Alterations in lipid metabolism and use of energy depots of gilthead sea bream (Sparus aurata) at low temperatures

Gilthead sea bream cultured along the northern Mediterranean coast are affected by the winter season when low temperatures reduce fish feed intake and growth. The coldest episodes can provoke a fish pathology known as ‘winter disease’. The effects of low temperatures, as well as concurrent fasting, have been studied by transferring three groups of gilthead sea bream from 16 °C to 14 °C, 12 °C and 8 °C. Fish at 12 °C and 8 °C refused food, whereas those at 14 °C were not fed following the temperature drop. Changes in body indices, organ composition, liver metabolism, and in particular, lipid fractions and their fatty acids were analysed on days 7 and 20 after the temperature shift. Only the rapid reduction of non-polar lipids in muscle was common for the three conditions. Fasting effects were linked to the maintenance temperature, being maximal after 20 days at 14 °C where fish body weight, hepatosomatic index, and perivisceral fat were reduced by 18%, 40%, and 60%, respectively. In this group, liver lipids did not change, as was the case for the enzymatic activities of liver glucose-6-phosphate and phosphogluconic acid dehydrogenases (G6PDH and PGADH) and lipoprotein lipase (LPL). In contrast, the liver of sea bream submitted to 8 °C accumulated large amounts of non-polar lipids (from 80 mg to 125 mg in 20 days), changing in size and aspect (bigger, pale, and friable). Simultaneously, liver LPL and hepatic lipase (HL) activities decreased. After 20 days at 8 °C, sea bream exhibited incipient acclimation responses to low temperatures: rising levels of unsaturation ratio in gill and liver polar lipids, of docosahexaenoic acid (DHA) in muscle polar lipids, and of G6PDH and PGADH hepatic activities.Fish at 12 °C presented some changes similar to those of the group at 14 °C (e.g., in morphological indices, and LPL and HL activities) and others like the group at 8 °C (increases in G6PDH and PGADH), suggesting a temperature threshold for gilthead sea bream (below 13 °C).