Seasonal variation in osmoregulatory and metabolic parameters in earthen pond-cultured gilthead sea bream Sparus auratus

Seasonal variations in osmoregulatory and metabolic parameters were assessed in juvenile gilthead sea bream ( Sparus auratus ) cultured in earthen ponds under a natural photoperiod and temperature. Specimens were sampled, and the plasma, gill, kidney and liver were collected during winter 2005 and 2006 (January), spring 2005 (April), summer 2005 (July) and autumn 2005 (October). Plasma osmoregulatory parameters showed higher values in summer, while metabolic parameters presented different patterns of variations. Gill Na⁺,K⁺-ATPase activity decreased significantly in winter, while gill metabolite levels showed different patterns of variations among seasons. The enzymatic activities tested did not present a clear pattern of variation (glutamate dehydrogenase (EC 1.4.1.2) (GDH) and hexokinase (EC 2.7.1.11) (HK)] or significant differences along seasons glucose 6-phosphate dehydrogenase (EC 1.1.1.49)]. Kidney Na⁺,K⁺-ATPase activity decreased during summer and autumn. Different patterns of variation were observed in kidney metabolite levels while all the enzymatic activities assessed lactate dehydrogenase-oxidase (EC 1.1.1.27) (LDH-O), HK and GDH] presented the highest values during summer. In the liver, metabolite levels and enzymatic activities did not show significant variations or present clear patterns of variation along different seasons. These results indicated seasonal variations in the osmoregulatory and metabolic parameters of different organs (blood, gill, kidney and liver) in earthen pond-cultured gilthead sea bream ( S. auratus ), which could be mainly attributed to seasonal changes in temperature.