Energy reserves and metabolic status affect the acclimation of gilthead sea bream (Sparus aurata) to cold

During winter, low temperatures induce a direct metabolic depression in gilthead sea bream, without any significant compensatory effect below 13 degrees C. The present study therefore focused on how to improve response to cold in these fish, looking specifically at the two factors of diet (high energy, HiE, and low energy, LoE) and activity (normal, -SW, and sustained activity, +SW) prior to exposure to cold. Following a preparatory period of 75 days water was adjusted to 10 degrees C and kept for 40 days. Enzymatic activities and store deposition revealed that the HiE-SW group had acquired an energy surplus whilst the LoE+SW group exhibited an energy deficit. Liver enzyme activities evidenced diet dependence: LoE groups showed greater glucose-6-phosphate dehydrogenase activity and HiE groups showed greater lipoprotein lipase and hepatic lipase activities. Moreover, the HiE-SW group's lower citrate synthase/cytochrome-c-oxidase ratio reflected the energy surplus available. Perivisceral fat mobilisation caused by cold stress affected liver integrity, resulting in a pre-steatotic condition for the HE-SW group. The differences in liver enzyme activities produced by pre-cold conditions disappeared at low temperatures and enzymatic activities did not compensate. Therefore any improvement that would enable gilthead sea bream to face up to winter must be achieved prior to the appearance of low temperatures.