Protected Areas and World Heritage Programme

NAME Agana Swamp

LOCATION 13^o28'N, 144^o45'E; the marsh is located in west-central Guam in the municipalities of Agana, Sinajana and Mongmong-Toto-Maite. It is found 250 m southeast of the intersection of Highway 4 and O'Brien Street (Highway 7).

AREA 112 ha.

ALTITUDE 1-3 m.

OVERVIEW Long referred to as the Agana Swamp, this is Guam's largest natural freshwater marsh. It is vegetated primarily with dense stands of Phragmites karka. Common Moorhens (Gallinula chloropus guami) nest here. The site is largely surrounded by urban and suburban development.

PHYSICAL FEATURES Randall and Tsuda (1974) described the physiography and geology of the Agana Swamp and its surrounding environment. The main body of the marsh lies in a broad low-lying basin along the Agana River, with several narrow fingers of marshland extending up to 1.5 km to the north, east, and south. Important sources of water for the wetland are run-off from surrounding hills, rainfall, several springs (including the Agana Springs) and the Chaot River, which enters from the south. Drainage occurs via a channelized section of the Agana River, 800 m long, which empties into Agana Bay. Inside the marsh, the Agana River channel is poorly defined and silted in. The marsh's water level ranges from 5-30 cm in the rainy season, but only depressions and the river channel retain standing water during the dry season (Randall and Tsuda, 1974). Soils consist of deep poorly drained muck derived from decomposed organic matter (Randall and Tsuda, 1974; Young, 1988).

The Agana Springs are located on the south side of the marsh at the base of a limestone ridge. A pair of retaining walls and a low dam are present, and were once part of a municipal water pumping station (Smalley and Zolan, 1981). Maximum depth at this site is 2-3 m. Water quality characteristics and hydrology of the marsh and Agana Springs are described and discussed by several authors (Smalley and Zolan, 1981; Ayers and Clayshulte, 1983; U.S. Army Corps of Engineers, 1985).

Low limestone hills and ridges border the marsh and project inward at a number of locations. Isolated limestone hills and hummocks occur inside the marsh and are 6-12 m high. The entire Agana-Chaot river basin covers 2,700 ha (U.S. Army Corps of Engineers, 1985).

ECOLOGICAL FEATURES The wetland is dominated by dense stands of Phragmites karka, with Hibiscus tiliaceus common along the edges and scattered throughout (Randall and Tsuda, 1974; Moore et al., 1977; Smith & Hedlund, 1978). A fairly large bed of Eleocharis ochrostachys is present in the northeast (Wiles, pers. obs.). A few small openings in the Phragmites exist, but only several have open water. The opening at the Agana Springs is choked with Eichhornia crassipes, while the channel for the Agana River at the O'Brien Street bridge is covered by Pistia stratiotes, Panicum muticum and a small amount of E. crassipes. Two small plots on the north and northwest sides of the wetland have been cleared of Phragmites and planted with taro (Cyrtosperma chamissonis and Colocasia esculenta). Other plants in the marsh include Acrostichum aureum, Thelypteris interrupta, Eleocharis geniculata, Polygonum minus, Cyperus polystachyos, Lemna perpusilla and Ipomoea aquatica. The habitat classification of the marsh is PEM1F, PFO3C, PEM1Ff, R2EM1Hx and R2AB4Hx (Cowardin et al., 1979).

Dryland vegetation on the boundaries of the marsh is variable and highly disturbed (Randall and Tsuda, 1974). Momordica charantia, Passiflora foetida, Mikania scandans, Chromolaena odorata and Antigonon leptopus are common in weedy openings. Scrubby forests contain Leucaena leucocephala, Heterospathe elata, Cocos nucifera, Pandanus tectorius, Triphasia trifolia and Morinda citrifolia.

LAND TENURE Most of the wetland is privately owned. The Government of Guam owns two tracts of property in the marsh totalling about 17 ha. Surrounding areas are mostly private.

CONSERVATION MEASURES TAKEN A small area around the Agana Springs was declared a conservation preserve in the 1970s by the Guam Department of Parks and Recreation, and developed as a park for nature education (Belk et al., 1971), but the facilities have fallen into disrepair in recent years. The entire marsh is considered a wetland of primary importance for Common Moorhens (U.S. Fish and Wildlife Service, 1991). In order to protect moorhen habitat, the U.S. Fish and Wildlife Service issued a "jeopardy opinion" on a filling and construction project that was proposed for a corner of the marsh in 1987.

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

A proposal to designate land owned by the Government of Guam in the marsh and along the Chaot River as conservation reserves was submitted by the Guam Department of Agriculture to the Guam Departments of Land Management and Parks and Recreation in 1989. Recent consideration has also been given by the government to acquire privately owned land in the marsh for conservation purposes.

LAND USE Colonial administrations dating back to at least the 1850s regularly considered or attempted to drain the marsh to encourage farming and for public health reasons (Safford, 1905; E.G. Johnston in Randall and Tsuda, 1974). The Spanish built a set of dikes in the marsh in the 1800s in an apparently unsuccessful effort to grow rice. In 1933-1934, a channel six metres wide was cut from the north side of the marsh to the Agana Springs, a distance of 1,750 m (Seabury, 1934; Randall and Tsuda, 1974). This resulted in two-thirds of the marsh being drained of standing water and allowed corn to be planted. Safford (1905) and Thompson (1947) both reported that some of the marsh was used for taro and vegetable gardening. The Agana Springs was used as a municipal water source from 1937 to 1957, with 3.8-9.5 million litres of water pumped daily (Smalley and Zolan, 1981). During the last 20-30 years, most of the marsh has been unused. Currently, taro is cultivated on two small plots.

