Upali S. Amarasinghe

An Analysis of Fish Species Richness in Natural Lakes

There is a growing recognition of the need to conserve biodiversity that has been conceptualised in the Convention of Biological Diversity. Maintenance of fish species richness is particularly important, because habitat degradation in inland waters continues to accelerate on a global scale. Here we develop empirical models for predicting fish species richness in natural lakes in various geographical regions of the world. In tropical lakes where fish biodiversity is richer than in temperate lakes, fish species richness can be predicted by a few variables such as lake area and altitude. Low fish species richness in most temperate lakes might be due to the effect of glaciation on colonisation and speciation of fishes. In US, Canadian and northern European lakes, lake acidification is one of the important factors influencing fish species richness. Although limnological characteristics influence fish species richness in temperate lakes, lake area and altitude have greater predictive power. This is in contrast to fish species richness in rivers, which can be reliably predicted by basin area. In the power curves, which describe the relationship between fish species richness and habitat size in lakes and rivers, the exponent is always greater in tropical regions than in temperate regions. Because fish biodiversity is greater in the tropics threats to fish biodiversity through habitat degradation are greater than those in temperate inland waters.

Alien Species in Aquaculture and Biodiversity: A Paradox in Food Production

Abstract Aquaculture is seen as an alternative to meeting the widening gap in global rising demand and decreasing supply for aquatic food products. Asia, the epicenter of the global aquaculture industry, accounts for over 90% of the global aquaculture production quantity and about 80% of the value. Asian aquaculture, as with global aquaculture, is dependent to a significant extent on alien species, as is the case for all the major food crops and husbanded terrestrial animals. However, voluntary and or accidental introduction of exotic aquatic species (alien species) is known to negatively impact local biodiversity. In this relatively young food production industry, mitigating the dependence on alien species, and thereby minimizing potential negative impacts on biodiversity, is an imperative for a sustainable future. In this context an attempt is made in this synthesis to understand such phenomena, especially with reference to Asian inland finfish, the mainstay of global aquaculture production. It is pointed out that there is potential for aquaculture, which is becoming an increasingly important food production process, not to follow the past path of terrestrial food crops and husbanded animals in regard to their negative influences on biodiversity.

Assessment of fishing effort in Parakrama Samudra, an ancient man-made lake in Sri Lanka

Abstract As in most artisanal fisheries, assessment of fishing effort is crucial to the management of the artisanal fishery of Parakrama Samudra, an ancient man-made tropical lake in Sri Lanka. Since there were few similarities between alternative ways of expressing effort for the gill-net cichlid fishery is the lake, this study aimed to find the most effective formulation of catch per unit effort (CPUE). The variability of preliminary estimates showed that, before analysis, the data needed to be stratified for different seasons with low and high water levels, and for three geographically distinct areas of the lake. Standardised catch per man was higher in boats operated by smaller numbers of fishermen, but catch per net did not change with the number of nets per boat, and is therefore recommended as the best measure of CPUE. The measure of fishing effort also needs to take account of fishermen who increased their catch by a water-beating technique. The incidence of beating varied with season and area, but could account for a significant portion of the catch. The highest catch efficiencies derived from one-man boats using this technique. Using this method of assessing effort, total annual yield from Parakrama Samudra was estimated at about 120 kg ha −1 , a lower value than previously reported.

Seasonality in dietary shifts in size-structured freshwater fish assemblages in three reservoirs of Sri Lanka

We studied seasonal changes in feeding habits of size-structured fish assemblages in three freshwater reservoirs in Sri Lanka. We obtained fish samples for diet analysis from three reservoirs during the months of rising water level (RIWL) and receding water level (REWL). During RIWL, peripheral areas with terrestrial/semi-terrestrial macrophytes get inundated, which resulted in increased food availability for macrophyte feeding fish. During REWL, detrital food sources increase due to decaying terrestrial plant material, which has inundated. Based on the dietary habits of individual species in the three reservoirs during RIWL and REWL, it is evident that detritivorous and phytoplanktivorous fish species, which belong to low trophic levels do not show significant variation in dietary habits between the two seasons (e.g. Oreochromis mossambicus, Oreochromis niloticus and Amblypharyngodon melettinus). Stenophagous species such as benthic invertebrate predators (Puntius chola and Puntius dorsalis), macrophytophagous species (Etroplus suratensis and Tilapia rendalli) and zooplanktivorous Hemirhamphus limbatus do not exhibit significant variation in dietary habits between seasons. Also their feeding habits do not vary with body size. Euryphagous species such as Puntius filamentosus on the other hand, exhibit variations in dietary habits between seasons as well as with body size. Stenophagous species, which feed on detritus and phytoplankton and euryphagous species, which exhibit temporal dietary plasticity, are therefore abundant in fish assemblages of these reservoirs.

