Time and Concentration Effects of Amino Acid Supplements on Stress Responses in the Gilthead Seabream (Sparus aurata)

It has been reported that phenylalanine (Phe) or tyrosine (Tyr) supplementations significantly affect fish stress and welfare biomarkers. The objective of this study was to determine the amount of Phe/Tyr in the diet and feeding time necessary to mitigate stress. Seabreams (Sparus aurata) were stocked at 5 kg m⁻³ in 500 L tanks. The experimental treatments consisted of different types of feeding: control, Phe-enriched (5%, 7.5%, 10% on dry food), and Tyr-enriched (5%, 7.5%, 10%) food for 2, 4, or 8 days each. At the end of each experimental treatment, fish were sampled for blood, and 10 fish from each treatment were previously subjected to stress by exposure to air (3 min) and sampled 30 min and 2 h later. Plasma glucose, lactate, proteins, cortisol, adrenaline, and noradrenaline concentrations were measured. Dietary Tyr supplements led to high mortalities in long treatments (8 days). The best results for attenuating stress biomarkers were found for the 10% Tyr supplements for 4 days since this treatment kept the maximum number of stress biomarkers with no significant changes. The correlations between biomarker concentrations and amino acid (AA) concentration/supply time were not always significant, though it seems that Tyr supplements present more consistent effects, the increase in Tyr concentration or feeding period being related to decreasing biomarker concentrations.