Dietary supplementation of glutamate and arginine to Atlantic salmon (Salmo salar L.) increases growth during the first autumn in sea

During the sea water phase, Atlantic salmon is exposed to shifting environmental conditions affecting the physiology and metabolism of the fish. Strategic dietary supplements may enable the fish to sustain a higher growth rate during particular periods. In this experiment, a combination of two amino acids (l-arginine and l-glutamate, with supplementation levels 1.1% and 0.75%, respectively) was used as dietary supplements since earlier observations have indicated that these amino acids have versatile functions, influencing growth, reproduction and the immune response in animals. In the case of arginine, the control diet was set at assumed requirement level, while the test diet was supplemented further. The effects of this supplementation on feed intake and growth parameters were studied, as well as selected organ weight, biochemical, molecular and hormonal indicators. During the first experimental period (May–July), few significant effects were found. However, in the second period of the experiment (July–September), a higher specific feeding rate (SFR), thermal growth coefficient (TGC) and specific growth rate (SGR) (p < 0.05) occurred in fish fed the supplemented diet, together with a trend (p < 0.1) for increased final body weight. The measured gene expression levels of Insulin-like growth factor (IGF) and growth hormone receptor gene did not correlate with the increased growth observed during the second period in fish fed the supplemented diet. Furthermore, no difference in total plasma IGF-I concentrations was found after the first period. After the second period, fish fed the supplemented diet had in fact a significantly lower plasma IGF-I level. Additionally, we found a higher relative weight of gastrointestinal sections and elevated levels of plasma arginine, urea and ornithine after the second period in fish fed the supplemented diet. We conclude that dietary supplementation of l-arginine and l-glutamate significantly increased feeding rate and growth during the second feeding period in autumn. This response appeared to occur when the two conditions of rapidly decreasing day lengths and high feed intake, were met.