Protected Areas and World Heritage Programme

NAME Masso Reservoir

LOCATION 13^o27'N, 144^o42'E; this pond is located in central Guam in the municipality of Piti. It lies about 600 m east of the intersection of Highways 1 and 6, or about 300 m east of the Guam Veterans Cemetery in Piti village.

AREA 2 ha.

ALTITUDE 12 m.

OVERVIEW A small man-made pond built as a potable water reservoir. Common Moorhens (Gallinula chloropus guami) occur on the site.

PHYSICAL FEATURES The reservoir was created by damming the Masso River. A dike and spillway, 65 m long, are present on the west bank. Water depth in the pond is about 4 m (Biosystems Analysis, Inc. 1989a) and falls slightly during the dry season. Hosmer (1982) reported on a variety of water analyses conducted here. The reservoir's watershed covers approximately 200 ha. Soils in the area are derived from argillaceous limestone and are shallow and well drained (Young, 1988).

ECOLOGICAL FEATURES This wetland has 75% open water, with large aquatic beds of Hygrophila difformis common. Dense stands of Phragmites karka grow along the southeastern and northeastern shores. The site is classified as PAB2Hh, PEM1F and POWHh (Cowardin et al., 1979). Small to medium-sized trees were planted on the dike in the late 1970s. Extensive grasslands grow on the east and south sides, while secondary forest occurs to the north and west.

LAND TENURE The wetland and surrounding property are owned by the U.S. Navy but have been pending transfer to the Government of Guam for several years.

CONSERVATION MEASURES TAKEN The Navy and Government of Guam signed a perpetual use agreement in 1976, allowing the Guam Department of Agriculture to manage the reservoir and immediate area (2.2 ha) for public use. The reservoir is listed as a wetland of secondary importance for Common Moorhens (U.S. Fish and Wildlife Service, 1991).

CONSERVATION MEASURES PROPOSED A recovery plan for moorhens lists the activities needed to protect this species on Guam (U.S. Fish and Wildlife Service, 1991). Actions that particularly apply to this wetland include: (1) the development and implementation of habitat management plans through cooperative management agreements between the Government of Guam and the U.S. Fish and Wildlife Service; (2) the conducting of regular censuses for moorhens; (3) the periodic monitoring of the site for toxic substances; and (4) the control of introduced predators such as Brown Tree Snakes (Boiga irregularis) and feral dogs and cats.

LAND USE The Masso Reservoir was constructed by the U.S. Navy in about 1945 for the storage of drinking water, however, excessive siltation caused it to be abandoned in 1951 (Tucker and Kock, 1979). In 1978, the Guam Division of Aquatic and Wildlife Resources renovated the site with the goal of turning it into a public park and fishing area (Tucker and Kock, 1979). This involved the removal of one ha of Phragmites, the repair of the spillway and the planting of exotic trees and shrubs on the banks of the reservoir. In 1980 and 1981, the Division stocked the pond with hybrid tilapia (Oreochromis mossambicus x O. niloticus), mosquitofish (Gambusia affinis) and tucunare (Cichla ocellaris) (Hosmer 1982). However, vandalism and illegal fishing with chlorine caused the management program to be terminated in 1983 (Molina, 1983). The reservoir and surrounding grasslands are currently unused.

POSSIBLE CHANGES IN LAND USE No changes in land use are foreseen at the reservoir, but a condominium complex is proposed for construction 1.3 km upstream of the reservoir.

DISTURBANCES AND THREATS Siltation of the reservoir is a chronic problem and is worsened by grassland fires that cause increased erosion in the catchment area. Proposed development is likely to worsen soil run-off. Illegal fishing with chlorine occurs infrequently in the pond and threatens aquatic animal life.

HYDROLOGICAL AND BIOPHYSICAL VALUES Useful in holding eroded soil sediments.

SOCIAL AND CULTURAL VALUES None known.

NOTEWORTHY FAUNA Common Moorhens are seen occasionally at the reservoir. Further observations may find that nesting occurs here. In 1978, the aquatic fauna of the reservoir included two species of eel (Anguilla marmorata and A. bicolor), three gobies (Awaous guamensis, Stiphodon elegans and Sicyopterus macrostetholepis), a sleeper (Eleotris fusca) and several shrimp (Macrobrachium lar, Atya sp. and Caridina sp.) (Tucker and Kock, 1978). In 1980 and 1981, the pond was stocked with hybrid tilapia, mosquitofish and tucunare (Hosmer, 1982). Some of these species may have since disappeared from the reservoir because of poisoning with chlorine.

NOTEWORTHY FLORA None known.

SCIENTIFIC RESEARCH AND FACILITIES Limnological and faunal surveys were made by Tucker and Kock (1978) and Hosmer (1982).

CONSERVATION EDUCATION There is little potential for education purposes under present conditions.

RECREATION AND TOURISM A minor amount of recreational fishing still occurs at the reservoir.

MANAGEMENT AUTHORITY AND JURISDICTION Management authority: Division of Aquatic and Wildlife Resources and U.S. Navy. Territorial jurisdiction: Territory of Guam and U.S. Navy. Functional Jurisdiction: Division of Aquatic and Wildlife Resources and U.S. Navy.

REFERENCES Biosystems Analysis, Inc. (1989a); Hosmer (1982); Molina (1983); Tucker and Kock (1979); U.S. Fish & Wildlife Service (1991); Young (1988).

REASONS FOR INCLUSION 2a. Habitat for moorhens.

SOURCE Gary J. Wiles and Michael W. Ritter.