Metabolic and Stress Responses in Senegalese Soles (Solea senegalensis Kaup) Fed Tryptophan Supplements: Effects of Concentration and Feeding Period

SIMPLE SUMMARY: In order to improve fish welfare in fish farms, feed additives (tryptophan, Trp) were assayed in a cultured species, the Senegalese sole (Solea senegalensis). It has been observed in previous studies that fish fed Trp supplements for several days prior to a stress challenge improved their physiological stress response. Therefore, this feeding strategy could be applied in commercial fish farms before submitting fish to stressful zootechnical procedures, such as grading, sampling, slaughter, and, consequently, fish welfare would be improved. ABSTRACT: The objective of this study was to assess the impact of different dietary Trp concentrations on the stress and metabolism response of juvenile Senegalese soles (Solea senegalensis). Fish (38.1 ± 1.9 g) were fed different Trp-enriched feeds (0%, 1% and 2% Trp added) for two and eight days, and later exposed to air stress for three min. Samples were taken pre- and 1 h post-stress (condition). Plasma cortisol, lactate, glucose and proteins were significantly affected by the sampling time, showing higher values at 1 h post-stress. Trp concentration in food also had significant effects on lactate and glucose levels. However, the feeding period did not affect these parameters. Post-stress values were higher than in the pre-stress condition for every plasma parameter, except for lactate in two days and 1% Trp treatment. Nevertheless, cortisol, glucose and lactate did not vary significantly between pre- and post-stress samplings in fish fed the 1% Trp-enriched diet for two days. The lack of variability in cortisol response was also due to the high pre-stress value, significantly superior to pre-stress control. The exposure time to Trp feeding did not significantly affect any enzyme activity; however, Trp added and condition influenced protein-related enzyme activities. In spite of decreasing stress markers, Trp-enriched diets altered the protein metabolism.