Physiological stress responses of European sea bass Dicentrarchus labrax related to changing operations of floating net cages in intensive farming conditions at the Moroccan M'diq Bay

The present study aimed to quantify and compare in situ the primary and secondary physiological stress responses, related to the changing operations of floating net cages, in both subadult (523 days post hatching [dph]) and adult (916 dph) European sea bass Dicentrarchus labrax under intensive farming conditions in the Moroccan M'diq Bay. The blood levels of cortisol, glucose, total cholesterol, total protein, and lactate, as well as the percentage of haematocrit, were measured before and after this operation. The results showed significantly elevated levels of cortisol and blood glucose in both age groups, whereas total cholesterol and protein levels were unaffected. In fact, blood lactate significantly decreased in subadults, whereas in adults this parameter was not affected by the operation. However, the haematocrit percentages measured after the operation were significantly higher than those found before the operation in both groups of fish, which is attributed to the increased rate of oxygen renewal in the new net cages and the lower water temperature inside the cages. With regard to the age-specific response during this essential operation plasma cortisol, blood glucose, and lactate concentrations, as well as plasma total protein levels, were significantly higher in subadults than in adults, in both pre- and post- stress measurement, with the presence of individual-specific response. It is concluded that aquaculture practices such as changing the aquaculture net cage could have repercussions in terms of the classic physiological responses to stress in D. labrax.