Assessment of Water Quality and Fish Producivity in Three Undrainable Reservoirs of Naromoru, Nyeri County, Kenya.

Abstract

Naromoru is a semi-arid area located on the leeward side of Mt. Kenya, and has more than 15 small sized reservoirs constructed to provide water for livestock, irrigation and domestic use. To increase their usefulness, some reservoirs have been stocked with fish, even though no studies have been carried out to establish their suitability and potential for fish culture. This study therefore aimed at assessing the water quality, fish food availability and fisheries productivity of three undrainable reservoirs of Naromoru, selected on the basis of their location, accessibility, size and fish availability. The study was carried out between September 2008 and February 2009. Water quality was determined by assessing the physical (temperature, water depth, transparency) and chemical (pH, dissolved oxygen, electrical conductivity and total dissolved solutes) characteristics. Water samples were also collected and concentrations of the major anions (bicarbonates, sulphates and chloride), cations (potassium, calcium, magnesium and sodium), heavy metals (copper and lead) and nutrients (nitrates and phosphates) measured using Atomic Absorption Spectrophotometer (ASS). Phytoplankton communities were sampled by filtering 15 litres of water through 0.063~km mesh plankton net, and their relative abundance, species composition and diversity determined. Similarly, zooplanktons were sampled by filtering 10 litres of water through 100~km, while benthic invertebrates were sampled using an Ekman grab sampler and their species composition, diversity and relative abundances determined. Fish were caught by seine and gill nets, identified and measured for standard lengths (cm) and weights (g). Length and weight measured were used in analysis of relative condition factors and length-weight relationship. Data was analysed by Kruskall-Wallis test, principal component analysis and Mann-Whitney test. Results showed that physical and chemical characteristics fell within the recommended guidelines for fish culture in all the three reservoirs, with the exception of water transparency, which measured 7.4±0.11cm at Kianda Dam. Lusoi Dam had the lowest water quality status with pH, TDS and conductivity measuring 9.6±0.05, 833±35.3mgl-1 and 1283±48.1~kScm-1, respectively. Concentration of heavy metals in the dams was generally low. The three reservoirs had abundant fish food supply, with phytoplankton dominated by the blue green algae, Microsystis aeruginosa while zooplanktons were dominated by rotifers and copepods. Cladocera only occurred in Kianda Dam. Benthic invertebrates were similarly abundant dominated by chiromids and oligochaetes. The fish had good relative condition which ranged from 1.7 to 1.9. A highly significant length-weight relationship was obtained with an R2 value of 0.9908, 0.9708 and 0.9495 at Gathathini, Lusoi and Kianda Dams, respectively, indicating an allometric fish growth. Fish productivity in the dams ranged from 47±29.6g/m2/hr to 259±150.2g/m2/hr. These results show that the undrainable reservoirs of Naromoru are generally suitable for fisheries production, even though management interventions are needed to deal with pollution problems and fish seed quality in order to achieve maximum fish productivity.