Fish oil replacement in rainbow trout diets and total dietary PUFA content: I) Effects on feed efficiency, fat deposition and the efficiency of a finishing strategy

The aim of this study was to evaluate the effects of varying total amounts of dietary C18 PUFA on growth performance and tissue fatty acid composition in rainbow trout (Oncorhynchus mykiss). The experimental feeding trial was implemented over an entire grow-out period with a subsequent finishing period up to commercial size. Overall performance was unaffected by the dietary treatments, whilst fillet fatty acid make-up was mirroring that of the diet. The different dietary C18 PUFA levels had different effects on the fillet fatty acid productive value (i.e. fatty acid retention rates) during the grow-out and finishing periods. Specifically, during the grow-out period, diets rich in C18 PUFA resulted in increased n−3 LC-PUFA content in fish fillets. However, from an n−3 LC-PUFA restorative viewpoint, during the subsequent finishing period, the high C18 PUFA content of fillets of fish previously fed with high C18 PUFA diets was detrimental to the success of the finishing strategy. Therefore, depending on the strategy to be implemented (maximal fatty acid bioconversion or a fish oil restorative period), dietary C18 PUFA inclusion can be viewed from both a positive or a negative perspective.