Effects of moderately oxidized dietary lipid and the role of vitamin E on the stress response in Atlantic halibut (Hippoglossus hippoglossus L.)

Lipid peroxidation of marine fish diets can affect the nutritional value of the diet and consequently fish health, especially in the absence of adequate amounts of anti-oxidants. In this study, diets with different levels of oxidized oil and dietary vitamin E were fed to juvenile Atlantic halibut for 16 weeks and the effects on the acute stress response were investigated after this period. Fish were fed diets containing either non-oxidized (POV = 0.6 meq kg-1, diet A, control) or oxidized fish oil (POV = 7.5 meq kg-1, diets B and C; 15 meq kg-1, diets D and E). Diets A, C and E were supplemented with vitamin E (300 IU kg-1). Following this period, Atlantic halibut were subjected to a 1-h heat shock (HS; from 12 to 18 °C). Plasma cortisol and glucose, and red blood cells (RBC) heat shock protein 70 (hsp70) levels were measured prior to, 0 (immediately after), 6, 12, and 24 h after stress. Two-way ANOVA, using dietary treatment and sampling point as main factors, was performed. In all experimental groups, Atlantic halibut showed increased plasma cortisol levels immediately after (0 h) heat shock, however these returned to pre-stress levels by 6 h. Similarly, plasma glucose level increased significantly immediately after heat shock and decreased to pre-stress levels by 6 h. Dietary treatment had a significant effect on plasma glucose levels. Fish fed the highly oxidized diet (diet E) showed lower overall plasma glucose levels than fish fed less or non-oxidized diets (diets A, B, and C). RBC hsp70 was detected in all treatment groups. However, no significant changes in hsp70 levels were observed after exposure to heat shock. The overall results indicate that juvenile halibut fed diets containing oxidized fish oil up to a peroxide value of 15 meq kg-1 were able to cope with temperature stress, regardless of dietary vitamin E content. The glucose results, however, suggest that highly oxidized diets decrease the overall glucose levels. Furthermore, plasma cortisol and glucose, but not hsp70, seemed to be adequate indicators of heat shock stress in juvenile halibut.