Lakes & Reservoirs: Research and Management 2005 10: 157–166 Blackwell Publishing, Ltd. Original Article Limnology and culture-based fisheries Limnology and culture-based fisheries in non-perennial reservoirs in Sri Lanka U. Asanka D. Jayasinghe, 1 Upali S. Amarasinghe 1 and Sena S. De Silva 2 * 1 Department of Zoology, University of Kelaniya, Kelaniya, Sri Lanka, and 2 School of Ecology and Environment, Deakin University, Warrnambool, Victoria, Australia Abstract This study was carried out to investigate the possibility of using the limnological characteristics of non-perennial reser voirs in Sri Lanka for the future management of culture-based fisheries. Forty-five reservoirs were randomly selected to study their limnology, out of which 32 were stocked with fish fingerlings of Chinese and Indian carps, tilapia and freshwater prawn at stocking densities ranging from 218 – 4372 fingerlings ha -1 . Of these, 23 reservoirs were harvested at the end of the culture period (6 –10 months). Thirteen limnological parameters were measured during the water retention period of each of the 45 reser voirs between November 2001 and Januar y 2004. The mean values of the limnological parameters were used to ordinate the reservoirs through principal component analysis. Ordination showed a productivity gradient among reservoirs where Secchi disc depth, total phosphorus, chlorophyll- a, inorganic turbidity and organic turbidity were identified as key factors. The total fish yield of culture-based fisheries was positively correlated to the scores of the first principal component axis. This study reveals that it is possible to classify non-perennial reservoirs in Sri Lanka based on the above limnological parameters in order to develop culture-based fisheries and that they could be applicable in comparable water bodies elsewhere in the tropics. Key words carp, culture-based fisheries, fish yield, limnology, reservoirs, Sri Lanka, tilapia. INTRODUCTION Water quality monitoring is an essential activity in regard to intensive and semi-intensive aquaculture systems, and much has been written on the subject. Culture-based fisheries in small, non-perennial water bodies are essentially an extensive form of aquaculture (De Silva 2003; Felsing et al. 2003). As extensive aquaculture systems are low-input systems, little emphasis has been placed on water quality- monitoring efforts. However, for large lakes and reservoirs, fish yield prediction models based on limnological data (Melack 1976; Hanson & Legget 1982; Nissanka et al. 2000; Gomez et al. 2002), morphometric data (Rawson 1952; Moreau & De Silva 1991), and combinations thereof (Ryder 1965, 1982; Henderson & Welcomme 1974), have been topical in many parts of the world, often being used to manage the fisheries. Unfortunately, only a few studies have reported on the limnological characteristics of small water bodies in relation to culture-based fisheries, and even these few are mainly restricted to oxbow lakes in Bangladesh (Hasan et al. 1999, 2001) and farmer-managed small reservoirs in Vietnam (Nguyen et al. 2001). An estimated 15 000 small, non-perennial reservoirs are found in Sri Lanka (Department of Agrarian Services 2000). They are located in the dry zone of the island, a region that receives < 187 cm of rainfall annually. These small, non-perennial reservoirs (< 60 ha) are rainfed from the north-east monsoon, retaining water for ≈ 7– 9 months, from November–December to July–October. There are no established commercial fisheries in these non-perennial water bodies, in contrast to large (> 200 ha) perennial reservoirs, where the fisheries are based mainly on naturally recruiting populations of exotic cichlids (De Silva 1988; Amarasinghe 1998). Since the early 1980s, the development of culture-based fisheries in non-perennial reservoirs has received considerable attention, being included in the national inland fisheries development plans (Thayaparan 1982; Chakrabarty & Samaranayake 1983). These culture-based fishery activities, however, were unsustainable. De Silva (1988, 2003) discussed the likely reasons for the overall *Corresponding author. Email: sena.desilva@deakin.edu.au Accepted for publication 18 May 2005.