Stress response in sea bream (Sparus aurata) held under crowded conditions and fed diets containing linseed and/or soybean oil

The physiological response to stressors in fish, including hormonal profiles and associated tissue responsiveness, is less documented. The aim of this study was to evaluate feeding gilthead sea bream (Sparus aurata) with diets containing linseed oil (LO) and soybean oil (SO) as substitutes to fish oil (FO) and their effect on fatty acid profile of head kidney and the consequent effect on stress response to a crowding challenge. Fish were fed 8 experimental diets with different levels of substitution 0% (FO), 70% (70LO, 70SO, 20LO50SO and 50LO20SO) and 100% (100LO, 100SO and 50LO50SO) over a period of 8 months. At the end of the feeding trial, samples of head kidney were collected for biochemical analysis and the fish were challenged by a crowding test. During the challenge, samples of plasma for cortisol analysis were collected at 0 h, 2 h, 5 h, 24 h, 48 h and 1 week in order to study acute and chronic stress responses. Results showed that fish fed vegetable oils (VO) had significantly decreased ARA, EPA, DHA and n-3 HUFA, while LA, LNA and total C18 PUFAs were significantly increased. The basal cortisol levels were significantly increased in fish fed 70LO, 100LO, 50LO20SO and 50LO50SO. The physiological response to crowding was significantly affected by the diet. After 2 h of crowding, all the treatments showed higher cortisol, with fish fed 100LO had significantly the highest response registering 131.38 pg/ml. After 5 h and 24 h, plasma cortisol was reduced in all treatments except in 50LO20SO. After 48 h of crowding, the plasma cortisol was increased in all treatments with the maximum value seen in fish fed 100LO (72.12 pg/ml). These levels were decreased in fish fed FO, 70LO, 100LO and 50L050SO after 1 week of crowding, but remained higher in fish fed 70SO, 100SO, 20LO50SO and 50LO20SO. In conclusion, fish fed LO diets showed the same response pattern as the control but with higher intensity regaining the basal levels after 1 week as the control, while fish fed SO had a slow response but changed the pattern characterized by a lower response at the beginning and longer recuperation without regaining the control value even after 1 week.