Stable isotope analysis to quantify contributions of supplementary feed in Nile Tilapia Oreochromis niloticus (GIFT strain) aquaculture

Nile tilapia (Oreochromis niloticus) is currently one of the most farmed freshwater fish and contributes significantly to total global aquaculture production. The genetically improved strain of O.niloticus (GIFT) was introduced to Papua New Guinea (PNG) in 1999 to improve food and income security. The high cost and low availability of commercial fish feed hinder the growth of GIFT farming in PNG. Stable carbon and nitrogen isotopes were used to determine the role of supplementary and natural food sources in the diet of GIFT in pond-based aquaculture. Two treatments were used: treatment 1 was daily feeding, and treatment 2 was weekly feeding, each with three replicates. Isotopic analysis of muscle tissue and all potential food sources showed that pellet feed contributed 7% to the growth of GIFT in daily-fed ponds and 33% in the weekly-fed ponds. Highly enriched N-15 values for chicken manure, compared to depleted values for GIFT and other natural food sources in both treatments, clearly indicate insignificant contributions of this input to production. After 90days of cultivation, the average final body weight of GIFT receiving daily feed inputs was 134g (average 19cm), while for weekly-fed it was 92g (17cm). The feed conversion ratio (FCR) was poor (6.4:1) in the daily-fed GIFT ponds compared to a better, and preferable, FCR (1:1) in the weekly-fed ponds. The findings of this study show that pelleted feed was not the major contributor to the growth of GIFT. Genetically improved farmed tilapia aquaculture should focus on enhancing natural food availability for fish production.