Progressive replacement of fishmeal by raw and enzyme-treated alga, Spirulina platensis influences growth, intestinal micromorphology and stress response in juvenile barramundi, Lates calcarifer

Seven isonitrogenous (47.0% crude protein), isolipidic (14.0% crude lipid) and isocaloric (22.0 MJ Kg(-1) gross energy) diets were formulated and tested in a feeding trial to evaluate the efficacy of raw and enzyme-treated alga, Spirulina platensis (SP) to replace fishmeal (FM) on growth, digestibility, biochemical response and health status of juvenile barramundi, Lates calcarifer. The control diet had fishmeal as the main protein source and was progressively substituted with raw (RSP) or enzyme-treated SP (ESP) protein at 10%, 20%, and 40% of FM. Diets were fed in triplicated groups of barramundi with an initial body weight of 9.20 +/- 0.3 g/fish, thrice a day for 8 weeks. The results showed that the weight gain (WG) and feed conversion ratio (FCR) were not negatively affected by the replacement of dietary ESP, while dietary 40% RSP level of FM significantly reduced (P < .05) the WG and FCR than the control. The apparent digestibility coefficients (ADCs) of protein and lipid were similar up to 20% replacement but decreased at 40% replacement by ESP and RSP. A significant (P > .05) reduction of intestinal fold height, microvillus height, and thickness of muscular wall were recorded in fish fed RSP20, RSP40, and ESP40 diets than the control. The histopathological alterations in liver and muscle appeared in fish fed both ESP and RSP diets at 40% replacement of FM. Except for the fish fed 10% of RSP diet, all other fish, the exposure to transportation stress increased the plasma glucose levels although the plasma cortisol levels were not changed in various feeding levels. Overall, this study demonstrates that up to 20% RSP or 40% ESP unaffected the growth performance of juvenile barramundi while replacement of 40% FM with SP either raw or enzymetreated impacted the health status in terms of the liver, muscle and intestinal morphology of fish.