A study was undertaken to determine the effect of dietary lipid level on growth, feed efficiency and body chemical composition of juvenile grass carp. Seven isonitrogenous diets (400 g kg−1 crude protein) containing seven dietary lipid level (0, 20, 40, 60, 80, 100 and 120 g kg−1 dry matter) were fed to triplicate groups of 40 fish with initial weight 6.52 g, for 70 days. No obvious and assured essential fatty acid deficiency symptom appeared in fish fed the lipid‐free diet. Excess dietary lipid level (100 and 120 g kg−1) resulted in decreased feed intake. The best growth performance and feed utilization was observed in fish fed 20–40 g kg−1 dietary lipid. The fish fed a lipid‐free diet had the lowest protein efficiency and protein retention. Growth performance and feed utilization increased with the increasing dietary lipid levels up to 40 g kg−1 dietary lipid. Higher dietary level (above 40 g kg−1) made growth performance and feed utilization decrease and no protein sparing effect was observed. Lipid retention decreased as dietary lipid level increased. Mesenteric fat index (MFI) increased, hepatosomatic index (HSI) decreased with dietary lipid level. The increased MFI and simultaneous decrease lipid retention can be explained by differences in growth. The effect of dietary lipid levels on the chemical composition of tissues was significant only for whole body and muscle. The excess lipid content of liver in all groups was regarded as a slight symptom of fatty liver, which was partly identified by microscopic structural study and lower plasma lipid indexes, comparing to the initial plasma data. In conclusion, grass carp is a fish with low energy requirement and excess dietary lipid level should be avoided.

An 8-week feeding experiment was conducted in floating cages (1.5 · 1.0 · 2.0 m) to determine the potential use of defatted soybean meal (roasted and solvent-extracted) as a partial replacement of fishmeal in the isonitrogenous (approximately 450 g kg )1 CP [crude protein]) diet for juvenile cobia with an initial average weight of about 8.3 g. Diets were formulated to include 0, 100, 200, 300, 400, 500 and 600 g kg )1 (diets D0, D10, D20, D30, D40, D50 and D60, respectively) of fishmeal protein being substituted by defatted soybean meal without methionine supplementation. The results showed that weight gain rate decreased significantly when the replacement level of fishmeal protein was increased from 400 g kg )1 to 500 g kg )1 , and the D60 diet was the lowest in all groups. These results indicate that up to 400 g kg )1 of fishmeal protein can be replaced by defatted soybean meal without causing significant reduction in growth. Feed conversion ratio (FCR) and protein efficiency ratio (PER) were significantly affected by the replacement level of fishmeal protein being substituted by defatted soybean meal, when the replacement level of fishmeal protein was 200 g kg )1 (diet, D20), FCR was the lowest and PER was the highest. There were no significant differences in the moisture, lipid, crude protein and ash content in whole body and muscle, while lipid content in liver increased as the dietary soybean meal replacement levels increased. There were significant differences in haemoglobin, haematocrit, red blood cell, plasma glucose and triglyceride concentration in fish fed diets with different soybean meal replacement levels. Results of this trial indicated that the optimum level of fishmeal protein replacement with defatted soybean meal, determined by quadratic regression analysis was 189.2 g kg )1 , on the basis of maximum weight gain. KEY WORDS

Pacific white shrimp Litopenaeus vannamei (1050 individuals with initial weight of 1.01 ± 0.001 g) were fed either control diet or one of six dietary astaxanthin (AX) concentration (25, 50, 75, 100, 125 and 150 mg kg−1) diets for 56 days in 35 tanks (30 shrimp per tank). After 56 days of culture, shrimp‐fed AX125 and AX150 diets had higher (P < 0.05) weight gain, specific growth rate, total antioxidant status and lower (P < 0.05) superoxide dismutase (SOD), catalase (CAT) than shrimp fed control diet. After low dissolved oxygen stress for 1 h, survival rate of shrimp fed AX75, AX100, AX125 and AX150 diets was higher (P < 0.05) than that of shrimp fed control diet. Hypoxia inducible factor‐1α (HIF‐1α), cytosolic manganese superoxide dismutase (cMnSOD) and CAT mRNA expression levels of shrimp fed seven diets were significantly down‐regulated under hypoxia than under normoxia, but their expression levels were higher under hypoxia in shrimp fed AX‐supplemented diets than in shrimp fed control diet. About 70‐kDa heat‐shock protein (Hsp70) mRNA expression level of shrimp fed seven diets was significantly up‐regulated under hypoxia than under normoxia, but its expression level was lower under hypoxia in shrimp fed AX‐supplemented diets than in shrimp fed control diet.

A growth trial was conducted to estimate the optimum concentration of dietary available phosphorus (P) for grass carp (Ctenopharyngodon idella). Triplicate groups of grass carp (5.59 ± 0.02 g) were fed diets containing graded levels (2.36, 4.27, 6.31, 8.36, 10.4 and 14.8 g kg )1 ) of available P for 8 weeks. Grass carp fed with the P-supplemented diets had significantly higher specific growth rate, weight gain, protein efficiency ratio and feed efficiency than fish fed with the basal diet. In whole-body composition, protein content increased, while lipid content decreased with the increase in P level in diet (P < 0.05). Fish fed with the P-supplemented diets had significantly higher whole body, vertebrae and scales mineralization (P < 0.05), but Ca/P ratios were not influenced. The blood chemistry analysis showed that dietary available P had distinct effects on P, Ca and Mg contents, as well as on the contents of triacylglycerol and total cholesterol. Brokenline analysis indicated that 8.49 g kg )1 dietary available P was required for maximal tissue storage and mineralization as well as optimal growth.

