The aim of the study was to evaluate the performance of Litopenaeus vannamei in an intensive photo-heterotrophic hypersaline system with minimal seawater replacement, and establish relationships between parameters of a stochastic production model and relevant water quality variables. Six experimental 1000 m 2 lined ponds were stocked at a density of 120 shrimp m À2 for a 105-day trial. Salinity increased from 37 to 45 AE 2 g/L, and the water level was maintained with the weekly addition of filtered seawater, equivalent to 1.6% per day. The stochastic model predicted that, at harvest, there is 95% confidence that the system produces between 12.1 and 14.7 t/ha with a mean final individual weight of 13.1 g and a mean survival of 84.2%. Sensitivity analyses showed that dissolved oxygen and individual final weight of shrimp were the main variables influencing yield variance.Nitrogenous compounds were maintained between optimal cultivation levels (NH 3 -NH 4 + = 0.73 AE 0.43 mg/L, N-NO 2 À = 0.09 AE 0.05 mg/L, N-NO 3 À = 3.22 AE 0.11 mg/L). Heterotrophic bacteria (6.6 AE 3.4 9 10 5 CFU/ml) and chlorophyll-a concentration (108.5 AE 80.2 lg/L) showed a similar development pattern, indicating a strong relationship between bacteria and microalgae during cultivation. Vibrio spp. concentrations were low (1.24 AE 1.42 9 10 3 CFU/ml). It was shown that the photo-heterotrophic system could be used in hypersaline conditions, typical of semi-arid regions, to consistently produce between 12.1 and 14.7 t/ha in 15 weeks.
K E Y W O R D Shypersaline, intensive cultivation, Litopenaeus vannamei, minimal seawater replacement, photo-heterotrophic, stochastic modelling
A stochastic bioeconomic model was used to estimate the production cost and risk factors affecting economic performance of juvenile redclaw crayfish, Cherax quadricarinatus, commercial nursery production, in a farm located in Baja California Sur, Mexico. At harvest, the biological submodel showed there is 95% confidence to obtain an average weight of 19.98 g, with 126,341/ha surviving organisms, representing 84% survival. Biomass was 2.5 t/ha, with a feed conversion ratio (FCR) of 0.98, reflecting the efficient adaptation of the species to culture conditions. The economic submodel showed that at Day 90, there is 95% confidence that the cost of producing a preadult will be US$0.090 or US$4.55/kg. With increasing demand and worldwide sale prices of US$9/kg, there is a potential market for 20–30 g redclaw reared in intensive lined ponds. Sensitivity analyses showed that the regression coefficients from a second‐order polynomial used for calculation of FCR associated with time (a2 and a1) were the most important parameters influencing the production cost. Mortality rate and parameters used to calculate individual weight also had considerable influence on production cost. The bioeconomic analysis contributes significantly to establishing the economic viability of the intensive three‐phase production system.
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