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Summary Phytate formed during maturation of plant seeds and grains is a common constituent of plant‐derived fish feed. Phytate‐bound phosphorus (P) is not available to gastric or agastric fish. A major concern about the presence of phytate in the aquafeed is its negative effect on growth performance, nutrient and energy utilization, and mineral uptake. Bound phytate‐P, can be effectively converted to available‐P by phytase. During the last decade, phytase has been used by aqua feed industries to enhance the growth performance, nutrient utilization and bioavailability of macro and micro minerals in fish and also to reduce the P pollution into the aquatic environment. Phytase activity is highly dependent on the pH of the fish gut. Unlike mammals, fish are either gastric or agastric, and hence, the action of dietary phytase varies from species to species. In comparison to poultry and swine production, the use of phytase in fish feed is still in an unproven stage. This review discusses effects of phytate on fish, dephytinisation processes, phytase and pathway for phytate degradation, phytase production systems, mode of phytase application, bioefficacy of phytase, effects of phytase on growth performance, nutrient utilization and aquatic environment pollution, and optimum dosage of phytase in fish diets.

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Jatropha curcas is a multipurpose and drought‐resistant tree, widespread throughout tropics and subtropics. Its seeds are rich in oil and protein. It is being promoted as a biofuel plant. Jatropha kernel meal obtained after oil extraction is an excellent source of protein [580–660 g kg−1 crude protein (CP)]. However, the presence of toxic and antinutritional constituents restricts its use in fish feed. Jatropha kernel meal was detoxified. Using Oncorhynchus mykiss juveniles, a 12‐week experiment was conducted to evaluate the nutritional quality of detoxified Jatropha kernel meal (DJKM). Oncorhynchus mykiss juveniles (36, av. wt. 4.2 ± 0.4 g) were distributed into three groups with 12 replicates and fed isonitrogenous (CP 450 g kg−1) diets: control [fish meal (FM)–based protein], J50 and J62.5 (50% and 62.5% of fishmeal protein replaced by DJKM). Growth performance, apparent lipid conversion, hepatosomatic index, nutrients and energy digestibilities; efficiency of digestible nutrients and energy; and glucose and creatinine levels in blood were statistically similar (P > 0.05) for control and J50 but were higher (P < 0.05) than for J62.5. Opposite trend was observed for phosphorus, sodium, albumin and total protein in blood. Feed conversion ratio, protein efficiency ratio, protein productive value, energy retention, gastro somatic index, muscle lipid peroxide; and red and white blood cells, haematocrit, haemoglobin, mean corpuscular volume, mean corpuscular haemoglobin, mean corpuscular haemoglobin concentration; alkaline phosphatase and alanine transaminases activity, globulin, total bilirubin, blood urea nitrogen, calcium and potassium in blood and lysozyme activity in serum did not differ significantly among the groups. Intestinal enzyme activities; and plasma and muscle cholesterol levels for control were higher (P < 0.05) than for DJKM groups, while opposite trend was observed for relative intestinal length (mm g−1). Conclusively, performance of J50 group was similar to control group and, thus, is recommended as the optimal diet for rainbow trout.

Bile is a yellow‐green liquid produced in the liver from cholesterol and stored in the gallbladder of vertebrates. Bile improves the efficacy of lipid digestion by acting as an emulsifier and is essential in activating bile salt lipase, an enzyme that has broad substrate specificity. Bile improves the absorption of lipid soluble nutrients while also facilitating the excretion of cholesterol and toxic metabolites, particularly bilirubin. Dietary fishmeal alternatives often disturb bile acid status in fish resulting in either increased excretion/decreased intestinal reabsorption and/or decreased bile acid synthesis. Saponins and high molecular mass proteins are believed to contribute to altered bile acid status, which may reduce fish productivity. This situation can worsen with increased incorporations of plant‐based proteins in aquafeeds, but also may be mitigated by processing of fishmeal alternatives as well as the dietary inclusion of some bile acids/salts. This area of research will likely increase due to the roles dietary bile acids/salts can have on protecting digestive organs and improving nutrient utilization. This, however, depends on the bile type, level and fish species. This review discusses these aspects in regard to fish nutrition to help increase the inclusion of dietary fishmeal alternatives and thus enhance aquaculture sustainability.