Increased aquaculture production is associated with a growing interest in improving fish welfare. For this reason, the search for strategies to mitigate stress has intensified, one of these strategies being food supplementation with amino acids. The objective of this study was to evaluate the effects of dietary phenylalanine (Phe) and Tyrosine (Tyr) on the stress response and metabolism of juvenile gilthead seabreams (Sparus aurata) and meagres (Argyrosomus regius). Fish batches were fed a control diet and two diets supplemented with 5% Phe or Tyr for seven days. At the end of the experiment fish were stressed by air exposure for 3 min and then sacrificed for the extraction of blood and brain. Classical plasma stress markers were analyzed (glucose, lactate, proteins, cortisol), as well as hormones derived from those amino acids (adrenaline, noradrenaline and dopamine). Despite interspecific differences, fish fed the diets supplemented with Phe or Tyr showed a reduction on several stress markers. However, interspecific differences were detected for many indicators. Concretely, hormonal stress markers were significantly attenuated in meagres fed the enriched diets. Moreover, the stress condition favored a mobilization of amino acids towards the brain, especially in supplemented diets, hence this amino acid excess could be used as an energy substrate to cope with stress.

The behavioural responses of fish to a stressful situation must be considered an adverse reaction caused by the perception of pain. Consequently, the handling prior to stunning and the immediacy of loss consciousness following stunning are the aspects to take into account during the slaughtering process. The most common commercial stunning method in seabream and seabass is based on hypothermia, but other methods such as electrical stunning, carbon dioxide narcosis or anaesthetic with clove oil, are discussed in relation to the time to reach the unconsciousness stage and some welfare indicators. Although seawater plus ice slurry is currently accepted in some guidelines of fish welfare well practices at slaughter, it cannot be considered completely adequate due to the deferred speed at which cause loss of consciousness. New methods of incorporating some kind of anaesthetic in the stunning tank could be a solution to minimize the impact on the welfare of seabass and seabream at slaughtering.

Two experiments were carried out. Firstly, 54 anestrous females were placed in contact with photostimulated males (Photo; n = 27) or with no photostimulated males (Natural; n = 27). Moreover, a group of bucks treated with artificial photoperiod and a group of bucks subjected to natural photoperiod without contact with females was used (Photo Isolated and Natural Isolated, respectively). In the Natural groups, the testosterone concentrations were similar except for three days after the introduction of the bucks to the does (19.72 ± 4.11 vs. 2.05 ± 0.25 ng/mL for Natural and Natural isolated bucks, respectively, p < 0.05). However, no differences were observed in the Photo groups during the entire experiment. The percentage of females showing estrous was higher in the group of females in contact with photostimulated bucks (96 vs. 74%, respectively, p < 0.05). In the second experiment, a GnRH agonist, deslorelin, was used to regulate the testosterone concentrations of the bucks. Seventy anestrous females were divided into five groups depending on the treatment received by the bucks to which they were exposed: photostimulated bucks (Photo group, n = 14); photostimulated bucks but treated with the agonist at the onset of the photoperiod treatment (Photo-Ago Long group, n = 13); photostimulated bucks but treated with the agonist at the end of the photoperiod treatment (Photo-Ago Short group, n = 15); bucks receiving no photostimulation but treated with the agonist at the end of the photoperiod treatment period (Natural-Ago Short group, n = 13) and bucks receiving no photostimulation nor agonist (Natural group, n = 15). The agonist treatment increased testosterone concentrations after the injection, which remained high for the entire experiment (p < 0.05). Six days after the introduction of the bucks to the does, the testosterone concentrations increased only in the Natural group reaching similar concentrations to the other groups (12.17 ± 6.55, 16.69 ± 4.53, 8.70 ± 0.61, 11.03 ± 1.45 and 14.42 ± 3.64 ng/mL for Photo, Photo-Ago Long, Photo-Ago Short, Natural-Ago Short and Natural bucks, respectively, p > 0.05). No differences in reproductive parameters were observed (p > 0.05). These results demonstrate that, at Mediterranean latitudes, anestrous females can stimulate the testosterone concentrations of bucks after a period of isolation. The high testosterone concentrations are not a prerequisite for an adequate response to the male effect.