• self-feeding;
  • size;
  • rainbow trout Oncorhynchus mykiss (Walbaum), aggression;
  • production;
  • welfare


This study examined the production and welfare effects of including a large self-feeder competent rainbow trout, Oncorhynchus mykiss (Walbaum) (~665 g) in groups of smaller self-feeder competent conspecifics (~234 g). Costs and benefits were examined for both welfare (aggression, fin damage, condition and mortality) and production (self-feeder utilization and growth). The 8-week experiment used six groups of small trout; three treatment groups containing a large trout and three control groups. After 4 weeks the large fish were removed from treatment groups and added to control groups, thus reversing the treatments. Whilst it was thought that the presence of a larger fish would suppress aggression in smaller conspecifics this did not occur. In fact aggression was significantly (= 0.036) higher when large trout were present during the first 4 weeks. No significant differences were found between other welfare indicators, self-feeder utilization or production parameters. From a production and welfare perspective these results suggest that with the exception of initially increasing aggression larger fish do not represent a significant benefit or risk to smaller conspecifics being cultured in self-feeder equipped tanks, when all fish are self-feeder competent.