As marine feedstuffs have transitioned to strategic ingredients in aquafeeds, plant protein feedstuffs now provide a majority of crude protein in most aquaculture diets. Although this shift often reduces the overall price of diets through incorporation of less expensive raw materials, specific attention must be given to the nutritional profile of diets high in plant protein feedstuffs, especially the amino acid (AA) ratios. In addition to dietary essential AA, traditionally classified nonessential AA have been recognized as potentially limiting as standard levels of fishmeal have decreased in carnivorous fish diets. Therefore, a 6-week comparative feeding trial was conducted to assess the effects of glutamine supplementation as a functional AA in soybean-meal (SBM)-based diets fed to hybrid striped bass (HSB). The basal diet (38% crude protein (CP), and 10% crude lipid) was composed of practical ingredients with SBM contributing 75% of total CP. Along with the basal diet, experimental diets were supplemented stepwise with L-glutamine (Gln) at levels 0.0, 0.5, 1.5, 2.0, and 4.0% of dry diet weight. Responses of HSB were statistically assessed via growth parameters, condition indices, whole-body proximate composition, histological condition of the distal intestine and various innate immune parameters.
Groups of 10 juvenile HSB (~8.18 g/ fish initial weight) were stocked into 24, 38-L aquaria fashioned as a recirculating aquaculture system with quadruplicate aquaria randomly assigned to each dietary treatment. Fish were fed to apparent satiation twice daily for 6 weeks. HSB fed the diets supplemented with glutamine exhibited a significant (P<0.05) positive quadratic response of weight gain (Figure 1). The optimal supplementation level was estimated at 1.64% of diet. Although not significant, a similar quadratic trend was also observed for feed efficiency. Body condition indices and nonspecific immune responses including neutrophil oxidative radical production in the whole blood were not significantly influenced by diet.
Analysis of AA fluxes in plasma and various other tissues of HSB is currently ongoing, along with determination of whole-body proximate composition. Histological analysis of HSB distal intestines is also currently ongoing to assess potential intestinal enteritis mitigation given this condition has been observed in some carnivorous species fed high levels of SBM. Nonetheless, preliminary results indicate beneficial effects of Gln supplementation in juvenile HSB.