| Summary: | The present study aims to evaluate methanol-extracted cottonseed meal (CSM) as a potential replacement for fishmeal (FM) in aquafeeds for juvenile cobia <i>Rachycentron canadum.</i> Five isonitrogenous (41% crude protein) and isolipidic (11% crude lipid) diets were formulated with 0 (i.e., the full fishmeal diet, as Control), 25%, 50%, 75% and 100% of the dietary protein from FM replaced by methanol-extracted CSM with L-lysine (L-Lys) and DL-methionine (DL-Met) and supplemented to the established requirement levels for cobia. Diets were fed to triplicate groups of juvenile fish with an average initial weight (±SEM) of 11.35 ± 0.23 g/fish for 9 weeks. Percent weight gain (WG), feed efficiency (FE) and protein efficiency ratio (PER) of fish fed diets with 25% and 50% of FM protein replaced by methanol-extracted CSM were higher or comparable to those of fish fed the Control diet. Those responses were gradually reduced with increasing levels of CSM substitution, resulting in significant (<i>p</i> < 0.05) negative linear trends. Condition factor (CF) and hepatosomatic index (HSI) values significantly decreased with increasing dietary CSM inclusion, as did whole-body protein and lipid composition. Activities of superoxide dismutase (SOD) of fish fed CSM diets were not significantly different compared to that of fish fed the Control diet. The glutathione peroxidase (GSH-Px) and malonaldehyde (MDA) levels, as well as serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT) activities from fish fed diets with 50% or greater of CSM were lower than those of fish fed the Control and CSM25 diets. These results indicated that the inclusion of CSM did not induce any apparent stress on juvenile cobia. Additionally, methanol-extracted CSM with Lys and Met supplementation was able to replace up to 20~30% of crude protein provided by FM in the diet of cobia without drastically affecting the growth performance or body composition.
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