In this study we examined the costs and effectiveness of using larvivorous fish, Poecilia reticulata, and a chemical larvicide, temephos, in anopheline mosquito control in the riverbeds below the major dams in Sri Lanka. Five riverbeds below the dams, namely Laxapana, Kotmale 1, Kotmale 2, Nilambe, Rantembe and Victoria, were selected. Riverbed pools in Laxapana and Kotmale 1 were treated with P. reticulata; Rantembe and Victoria were treated with temephos; and Kotmale 2 and Nilambe were kept as controls. In each area, the anopheline larval density, before and after application of fish/temephos, was estimated. The cost of application of fish/temephos was estimated by activities involved for each treatment. After intervention, there was a significant reduction in anopheline larval density in the fish-treated areas compared with the temephos-treated and control areas. Application of P. reticulata was 2.67 times less costly than that of temephos. The cost of fish application can be further reduced if the community is involved in the application.

The larvivorous potential of Poecilia reticulata was studied in the laboratory and in the field. In the laboratory, the number of anopheline larvae consumed per fish per day and the number of larvae in fecal matter per fish were determined. In the field study, 29 of 60 selected riverbed pools in Kotmale oya, below the Kotmale dam, were stocked with P reticulata, whereas the rest served as controls. Anopheline larval surveys were carried out starting from 1 day prior to stocking of fish and on selected days subsequently. P. reticulata consumed an average of 117 +/- 32.33 larvae per fish per day. Fecal matter of P. reticulata contained an average of 2.7 +/- 2.68 larvae per fish . There was a significant reduction in the number of pools positive for anopheline larvae (P < .001) after stocking fish and the number of larvae per pool (P < .001) and per 100 dips (P < .001) as compared with controls.

Benthic animals are generally considered to be good indicators of environmental conditions of aquatic ecosystems. The present study was undertaken to study the spatial variation of macrobenthic community in Negombo estuary (7° 6' - 7° 12' N; 79° 49' - 79° 53' E), which is subjected to a variety of anthropogenic activities. Benthic samples were obtained from 25 randomly selected sites in Negombo estuary in March 2003 and the macrobenthos were separated by wet sieving and identified as much as possible. The diversity of macrobenthos was determined using Shannon-Wiener index and the similarities among the macrobenthic communities at different sites were determined using Bray-Curtis similarity coefficient. Seventy-six species of benthic invertebrates belonging to 41 families were recorded during the study. Of the gastropods, those belonging to families Assimineidae, Cerithiidae, Haminoeidae and Hydrobiidae were the most abundant. The most abundant polychaete families were Pilargidiidae and Heterospionidae. Among amphipods, aorids were recorded in most of the sampling sites. The sampling sites were separated into several clusters based on the diversity of macrobenthos. The major environmental factors that influenced the separation of these clusters appeared to be salinity, presence of mangroves and soil texture. DOI: 10.4038/sljas.v11i0.2222Sri Lanka J. Aquat. Sci. 11 (2006): 43-61

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