Impact of nutrition and season on pond culture performance of mono-sex and mixed-sex Nile tilapia ( Oreochromis niloticus)

Abstract

A 2×2×2 factorial experiment was carried out in a 16 earthen pond system to determine the effect of poor and improved nutrient inputs on the performance of mixed-sex and mono-sex Nile tilapia over both wet and dry seasons in Central Thailand. The poor nutrient level consisted of fresh ruminant manure only (50 kg dry matter ha −1 day −1) and the improved also received inorganic fertilisers and ricebran as supplementary feed. Artificially incubated first feeding hatchlings were nursed under similar conditions to produce mixed-sex and mono-sex fry before stocking in 200 m 2 ponds at 3 fish m −2. Growth and net yields after 5 months were affected by nutrient level but not by sex (mono- or mixed-sex). However, ponds stocked with mixed-sex fish had a significantly greater proportion of small fish (<15 cm TL) than mono-sex that had more large size class fish (>15 cm TL). Mean weights of fish in each size class were not different overall but if ponds with improved nutrition were considered alone, survival and use of mono-sex affected the mean size of recruits (i.e. individuals <10 cm) and stocked fish (10–20 cm) harvested. Individual harvest size was inversely related to survival. The lack of major differences in performance of mono-sex and mixed-sex was linked to the presence of piscivorous fish and the method of seed production. The pattern of survival among the different treatments was possibly related to the movement of the air-breathing fish between ponds. The homogeneous and young age of the mixed-sex seed may explain the relatively low levels of reproduction, indicated by numbers of fry, observed in the trial. The importance of nutrient levels, rather than the use of mono-sex fish in the Chitralada strain, was demonstrated, provided young mixed-sex seed are used. The study has implications for promoting smallholder tilapia production; both the nature of demand for fish and the resource base need to be understood before developing tilapia hatchery strategies.