Effect of rearing density on thyroid and interrenal gland activity and plasma and hepatic metabolite levels in rainbow trout, <i>Salmo gairdneri</i> Richardson

Abstract

There were significant inverse correlations between rearing density of rainbow trout, Salmo gairdneri Richardson, and final body weight, plasma L‐thyroxine (T 4 ), trüodo‐L‐tryronine (T 3 ), cortisol and protein concentrations, plasma T 4 /T 3 ratios and thyroid epithelial cell height. In addition, hepatosomatic indices and plasma free fatty acid concentrations were higher in fish reared at low (134 g 1 −1 ) density compared with groups reared at medium (210g1 −1 ) or high density (299g 1 −1 ), and the post‐feeding (3.5‐4h) elevation in plasma glucose and triglyceride levels evident in trout maintained at low rearing density was not found in those fish reared at higher densities. There were no significant effects of rearing density on hematocrit, carcass composition, hepatic glycogen and lipid levels and interregnal nucleus size.