Anaiwilundawa Tank Sanctuary

Location: 70 45’ 0 N and 790 47’ 0 E to 70 42’ 0 N and 790 59’ 0 E; 856852 N and 365834 E to 851308 N and 371333 E; 15 km North of Chilaw, in the Puttalam District of the North-western Province.

Area: Total area is 1,397 ha while the tank area is 188 ha

Altitude: 0 – 1 m

Overview: An ancient system of seven cascading tanks dating back to the 12th century; the site is a unique wetland inthe Indian Ocean. The tanks range between 12-50 ha in size and are surrounded by traditional rice fields,scrubland, homesteads, coconut plantations and aquaculture farms. Based upon its unique biodiversity,cultural heritage and functional values, the Anaiwilundawa Sanctuary was declared a Ramsar site in 2001.In addition, BirdLife International recognises this site as an IBA.

Physical features: The tanks, which are less than 4m deep, vary in size between 12-50 ha, totalling an area of approximately188 ha. The seven tanks are separated from each other by low earthen bunds, which are now largelyovergrown. The tanks sustain about 412 ha of traditional paddy fields, which are interspersed with islets ofnatural vegetation. Underlain by crystalline rocks of Precambrian age, the tanks are covered by Quaternarydeposits that include sand dunes, clay, silt and red earth. The wetland complex sits on the side of an ancientlagoon that formed behind a barrier beach. Climatically, the area is located in the Dry Zone, receiving 1000-1500mm of rainfall annually. Mean annual temperature is approximately 260C. The major water suppliesare derived from the surface runoff from the Rathambala Oya basin (215 km2) and spill water from theKatupotha tank located upstream

Ecological features: Major wetland habitat types include fresh water wetlands (cascading tanks, rivers, irrigation canals,marshes, paddy lands), salt-water wetlands (mangroves, salt marshes and maritime grasslands, brackishwater canals, beach and sea shore vegetation) and seasonally inundated vegetation mosaics around tankfringes. Some of the tanks in the cascade dry out completely during the dry season. A series of distinctiveplant assemblages occur in the mosaic of tanks, which are influenced by salinity level, biological impacts,water level fluctuations, depth and other ecological parameters. The lentic zone of a tank in the cascadeincludes open areas, areas covered with floating macrophytes (native species such as Nelumbo nuciferaand Nymphaea pubescens, and exotics such as Eichhornia crassipes) and areas with a mixture ofsubmerged, rooted and floating vegetation (consisting of Nymphaea pubescens, Utricularia sp.,Ceratophyllum demersum, Polygonum spp., and Ludwigia adscendens). The peripheral belt of tanksconsists of seasonally flooded grasslands with short grass species such as Cynodon dactylon, and sedgessuch as Cyperus spp., with Fimbristylis spp. being common.A total of 290 plant species (24 aquatic plants, 120 woody plants, 61 shrub, 49 herbaceous, 34 climbers)and 281 vertebrate species (47 fish, 11 amphibians, 34 reptiles, 168 birds, 21 mammals) have beenrecorded from the habitats associated with this wetland.

Noteworthy fauna: Among the vertebrate species recorded, 10 are endemic (3 fish, 1 amphibian, 3 reptiles, 2 birds, and 1mammal) while 21 are nationally threatened (2 fish, 1 amphibian, 6 reptiles, 8 birds and 4 mammals). Thethreatened and/or locally declining vertebrates include several wetland dependent species (Fish - Esomusthermoicos, Clarias brachysoma; Amphibians - Rana gracilis; Reptiles - Lissemys punctata, Melanochelystrijuga, Crocodylus porosus; Birds - Pelecanus philippensis, Phalacrocorax carbo, Porzana fusca, Fulicaatra; Mammals - Lutra lutra, Prionailurus viverrinus). Among the bird species, there are 37 winter visitors,including several species of waders and waterfowl. Species of economic importance include fish(Oreochromis mossambicus, O. niloticus, Anguilla bicolour, Channa spp., Etroplus spp., Mystus spp.) andprawns (Macrobrachium spp., Penaeus spp.).

Noteworthy flora: One endemic species (Vernonia zeylanica) and two nationally threatened species (Aponogeton natans andDiospyros ebenum) have been recorded from the site. The seasonally inundated vegetation mosaic foundaround the tank fringes is a unique habitat type associated with ancient irrigation tanks and is dominated bylarge trees such as Terminalia arjuna, Diospyros malabarica and Vitex leucoxylon. Mangrove species(Avicennia marina, Lumnitzera racemosa, Rhizophora mucronata) are noteworthy as this is a rapidlydepleting vegetation type in the area. Hydrophila spinosa is a commonly used indigenous medicinal plant.

Land use: Major types of land and wetland use include prawn farming, coconut and paddy cultivations and the buildingof homesteads.

Possible changes in land use: Reclamation of tank beds for homesteads and coconut plantations, expansion of shrimp farms, extractionof clay from tank beds and unregulated cattle grazing.

Hydrological and biophysical values: Anaiwilundawa is a rain fed tank system. Its main sources are surface run-offs from the Rathambala Oyabasin with a catchment of 215 km2, and spill water from the Katupotha tank located upstream. The tanksstore rainwater that is used for irrigation. In addition they play a major role in flood control, aquifer recharge,absorption or retention of pollutants or sediments, and nutrient export.

Social and cultural values: Historical records indicate that this cascading tank system was established in the 12th century. The localcommunities in the area consist mainly of traditional farmers and fishermen, who have been involved inthese livelihoods since historic times. Approximately 90% of the estimated 2,500 families in and around thesanctuary depend on agriculture, primarily paddy cultivation, using water from the tanks. The traditionalfishing community carries out their activities in a sustainable manner, owing to the high productivity of thissystem.

Scientific research and monitoring: At present, no facilities exist for research. However, regular monitoring of waterfowl is being conducted bythe CBC.

Conservation education: University students (both undergraduate and postgraduate) are taken to the site for field courses on wetlandecology

Recreation and tourism: Although few visitors (both local and foreign) visit the area, no planned activities exist at present.

Conservation measures taken: The area was declared a sanctuary under the FFPO in 1997. In 1994, the CEA prepared a wetland sitereport and conservation management plan for this site.

Conservation measures proposed: The wetland site report and conservation management plan prepared by the CEA and the biodiversityassessment conducted by IUCN made recommendations for the conservation and management of thissite.

Disturbance and threats: The site was a waterfowl hunter’s paradise before it was declared as a Sanctuary. Although this has considerably reduced poaching, illegal hunting is still occasionally reported. The mangroves are being cleared for prawn culture, while several other woody plants especially in the seasonally inundated forests are exploited for domestic purposes. Spread of invasive alien aquatic plants such as Eichhornia crassipes and Salvinia molesta has resulted in the displacement of native floating plants and facilitates siltation of the tanks. Excessive use of chemical fertilizer and biocides in the cultivation of paddy and coconut is a further threat to the tank. Reclamation of tank beds for homesteads and coconut plantations, mining of clay from the tank beds, and unregulated cattle grazing are also issues of major concern.