of the Department of Agrarian Reform - Caraga region (DAR-13) and the Lake Mainit Development Alliance (LMDA) toward the sustainable fisheries management and biodiversity conservation of one of the Philippines’ major freshwater ecosystems. Figure 2. Bathymetric map of Lake Mainit in northern Mindanao (Source: Tumanda et al., 2004). METHODS An inventory of finfish, invertebrates, and aquatic vegetation occurring within Lake Mainit and along its single outlet, Kalinawan River, was conducted from August to November 2007 through field survey and collection, interview with fishers, and survey of local markets. Voucher specimens of the different fish, invertebrates, and plant species were collected during the surveys. Species reportedly present in the area, but without voucher specimens, were also listed and noted. The resulting fish and invertebrate inventories were matched against earlier works (Pauly, et al. 1990; Abrea 2003) to determine if any changes occurred in the biodiversity of the aquatic organisms. Classification of finfish species adopted the system of Greenwood, et al. (1966) and FAO “Identification Sheets” (FAO 1975). Common English names of fish species were adopted from De Bruin, et al. (1994) and Rainboth (1996) while the local names were based on the vernacular names used around the lake area. 4 J. Environmental Science and Environment, Vol. 3 (2015) Assessment of aquatic vegetation in lake was conducted through a spot check survey using a random quadrat sampling method. At least five quadrats (measuring 0.5m x 0.5m) were thrown randomly in each sampling point. Distance between each sampling point was approximately 300m. Percent cover of submerged plants was estimated, while counts of associated fauna, particularly the mollusk, were obtained in each quadrat. For broader estimates of aquatic vegetation, percent cover was read on board the boat while cruising and specific points were marked by a global positioning system (GPS). In areas with very low visibility, however, no estimates of percent cover were obtained. An assessment of plankton communities was conducted earlier by Tumanda, et al. (2006) and was, therefore, not included in this study. RESULTS AND DISCUSSION Finfish Diversity At least 41 species of finfish belonging to 8 orders, 24 families and 29 genera were identified in Lake Mainit and Kalinawan River based on collected samples from fishermen’s catch and market surveys (Table 1). This number exceeds the 37 species reported by Pauly et al. (1990) suggesting that more species are now found in the lake. On the other hand, 13 species listed in Pauly et al. (1990) are not reported in the present study. Notably absent from the current list is the tarpon Megalops cyprinoides (“bulan- bulan”), the Hawaiian ten-pounder Elops hawaiiensis, and the spotted grunt Pomadasys hasta. Pauly et al. (1990) also listed five species of gourami whereas only one species is reported in the present study, although Lake Mainit fishers allege that at least three kinds of gourami still occur in the lake, albeit rarely encountered. At least four more fish species remain unidentified as no voucher specimens were collected (Table 1). A stylized distribution map of finfish resources in Lake Mainit is shown in Fig. 3. This difference in species diversity across time could be attributed to a number of factors. Some species may have become locally extinct due to overfishing or the increased prominence of introduced or exotic species (such as tilapia, carp, and catfish). Conversely, the apparent increase in species diversity of fish in Lake Mainit could be due to the migratory nature of many species that follow different temporal patterns. At least 11 of the 41 finfish species (27%) identified in the lake are freshwater fish and the rest (73%) are mostly migratory fish that either enter freshwater bodies or move out to sea at certain parts of their life cycle in order to spawn (Pauly et al. 1990). Several known marine species in the list are caught in Kalinawan River near Santiago and Tubay, Agusan del Norte. Many