The Effect of Stocking Density in Hapas on Labeo victorianus Production in Ponds with High or Low C:N Ratio

Abstract

Culture of Labeo victorianus in Kenya can be used as measure of improving food security and as a means of supplementing income for rural families. This study was carried out in 18 hapas suspended in six, 150 m2 earthen ponds to investigate the effects of C/N ratios of 10 and 20 of feed inputs and stocking density on Labeo victorianus growth, water quality, total heterotrophic bacteria counts, sediment quality and pond productivity. L. victorianus juveniles were stocked in hapas at densities of 10, 15 and 25 fish m-2. All treatments were carried out in triplicate during a time period of seventy two days. A locally formulated and prepared feed containing 30% crude protein with a C/N ratio of 10 was applied. Maize flour was used as the carbohydrate source for manipulating C/N ratio and applied to the water column separately from feed. Increasing C/N ratio from 10 to 20 reduced (P<0.001) the total ammonia nitrogen (TAN), nitrite-nitrogen (NO2-N) and nitrate-nitrogen (NO3-N) in the water column and total nitrogen in the sediment (P<0.001). It also raised sediment pH, organic matter and total phosphorus (P<0.001). Increasing C/N ratio also increased phytoplankton by 13% and zooplankton biovolume by 25% in the water column (P<0.001), which was not to be used by bottom feeder Labeo victorianus. Total benthic macro-invertebrates biovolumes were also 30% higher (P < 0.05) with a C/N ratio of 20 compared to 10. The lowest protein efficiency ratio (PER), specific growth rate (SGR) and the highest food conversion ratio for the feed were recorded with a C/N ratio of 10 (P<0.05). Raising the C/N ratio from 10 to 20 increased the net yield by 15% from 1534 (C/N 10) to 1821 (C/N 20) kg ha-1 72 d-1. Based on highest growth, survival, production and net benefits, C/N ratio of 20 and a stocking density of 15 fish m-2 is optimal. Therefore, carbohydrate addition in L. victorianus culture is a promising option for sustainable aquaculture. However the underutilization of pond communities exhibited in this study indicates possibility for inclusion a water column feeder to further improve the total production per unit surface area.