Conventional feed additives or red claw crayfish meal and dried microbial biomass as feed supplement in fish meal-free diets for rainbow trout (Oncorhynchus mykiss): Possible ameliorative effects on growth and gut health status

Ensuring efficient growth performance and fish welfare, while improving aquafeed sustainability is a major challenge of the aquaculture sector. To reduce the dependence from unsustainable marine-derived raw materials and to counteract the negative side-effects of vegetable protein sources, functional feeds represent an interesting solution. The present study explored the nutraceutical effects of low dietary inclusions of conventional feed additives (nucleotides and sodium butyrate) and novel potential feed supplements such as Louisiana red claw crayfish (Procambarus clarkii) meal (RCM) and dried microbial biomass from Tetraselmis suecica (TS) and Artrhospira platensis (AP) during a 104-day-feeding trial performed on juvenile rainbow trout (Oncorhynchus mykiss). Four test diets were formulated starting from a basal diet rich in vegetal protein sources (CV) including 0.25% of conventional feed additives (CVplus) or replacing 10% of dietary crude protein supplied by plant protein-rich ingredients of the basal diet CV with the test ingredients (RCM, TS and AP). Through a multidisciplinary approach, fish responses to the different dietary formulations were evaluated in terms of growth performance, gut welfare and immune response. Results obtained showed that all the test diets exerted an ameliorative effect on fish responses compared to CV one. CVplus and AP diets did not impair growth but resulted only in a marginal amelioration of gut health status that remained highly affected. Differently, TS and RCM diets led to a slight worsening of zootechnical parameters compared to the CV diet but were able to improve the overall welfare and preserve the structural integrity of distal intestine.