We used a factorial approach to distinguish maintenance from growth requirements for protein, lysine and methionine in the black tiger shrimp, Penaeus monodon. Juvenile P. monodon (initial weight 2·4 g) were fed during 6 weeks one of ten semi-purified diets based on casein and purified amino acids (AA) as N source. The diets contained four levels of crude protein (CP, from 5 to 54 % DM diet) with two levels (% CP) of lysine or methionine (normal or 30 % deficient). Requirements were determined using linear and non-linear regression models. We could thus obtain the first ever data on maintenance (N equilibrium) requirements for CP and AA in P. monodon. CP requirements for maintenance (4·5 g/kg body weight (BW) per d) represented approximately 19 % of the CP requirement for maximal N gain (23·9 g/kg BW per d). The marginal efficiency of utilisation reached a maximum of 38 % for N, 0·77 for lysine and 1·62 for methionine using N gain as response. Lysine requirements were 0·20 g/kg BW per d for N maintenance and 1·40 g/kg BW per d for maximal N gain. Methionine requirements were 0·11 g/kg BW per d for N maintenance and 0·70 g/kg BW per d for maximal N gain. The lysine (5·8 %) and methionine (2·9 %) requirements for maximal N gain, expressed as percentage of protein requirement, agree with literature data using a dose -response technique with smaller P. monodon. The observed interaction between dietary CP and methionine for N gain demonstrates that requirements for indispensable AA (expressed as % CP) cannot be evaluated separately from CP requirements. Crustaceans: Protein requirement: Indispensable amino acids: Logistic model: Marginal utilisation efficiencyThe black tiger shrimp (Penaeus monodon) is the second most cultured crustacean species worldwide (1) . Due to the importance of protein for shrimp growth, its high cost in formulated feeds and the environmental implications of N losses, it is essential to gain a better understanding of N requirements and N utilisation in P. monodon. Available data on crude protein requirements (CP, % DM diet) of P. monodon show a large degree of variability, i.e. from 36-40 % (2) up to 50 % (3) . Several factors, e.g. differences in protein source, dietary energy level, life stage, rearing conditions and, in particular, differences in feed intake (FI), can explain some of this variation (4) . The confounding effect of FI on dietary protein requirement estimates has also been illustrated with the pacific white shrimp, Litopenaeus vannamei (5,6) . The latter authors (6) demonstrated that maximum weight gain could be obtained by a wide range of dietary CP levels (30-40 % DM diet) at different feed allowances (50, 75 and 100 % of typical daily intakes), underlining the importance of expressing protein requirements on absolute basis rather than as a percentage of the diet. The use of the factorial approach, which allows the distinction between maintenance and growth for estimating protein requirements, has been initially developed for terrestrial animals (7,8) and has also been appl...

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Closed recirculation aquaculture systems (RAS) in combination with integrated multi-trophic aquaculture (IMTA) are considered best management practices, but high material costs and difficult maintenance still hinder their implementation, especially in developing countries and the tropics. Few case studies of such systems with tropical species exist. For the first time, an extremely low-budget system was tested combining the halophyte sea purslane Sesuvium portulacastrum and a detritivore, sandfish Holothuria scabra, with finfish milkfish Chanos chanos over 8 wk on Zanzibar, Tanzania. In a 2 m3 RAS, milkfish and sea purslane showed good growth, producing an average (±SD) of 1147 ± 79 g fish and 1261 ± 95 g plant biomass, while sea cucumber growth was variable at 92 ± 68 g. The system operated without filter units and did not discharge any solid or dissolved waste. Water quality remained tolerable and ammonia levels were reliably decreased to <1 mg l-1. A NO2- peak occurred within the first 30 d, indicating good biofilter performance of the different system compartments. Changes in dissolved inorganic nitrogen (DIN) species support the notion that the sea cucumber tank was the main site of nitrification, while the hydroponic halophyte tank acted as a net sink of NO3-. A nitrogen budget accounted for 63.7 ± 5.3% of the nitrogen added to the system as fish feed. Increasing the plant to fish biomass ratio to 5:1 would fully treat the DIN load. The experiment provides proof-of-concept of a simple pilot-scale RAS, integrating tropical species at 3 trophic levels.