Six isonitrogenous (390 g kg )1 ) and isoenergetic (16.2 kJ g )1 ) diets with varying carbohydrate : lipid (CHO : L) ratios (202.5-1.74), were fed to triplicate groups of 25 fish in indoor recirculation system. Over 8-week-growth trial, best weight gain (WG), specific growth rate, feed conversion ratio, protein efficiency ratio and protein production value (P < 0.05) were observed in fish-fed diets with CHO : L ratio of 7.5. Fish fed either the lowest (1.7) or highest (202.5) CHO : L ratio tended to produce lower (P < 0.05) growth and feed conversion efficiencies. The values of viscerosomatic index, hepatosomatic index and intraperitoneal fat ratio increased as dietary CHO : L ratios decreased. There were no significant differences in whole body and liver crude protein among dietary treatments. Whole body and liver lipid increased as CHO : L ratios decreased. Plasma cholesterol and triacylglyceride levels increased linearly as dietary CHO : L ratios decreased. Activities of glucokinase and pyruvate kinase were stimulated by elevated levels of dietary carbohydrate; however, activities of lipase (LPS) and alkaline phosphatase were stimulated by elevated levels of dietary lipid. Based on a second-order polynomial regression analysis of WG against dietary carbohydrate and lipid levels, 275 g kg )1 of carbohydrate and 59 g kg )1 of lipid, corresponding to a CHO : L ratio of 4.7, in a diet holding 390 g kg )1 of crude protein and 16.3 kJ g )1 of gross energy, proved to be optimal for grass carp. These results indicated that utilization of dietary lipid and carbohydrate was moderate in grass carp, but the fish were a little more capable of utilizing lipid compared with carbohydrate. KEY WORDS

Four isonitrogenous and isolipidic diets containing fresh fish oil (peroxide value, POV: 11.5 meq kg−1, diet FR) and three degrees of oxidized fish oil (POV: 132, 277 and 555 meq kg−1, diet OX132, OX277 and OX555, respectively) were formulated to investigate the effects of dietary oxidized fish oil on growth performance, body composition, antioxidant defence mechanism and liver histology of juvenile largemouth bass. After a 12‐week feeding trail, a proportion of approximately 9% of Micropterus salmoides showed inflammation and haemorrhage at the base of dorsal, pectoral and tail fin in both groups OX277 and OX555. Fish fed oxidized oil diets obtained significantly higher (P < 0.05) weight gain and specific growth rate because of their remarkable higher feed intakes, compared with the fresh oil receiving group. The analysis of biometric parameters and body composition indicated significant differences (P < 0.05) in various test diets. The activities of hepatic catalase and superoxide dismutase were significantly stimulated (P < 0.05) by oxidized oil ingestion. Hepatic glutathione peroxidase, glutathione reductase and glutathione‐S‐transferase activities were significantly higher (P < 0.05), and liver glutathione content was markedly lower (P < 0.05) in group OX555 than the other treatments. Oxidized oil consumption resulted in marked depletion (P < 0.05) of vitamin E concentration in plasma, liver and muscle tissue, increased plasma and muscle malondialdehyde content along with decreased haematocrit value. Histological examinations indicated that hepatocytes with lipid vacuoles and nuclear migration were shown in groups OX277 and OX555. The overall results in this study suggested that an increased oxidative stress in M. salmoides fed oxidized lipid may account for their stimulated hepatic antioxidant defences, vitamin E depletion in plasma and certain tissues, and pathological changes. The detrimental effect of oxidation products on fish health and the unexpectedly enhanced feed intake of oxidized feeds in M. salmoides underline the importance that cares should be taken to minimize dietary oxidation products to the greatest extent possible.

The ability of Litopenaeus vannamei (initial mean weight: 0.96 ± 0.02 g) to utilize different levels of cornstarch was examined in terms of growth indices, body composition, digestibility and microscopic structure of the hepatopancreas. Six isonitrogenous semipurified diets were fed to satiation to shrimp for 8 weeks in triplicate tanks (30 shrimps per tank) connected to a natural brackish water (6–8 g L−1) recirculating system. Diets contained different levels of cornstarch (100, 150, 200, 250, 300 and 350 g kg−1) as the source of carbohydrate and were balanced using cellulose. Weight gain (WG), survival rate and feed conversion rate (FCR) were considerably affected by cornstarch levels of diets. The highest WG (453.6 g kg−1) and best FCR was observed in shrimp fed the 150 g kg−1 (cornstarch level) diet and was significantly (P < 0.05) higher than those fed diets containing 250–350 g kg−1 cornstarch. However, the survival rate reached maximum in shrimp fed the 100 g kg−1 diet (96.7), some 30% higher than the lowest rate, which was found in shrimp fed the 250 g kg−1 diet. Body lipid tended to be higher in shrimp fed diets with higher cornstarch levels. The apparent digestibility of dry matter and crude fat increased with increasing levels of cornstarch and, hence, decreasing levels of cellulose. In addition, histological study on shrimp fed 10–350 g kg−1 diets exhibited histological changes. The overall conclusion was that the optimum cornstarch level may be set at 100–200 g kg−1 when the diets contain 380 g kg−1 protein.

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