A comparative assessment of the fisheries in lacustrine inland waters in three Asian countries based on catch and effort data

Abstract The catch and effort data of the artisanal fisheries of the natural lakes and reservoirs in the Philippines, and reservoirs in Sri Lanka and Thailand are evaluated. The yield (kg ha −1 year −1 ) to effort relationship of the Philippines lakes and Thai reservoirs conformed to a second-order curve, and that of Sri Lankan reservoirs was linear. Similarly, the catch per unit effort ( CPUE ) to effort per unit area relationships for the Philippines and Thai fisheries were similar and conformed to the Fox model. The relationship of the mean annual yield (kg ha −1 ) to mean effort in the Sri Lankan reservoirs when considered as a whole, on a yearly basis, conformed to the Schaefer model. On the basis of these relationships the effort which gave the maximum yield per annum was computed for each fishery. The yields were 515, 256 and 93 kg ha −1 for efforts of 20, 6.4 and 10 fishermen for the fisheries of the Philippines, Sri Lanka and Thailand, respectively. An attempt is made to explain the reasons for the apparent differences in these three artisanal fisheries, and the importance of the models in the management of the respective fisheries are discussed.

Minor cyprinid resources in a man-made lake in Sri Lanka: a potential supplementary source of income for fishermen

Abstract The reservoir fishery in Sri Lanka is based mainly on the exotic species Oreochromis mossambicus, and indigenous cyprinid resources are not exploited. Four species of minor cyprinids, Amblypharyngodon mellettinus, Barbus chola, B. dorsalis and B. filamentosus, are abundant in Parakrama Samudra, a man-made lake in Sri Lanka. Experimental fishing trials with gill nets of 18-, 38-, 52- and 64-mm mesh sizes indicate that the mesh sizes below 52 mm can be used for exploiting minor cyprinids without detrimental effects on the existing O. mossambicus fishery, confirming the findings of earlier studies. The annual potential yield of minor cyprinids in Parakrama Samudra (632 kg ha−1), estimated by comparing catch per unit effort values of minor cyprinids with those in southern reservoirs in the country, is appreciably higher than that of O. mossambicus. The value of minor cyprinid resources to fishermen as a supplementary source of income is highlighted.

EVALUATION OF COMMUNITY PARTICIPATION FOR THE DEVELOPMENT OF CULTURE-BASED FISHERIES IN VILLAGE RESERVOIRS OF SRI LANKA

In this paper, an attempt is made to identify the socioeconomic characteristics of a community that influence the development and management of culture-based fisheries in village reservoirs of Sri Lanka. Socioeconomic data were collected from 46 agricultural farming communities associated with 47 village reservoirs in Sri Lanka. Principal component analysis indicated that scores of the first principal component were positively influenced by socioeconomic characteristics that are favorable for making collective decisions. These included leadership of the officers, age of the group, percentage of active members of the group, percentage of kinship of the group, percentage of common interest of the group, and percentage of participation of the group. The size of the group had a negative effect on the first principal component. The principal component scores of communities were positively related to willingness to pay (P < 0.001). The communities with socioeconomic characteristics favoring collective decision making were in favor of culture-based fisheries. Homogeneity of group characteristics facilitated successful development of culture-based fisheries.

Enhancing Rural Farmer Income through Fish Production: Secondary Use of Water Resources in Sri Lanka and Elsewhere

The inland fishery of Sri Lanka has been essentially a capture fishery from major and medium scale irrigation reservoirs. However, small-sized (<100 ha) minor irrigation reservoirs are also frequent in the country. Naturally, these water bodies are incapable of supporting self-recruiting fisheries, but can be utilized to enhance the fish production significantly through development of culture-based fisheries (CBF), without causing impediment on their primary use.

Promotion of sustainable capture fisheries and aquaculture in Asian reservoirs and lakes

A collaborative international project funded by the European Unions INCO-DC programme is undertaking limnological, fish biological, environmental and socio-economic research in five tropical lakes and reservoirs in Sri Lanka, Thailand and the Philippines over the period 1998–2001. The aim is to determine their trophic structure and their capacity to sustain both their existing fisheries and present and future aquaculture. In some cases, these activities could potentially be expanded for the benefit of rural communities and of the local market within the bounds of social and environmental sustainability. This paper describes the concepts and methods involved in this innovative multidisciplinary project which aims to integrate limnological, fisheries and socio-economic issues in a comparative approach involving Asian and European research teams.

Some aspects of photosynthetic characteristics in a set of perennial irrigation reservoirs located in five river basins in Sri Lanka

Phytoplankton primary productivity of eleven irrigation reservoirs located in five river basins in Sri Lanka was determined on a single occasion together with light climate and nutrient concentrations. Although area-based gross primary productivity (1.43–11.65 g O2 m−2 d−1) falls within the range already established for tropical water bodies, net daily rate was negative in three water bodies. Light-saturated optimum rates were found in water bodies, with relatively high algal biomass, but photosynthetic efficiency or specific rates were higher in water bodies with low algal biomass, indicating nutrient limitation or physiological adaptation of phytoplankton. Concentrations of micronutrients and algal biomass in the reservoirs are largely altered by high flushing rate resulting from irrigation release. Underwater light climate and nutrient availability control the rate of photosynthesis and subsequent area-based primary production to a great extent. However, morpho-edephic index or euphotic algal biomass in the most productive stratum of the water column is not a good predictor of photosynthetic capacity or daily rate of primary production of these shallow tropical irrigation reservoirs.