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A 60-day growth experiment was conducted to investigate the effect of dietary replacement of fishmeal by Spirulina platensis on growth performance, digestion and physiological parameters in juvenile gibel carp (5.0 AE 1.0 g). Four diets were formulated to replace 0 (SP0), 50% (SP50), 75% (SP75) and 100% (SP100) of dietary fishmeal protein by S. platensis respectively. Other two diets based on SP100 were supplemented with 2% dicalcium phosphate (SP100 + P) or 0.28% coated lysine (SP100 + Lys). The results showed that no significant differences of specific growth rate (SGR), feed efficiency (FE), feeding rate (FR) and protein efficiency ratio (PER) were observed between SP0 group and the replacement groups (SP50, SP75 and SP100). FE and PER of fish-fed SP100 + Lys diet were significantly higher than the fish-fed SP100 diet (p < .05). Apparent digestibility coefficients (ADCs) of dry matter, protein, energy and phosphorus increased significantly with the increasing S. platensis inclusion. Meanwhile, fish from SP100 + P or SP100 + Lys group had higher ADCs of nutrients compared to the SP100 group (p < .05). Compared with SP0 diet, fish-fed S. platensis diets had higher activities of plasma superoxide dismutase, hepatopancreas total antioxidant capacity, plasma alkaline phosphatase and plasma lysozyme, as well as the lower content of plasma malondialdehyde. The present results indicated that dietary fishmeal could be fully replaced by S. platensis without any negative effects on growth performance of gibel carp. Supplementation of lysine in the S. platensis replacement diet could further improve the feed utilization of gibel carp. K E Y W O R D Sdigestibility, gibel carp, growth performance, protein replacement, Spirulina platensis | INTRODUCTIONAs the traditional protein source in aquafeeds, fishmeal gets extensive use for its high contents of essential amino acids, essential fatty acids, mineral elements, bioactive substance and low level of anti-nutritional factors (Hardy, 2010). With the development of aquafeeds industry, the demands for fishmeal and fish oil are still rising (FAO, 2012). Although the worldwide fishmeal production has obtained a relatively high and stable level, even some of which are acquired through overfishing, the production still could not match

In recent decades, due to the continuous increasing demand for aquatic products, the global fishery industry has been expanding rapidly, and aquaculture output has been increasing at an unprecedented speed (Cole et al., 2009). High-density aquaculture is the current mainstream aquaculture modes worldwide. While the environmental stress induced by high rearing density results in poor resistance to disease and vulnerability to infection by pathogenic microorganisms in fish (Costas, Aragão, Mancera, Dinis, & Conceicão, 2008). For a long time, many antibiotics and chemotherapeutic agents have been used to improve non-specific immunity and to treat some bacterial infections in aquatic animals, but these agents also lead to undesirable effects, such as antibiotic resistance, pharmaceutical bioaccumulation and environmental pollution (Bilen, Bulut, & Bilen, 2011;Harikrishnan, Balasundaram, & Heo, 2011). In recent years, many harmless therapeutic techniques and

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An 8-week feeding trial was conducted to investigate the impacts of dietary leucine on growth, hepatopancreas and muscle protein content and relative expressions of protein synthesis-related genes in juvenile gibel carp (5.2 g). Triplicate tanks of fish (22 fish per tank) were fed with a low (0.82%), middle (1.90%) or high (2.99%) leucine diet to apparent satiation three times a day. The results showed that the weight gain (WG) and feed efficiency (FE) of the fish fed with the 1.90% leucine diet had the highest values compared to those of other two groups (p < .05). Dietary leucine significantly affected the protein and lipid contents of the hepatopancreas (p < .05), but not muscle or the whole body composition of the fish. The amino acid contents (Arginine, Leucine, Methionine, Phenylalanine, Alanine, Aspartic acid, Glutamic acid and Proline) of the dorsal muscles of fish fed with a 1.90% leucine diet were significantly higher than those fed with a 0.82% leucine diet. Moreover, transcriptional levels of target genes of rapamycin (TOR), ribosomal protein S6 kinase 1 (S6K1), eukaryotic translation initiation factor 4E (eIF4E) and leucyl-tRNA synthetase (LARS) in the hepatopancreas were significantly up-regulated (p < .05), but eukaryotic translation initiation factor 4E-binding protein 2 (4E-BP2) was significantly down-regulated (p < .05) with the increasing dietary leucine levels. However, dietary leucine had no effects on the gene expressions of 4E-BP2 and S6K1 in the muscle of gibel carp. The present results indicate that dietary leucine led to a marked tissue-specific response on protein content in the muscle and hepatopancreas of gibel carp that is related to the TOR signalling pathway.