World Aquaculture 57 Improved feed management strategy for Litopenaeus vannamei in limited exchange culture systems Susmita Patnaik1 and Tzachi M. Samocha1 Beside great losses to viral disease outbreaks, the shrimp farming industry is under increasing pressure by the regulatory agencies to meet effluent water quality requirements and to limit potential negative impacts on receiving streams. Shrimp farmers and researchers are looking for cost-effective and sustainable culture methods to minimize environmental impact and reduce their crop losses to diseases. Production of shrimp in limited or zero water exchange systems can provide more biosecurity while addressing both issues (Thakur and Lin 2003, Cohen et al. 2004). Feed is a primary source of macronutrients for shrimp and a major source of pollution in pond effluent (Tacon and Forster 2003). Protein is an important component of shrimp feed, contributing a substantial portion of the manufacturing cost (Kureshy and Davis 2002). With the recent trend in shrimp farming intensification under limited discharge, the use of suitable feed ingredients, feed formulations and feeding practices are key factors affecting the viability of these systems. A study by McIntosh et al. (2001) in a small tank system with limited discharge showed significantly better survival, mean final weight, yield and feed conversion ratio (FCR) for shrimp fed a commercial 31 percent crude protein (CP) diet over those fed the 21 percent CP diet. These results suggest that under the conditions of the study, shrimp utilized the higher protein feed more efficiently. Protein in feed has also been associated with the nitrogen load in the effluent from aquaculture activities (Cho et al. 1994). A substantial portion of the feed goes unutilized and, subsequently, ends up adding to the organic load of the culture system (Thakur and Lin 2003). Sustainable growth of the shrimp farming industry requires placing greater emphasis on feed quality and feeding practices that can reduce cost and pollution without compromising productivity. Only limited information is available concerning the effect of feeding high-protein feed, when it is fed at lower ration sizes to match the protein level of low-protein feed, on shrimp and fish performance. Cho and Lovell (2002), while working with channel catfish, did not find significant differences between 28 percent CP feed offered at satiation (100 percent) and a diet with 32 percent, which was fed at 87.5 percent of the satiation rate. In another study, Kureshy and Davis (2002) reported significantly better weight gain for L. vannamei juveniles when fed a 32 percent CP feed compared to 16 percent and 48 percent CP feeds when the feeds were offered on an isonitrogenous basis. A few studies have also been carried out where shrimp were fed based on their dietary energy and protein requirements. The optimal digestible energy:crude protein raatio (DE:CP) was reportedly 11.9 kcal/g protein for L. vannamei (Cousin et al. 1993). Working with the same species, Lawrence et al. (1993) reported the protein dietary requirement at 15 percent and the optimum DE:CP ratio at 28.57 kcal/g protein. The current study evaluated the effect of feeding commercial 30 percent and 36 percent CP diets on Pacific white shrimp L. vannamei growth, survival and FCR in tanks and ponds under limited water discharge conditions when the high protein feed was fed on a isonitrogenous basis to the low protein feed. Tank Study A 118-day study was conducted in 22 outdoor tanks (10.5 m2 bottom area, 6.8 m3 working water volume) at the Texas Agricultural Experiment Station, Shrimp Mariculture Research Facility, Corpus Christi, Texas USA. Tanks were filled with chlorinated water two days prior to stocking. Experimental tanks were positioned in a shaded area with 73 percent light reduction. Aeration was provided by air stones (10 air stones/tank with air flow of 6 to 8 L/min/air stone). Tanks were stocked with juvenile L. vannamei (0.8 g) at a density of 75/m3. Each tank was covered with plastic netting to prevent shrimp from jumping out. Except for emergency releases because of heavy rains, no water was discharged from the system. Municipal water was added to the tanks to compensate for evaporation and to maintain salinity. Two commercial shrimp diets, 30 percent and 36 percent CP (Rangen Inc., Buhl, Idaho), were tested at different ration sizes. The experimental design for the study is summarized in Table. 1. The higher protein diet (36 percent CP) was fed at lower ration size and was calculated to provide the shrimp with an equal nitrogen level to the lower protein diet (30 percent CP). Thus, for each 100 g of the 30 percent CP
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