*Serotonin (5-HT) levels were higher for reactive than proactive ones after stress in the brain stem of rainbow trout (Ă˜verli et al.2001). However, it is important to note that the serotonergic activity (measured as the ratio between 5-HT and its metabolite; (Shannonet al. 1986)) was found lower for reactive than for proactive in the whole brain of sea bass (Alfonso et al. 2019) and higher in the amygdala homologue of Atlantic salmon (Vindas et al. 2017a).