Optimizing rearing and welfare in Senegalese sole (Solea senegalesensis) broodstock: Effect of ambient light intensity and handling time on stress response

Abstract

Broodstock rearing conditions and handling procedures should be optimized in aquaculture species in order to benefit fish welfare and guarantee optimal conditions for spawning. In teleosts, basal cortisol levels display daily rhythms, oscillating along the 24 h of the day. In this sense, handling fish at different moments of the day may lead to different stress responses. The present study aimed at investigating the optimal rearing conditions for Senegalese sole broodstock, considering ambient light intensity and handling time. The optimal light intensity (50, 100 or 200 lx) was investigated by measuring fish cortisol levels and monitoring locomotor activity rhythms under each intensity tested. Results showed a significant increase in cortisol levels of fish exposed to 200 lx, when compared to values obtained under 100 lx, accompanied by changes in locomotor activity rhythms in both tanks under study. These results suggested that 200 lx may be too high as light intensity for this species, whereas 100 lx seems to be more adequate. Also, daily rhythms of stress response were investigated in breeders from different origins (Wild and first generation, G1). Basal cortisol levels and cortisol stress response after an acute stressor (air exposure) were monitored at two distinct moments of the day (Mid-Light and Mid-Dark). Basal levels were higher during the day in the wild group, while G1 fish seemed to have lost the daily fluctuations in basal cortisol plasma levels, as well as their daily rhythms of locomotor activity. Both groups showed lower stress responses during night-time, an indication that this is an adequate period of the day to handle this species. Senegalese sole breeders born in captivity presented more pronounced stress responses when compared to wild fish, reflecting their different life history in terms of stress challenges.