Effects of feeding diets containing various dietary protein and lipid ratios on the growth performance and pigmentation of post-juvenile coho salmon Oncorhynchus kisutch reared in sea water

Six extruded dry diets formulated to contain one of two levels of digestible protein (37% or 44%) and one of three levels of digestible lipid (16%, 23% or 30%) on a dry weight basis and a seventh diet (commercial control) were used to feed triplicate groups of post‐juvenile coho salmon Oncorhynchus kisutch in sea water. Fish were fed to satiation twice daily for 168 days. Growth performances were monitored every 28 days. On day 168, samples were taken from each replicate group per dietary treatment for determinations of whole‐body and muscle proximate compositions. Fatty acid compositions and astaxanthin concentration in both the experimental diets and fish flesh were assessed by gas chromatography (GC) and high‐performance liquid chromatography (HPLC) respectively. Coho salmon fed diets containing the higher lipid levels (23–30%) exhibited improved feed efficiency, protein efficiency ratio, percentage protein deposition and percentage gross energy utilization. Higher protein content diets supported better growth than those that had lower protein content, but the former led to lowered protein efficiency ratio, percentage protein deposition and gross energy utilization. Fish fed the diets with high lipid levels (23% or 30%) also had higher astaxanthin content in raw flesh.