Economic values of growth rate, feed intake, feed conversion ratio, mortality and uniformity for Nile tilapia

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إنجليزي. The aim of the study was to derive the economic value (s) (EVs) of growth rate, feed intake, feed conversion ratio, mortality and uniformity for Nile tilapia (Oreochromis niloticus). A smallholder production system where fish are cultured in earthen ponds and oxygen is a limiting factor for production, was simulated using a deterministic bio-economic model. Traits of interest were: thermal growth coefficient (TGC), thermal feed intake coefficient (TFC), feed conversion ratio (FCR), mortality rate (M) and uniformity of harvest body weight (U). Two breeding objectives were considered: a breeding objective with TGC, TFC, M and U (H-1 model), and a breeding objective with TGC, FCR, M and U (H-2 model). Gross margin (GM) was simulated before and after one genetic standard deviation improvement (sigma(a)) on a trait while other breeding objective traits were kept unaltered. EVs (US$/kg/sigma(a)) were derived as: change in GM/(farmproduction before genetic improvement). Results show that EVs of TGC differ depending on the definition of the breeding objective. The EV of TGC was 1.19 when TFC was in the breeding objective. In contrast, EV was only 0.02 when FCR was in the breeding objective. This difference is caused by the way EVs are calculated: the increase in gross margin resulting from a marginal increase in TGC while keeping other traits (FCR or TFC) constant. EV of TFC was 1.25 and the EV for FCR was 0.41. EVs for M and U were 0.06 and 0.02, irrespective of the model used. Improving TGC reduced the overall grow-out period, increasing the number of production cycles in the farm. In H-1 model, reduced grow-out period was accompanied by a decrease in the amount of feed used in the farm (-272.24 kg/year) and in individual fish oxygen consumption (-1.67 g/fish/year), resulting in an increase in gross margin. In H-2 model, increasing TGC resulted in a reduced grow out period but also increased feed used in the farm (5.73kg/year) and increased individual fish oxygen consumption (0.57 g/fish/year). We conclude that the EV of TGC depends on which breeding objective is used. Faster growing fish consume more oxygen, and unless faster growth is accompanied by improved FCR, this will lead to oxygen limitations, necessitating lower stocking densities. Our results thus strongly confirm the economic importance of reducing FCR, irrespective of the model used.