The Agana Swamp occurs in a large urban centre. Land surrounding the marsh is occupied by a number of businesses and homes. Only the hills on the southeast side of the marsh remain relatively undeveloped.

POSSIBLE CHANGES IN LAND USE No development of the marsh is currently planned. However, continued commercial and residential growth on adjacent lands is expected in the future.

DISTURBANCES AND THREATS Over the years, drainage patterns have undoubtedly been modified by development in and around the marsh. Two important factors in this process have been small- and large-scale filling along the edges (Stinson et al., 1991) and siltation caused by human activity and the deposition of decaying wetland vegetation. An electrical powerline and gravel access road bisect the north end and have reduced drainage except through one depression. The Agana Shopping Center was built in 1978 on a four-hectare fill in the northwest corner of the marsh. Proposals were put forth in the 1970s and again in the 1980s to construct large flood control projects inside the marsh and along the lower Agana River (U.S. Army Corps of Engineers, 1985). The projects would have resulted in the building of several levees along the northern boundaries of the marsh. Neither project was initiated but the plans could be revived in the future.

Thick beds of Phragmites clog the wetland and prevent other more favourable emergent plants from becoming established. This probably reduces the attractiveness of the wetland for moorhens and other wildlife. Intense dry season fires occasionally burn off large areas of Phragmites. Four major fires (in 1983, 1987, 1988 and 1990) occurred between 1981 and 1991. Fires during the 1960s may have played a role in the decline of the Nightingale Reed-warbler (Acrocephalus luscinia) at this site (Reichel et al., 1992).

The Chaot River has been chronically polluted with raw sewage since at least 1981. Other probable sources of pollution are run-off from adjacent urban areas and illegal dumping of garbage. A military fuel pipeline crosses two upper sections of the wetland and is a pollution threat should a leak occur.

In 1989, the Public Utility Agency of Guam installed two new water wells near the Agana Springs. A total of 5.7 million litres/day is removed from the wells. This could have some affect on the amount of water entering the marsh.

HYDROLOGICAL AND BIOPHYSICAL VALUES The marsh is valuable for a number of reasons including sediment trapping, flood control and maintenance of water quality.

SOCIAL AND CULTURAL VALUES The dikes dating from the Spanish era remain in the marsh, but are heavily overgrown with Phragmites. The dikes are listed on the National Register of Historic Places. Potsherds and shell artifacts have been found on hummocks in the swamp, suggesting prehistoric use by Chamorros.

NOTEWORTHY FAUNA Common Moorhens were commonly recorded in the Agana Swamp in 1945 (Baker, 1951), but the population has been relatively small during the last 20-30 years. This may result from the presence of large Phragmites stands. Regular nesting occurred in one of the taro plots in the 1980s. The marsh is believed to be an important roosting site for significant numbers of Yellow Bitterns (Ixobrychus sinensis). It is the only recorded location for the White-browed Crake on Guam (Porzana cinerea) (Baker, 1951), which disappeared sometime after World War II. It is also the last site inhabited by the Nightingale Reed-warbler, which became extirpated in about 1969 (Reichel et al., 1992). The preservation of the marsh would assist in reintroductions of these species from other islands, should such efforts ever be attempted. Feral pigs (Sus scrofa) were a nuisance to taro farmers in 1991 and may be fairly common in parts of the marsh.

Randall and Tsuda (1974) and Smith and Hedlund (1978) describe the aquatic fauna of the marsh. Native species include shrimp (Macrobrachium lar), freshwater eels (Anguilla marmorata and A. bicolor), two gobies (Awaous guamensis and Stiphodon elegans) and a sleeper (Eleotris fusca). Introduced fish include tilapia (Oreochromis mossambicus and Tilapia zilli), catfish (Chlarius macrocephalus), mosquitofish (Gambusia affinis) and a guppy (Poecilia reticulatus).

NOTEWORTHY FLORA Checklists of plants are given by Randall and Tsuda (1974), Moore et al. (1977) and Smith and Hedlund (1978).

SCIENTIFIC RESEARCH AND FACILITIES Limnological, hydrological and biological surveys were made by Randall and Tsuda (1974), Moore et al. (1977), Smith and Hedlund (1978), Smalley and Zolan (1981) and Ayers and Clayshulte (1983).

CONSERVATION EDUCATION Efforts were made in the 1970s to use the Agana Springs Nature Preserve as a teaching site for conservation education (Belk et al., 1971), but the program was abandoned by 1980. The marsh's location in the population centre of Guam makes it an ideal site for future education efforts.

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

REFERENCES Ayers & Clayshulte (1983); Baker (1951); Belk et al. (1971); Moore et al. (1977); Randall & Tsuda (1974); Reichel et al. (1992); Safford (1905); Seabury (1934); Smalley & Zolan (1981); Smith & Hedlund (1978); Stinson et al. (1991); Thompson (1947); U.S. Army Corps of Engineers (1985); U.S. Fish & Wildlife Service (1991); Young (1988).

REASONS FOR INCLUSION 1a, 2a, 2b. This is the largest freshwater marsh on Guam. It contains habitat for Common Moorhens.

SOURCE Gary J. Wiles and Michael W. Ritter.