WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 39 water quality dynamics and fish growth. A maximum net yield of large (> 550 g) channel catfish of 9.3 kg/m3 has been attained. In addition, BFT may confer an increased resistance to bacterial and/or viral infection to cultured fish. Disease challenge research will be conducted to determine if channel catfish reared in the BFT system are more resistant to columnaris disease or ESC. Dissolved oxygen (DO) is of particular importance in earthen pond production systems, especially as water temperatures rise in response to global climate change. There is a curvilinear response between net yields of hybrid catfish (Fig. 4) and hybrid striped bass as DO concentration increases. Maintaining a minimum DO concentration of 40 percent of saturation in ponds would result in fast growth. Allowing DO concentration to drop to 25 percent of saturation results in a significant reduction in growth and feed utilization efficiency. Novel Ingredients and Diets for Hybrid Striped Bass There are three current objectives of the fish nutrition research program at SNARC: 1) refine nutritional requirements, 2) evaluate alternate sources of protein and, 3) develop practical feed formulas for Morone culture that conserve marine fishmeal. There are significant gaps in digestibility and performance data for alternate protein sources in carnivorous fish, particularly for hybrid striped bass. Increasing demand, uncertain supply, and volatile pricing for fishmeal and fish oil constrains expansion of commercial fish production. Therefore, alternative ingredients need to be evaluated for use in aquafeeds. Researchers at SNARC conduct feeding trials aimed at developing a database of nutrient composition and digestibility coefficients for a variety of common and emerging feed ingredients for different cultured fish (Barrows et al. 2011). The current database is assembled into a CD and is also posted and regularly updated on a federal website (www.ars.usda.gov/Main/ docs.htm?docid=21905) for use by the commercial and research communities. It also provides commercial producers and aquafeed manufacturers with critical and timely data for developing new fish diets. Another area of research addresses gaps in nutrient requirement data for hybrid striped bass based on practical ingredients. In other words, fish performance differs between purified and practical ingredients and between amino acids provided in crystalline form or as intact protein. Nutritionists at SNARC used levels of essential amino acids in the muscle of hybrid striped bass, based on ideal protein theory, to optimize the level of lysine in a fishmealfree commercial diet containing a high level (45 percent) of soybean meal (Rawles et al. 2013). Dietary protein sources included soybean meal, corn gluten meal and poultry byproduct meal. Nonlinear models were fit to growth performance and nutrient retention data after 12 weeks of feeding. Hybrid striped bass muscle was a reasonable model to target for optimizing essential amino acid ratios and levels in commercial fishmealfree diets for this fish. It was also a sensitive indicator of the order of the first four limiting amino acids (methionine, lysine, threonine and histidine) in the diet. There were no signs of soyderived anti-nutritional factors in the gastrointestinal tract. Feed mills can use the ideal protein philosophy of feed formulation for devising alternative diets for hybrid striped bass with confidence. High levels of soybean meal can be tolerated in hybrid striped bass diets assuming other essential nutrients are balanced. SNARC scientists are investigating how alternative feed formulas affect protein turnover rates and feed efficiency by evaluating gene expression. One facet of this research is to determine how amino acid supplementation of fishmeal-free diets and summer diets with varying protein-to-energy ratios affects gene expression for protein turnover in the muscle and liver. The first step is identifying genes that affect protein synthesis and degradation. A two-pronged approach is currently underway, the first being isolating and purifying RNA for both RNA sequencing (RNAseq) that targets all genes expressed in fish fed the different test diets. For likely candidate genes, RNA sequencing data is being validated while evaluating their expression using real time PCR. For example, muscle myostatin expression, a gene that inhibits protein synthesis, was lower in hybrid striped bass grown at high temperature (30.5 C) when fed a diet containing 40 percent digestible protein and 18 percent fat when compared to fish fed more (47 percent) or less (33 percent) digestible protein or less (10 percent) fat (Fig. 5). This finding correlates well with better growth and protein retention of fish fed the 40/18 diet than other diets. With identification of genes that correlate with increased TOP, FIGURE 4. Relationship between net yield of channel x blue hybrid catfish and DO-minutes (a measure of actual cumulative exposure to nocturnal DO conditions in the pond) for fish reared in 0.1-ha earthen ponds where minimum nightly DO concentration was maintained at 12%, 24%, 36% or 48% of saturation. BOTTOM, FIGURE 5. Muscle myostatin expression in hybrid striped bass fed two dietary lipid (10%, 18%) and three digestible protein (DP; 33%, 40%, 47%) levels at elevated temperature (30.5 ± 0.5ºC). (CONTINUED ON PAGE 40)
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