Materials and animals
There were 4,000 fries allocated into 3 tanks after 3 days, and 3,000 individuals were selected for 3 experimental tanks at 3 protein levels: 35, 40 and 45% crude protein (CP). The fry (30 after hatchery days - AHD), which were supplied from the brood stock station, had an average weight of 0.036 ± 0.50 g fish-1 and an average length of 18.25 ± 0.15 mm fish-1. The Vietnam National Science and Technology Project Fund (No. NVQG-2019/DA.18) supported this experiment, and the fish were nurtured in each composite tank with a volume of 1.0 m3 (water volume = 800 L – 80%). The experiment used a commercial feed sources with different crude protein contents (35, 40 and 45%) in the three diets.
Experimental setup
Nursery conditions: The initial water quality in the experimental tanks was uniform and suitable for rearing fish as were as follows: temperature 28-290°C, salinity 20-25 ‰, pH 7.8-8.5; dissolved oxygen (DO) > 5 mg L-1; stocking density: 1,000 fish m-3. The nursing period was 30 days. Experimental conditions: The experiment was arranged on three triets with 3 replicates. For each trial using industrial diets with 3 distinct levels of protein trial 1: 35% protein trial 2: 40% and trial 3: 45% crude protein (CP). The amount of feed consumed in this experiment was 10% during the first 10 days and 9% during the next 10 days. After reaching 8% body weight, the fish were fed 3 times per day. The experiment was arranged in a completely randomized design, and each experimental treatment was repeated three times.
Water management and variables in tanks
The fluctuation of environmental factors, such as pH, was checked daily during the hatchery process; these factors include: temperature, dissolved oxygen, total NH4/NH3 and salinity, in which water environment parameters such as water temperature were measured twice a day. Temperature was measured with a mercury thermometer, pH (measured twice a day) with a handheld pH meter HANNA (HI98107, Romania), dissolved oxygen content (DO, twice a day),and NH4/NH3 was measured once daily by German Sera test kits, salinity was measured twice a week using an EXTECH refractometer (RF 20, China).
Variables and methods
Growth performance of fish was determined at the time of initial stocking (30 days) and during the last day (60 days in total). 30 fish tank-1 were collected to determine the weight using a hand-held electronic scale APTP453 (Japan), and the length was determined using a graduated ruler. The survival rate of fish was determined after the end of the experiment.
Dietary formulation
Growth performance indices in terms of percent survival rate, daily weight growth (DWG, g day1), daily length growth (DLG, g day-1), specific growth rate (SGR, % day-1), Weight gain (g), length gain (cm), coefficient of variation (%) and feed conversion ratio (FCR) were determined following the equation for weight and length gain in Rahman et al. (2009) and Yousif et al. (2005): Weight gain (g) = (final body weight – initial body weight) Length gain (mm) = (final body length – initial body length) Daily weight growth (g day-1) = (final body weight – initial body weight)/days Daily length growth (mm day-1) = (final body length – initial body length)/days SGRw (% BW day-1) = [(ln final body weight – ln initial body weight)/(days)] *100 SGRL (% BL day-1) = [(ln final body length – ln initial body length)/(days)] *100 CVw (%) = (δ/M) *100 (δ, standard deviation; M, average value of the weight). Survival rate (%) = [(final number of fish – initial number of fish) – (initial chamber of fish)] *100
Statistical analysis
The data were collected, and the mean value (mean) ± standard error (SE) was calculated to identify the range of means. All data were analyzed using one-way analysis of variance (ANOVA) and t test through Microsoft Office Excel™ 2010, and SPSS ver. 16.0 software was applied to determine significant differences among dietary CP level treatments; finally, Tukey’s HSD ranking test was used when significant t-value differences were found (p<0.05).