Effects of dietary protein level on growth performance, nitrogen and energy budget of juvenile hybrid sturgeon, Acipenser baerii ♀ × A. gueldenstaedtii ♂

Abstract

An 8-week growth trial was conducted to estimate the optimal dietary protein level for juvenile hybrid sturgeon, Acipenser baerii ♀×A. gueldenstaedtii♂ (initial body weight, 25.1±0.12g). Effects of protein levels on nitrogen and energy budget of the fish were studied. Seven isoenergetic diets were formulated to contain seven levels of crude protein (250, 300, 350, 400, 450, 500 and 550gkg−1) and each diet was fed to triplicate groups of fifty fish at 21.4±0.6°C in twenty one 1m3 concrete tanks.The results showed that specific growth rate (SGR) increased when dietary protein levels increased from 250 to 400gkg−1 and then decreased significantly. Feed intake (FI) decreased steadily with increasing dietary protein levels. Fish fed 250gkg−1 protein diets showed the lowest feed efficiency (FE), and those fed 500 and 550gkg−1 protein diets showed significantly higher values than the other diets except 450gkg−1. Protein retention efficiency (PRE) was highest in 250–300gkg−1 groups, followed by 350–500gkg−1 groups and was significantly lower in 550gkg−1 group. Proportions of nitrogen intake (CN) allocated to excretory nitrogen (UN) were lowest in 250–300gkg−1 protein diets, followed by 350–500gkg−1 protein diets and highest in 550gkg−1 protein diet, whereas recovered (growth) nitrogen (RN) followed the inverse pattern with UN. Percentages of gross energy intake (IE) used for excretory energy (UE) increased steadily when dietary protein levels increased. Fish fed 400–550gkg−1 protein diets showed significantly higher proportion of metabolizable energy (ME) than those fed 250–350gkg−1 protein diets. The lowest and highest percentages of recovered (growth) (RE) were observed in 250 and 550gkg−1 groups. Nitrogen and energy budget equation of the fish fed 350gkg−1 protein diet was 100CN=11.72FN+58.78UN+29.50RN and 100IE=35.08FE+4.53UE+34.16ME+26.23RE.Five-parameter saturation kinetic model and second-order polynomial regression analysis indicated that the optimal dietary protein level for maximal growth of the fish was 340 and 370gkg−1. Diets with excessive protein contents not only resulted in inferior growth performance but also high proportions of protein and energy used for excretion.