THE USE OF DIETARY ADDITIVES AS A MEANS OF COUNTERACTING ELEVATED TEMPERATURE IN RAINBOW TROUT: GROWTH, EFFICIENCY AND PHYSIOLOGICAL PERFORMANCE

Global surface temperatures are projected to rise 2-4°C by the year 2100. The consequences of this environmental change on physiological function will alter production efficiency in the rainbow trout (Oncorhynchus mykiss) industry with concomitant regional and economic impacts. Here, we examine the effects of warming on growth, feed efficiency, and performance, and whether dietary additives can ameliorate the metabolic costs associated with elevated temperature.  

Rainbow trout (44 g) were stocked into one of two recirculating systems maintained near trout's thermal optima of 15°C or at an elevated temperature of 18°C. Each dietary treatment was stocked into triplicate 440 L polyethylene tanks with 15 fish per tank. Individuals were fed a control fishmeal-based diet or one of three treatment diets including either a prebiotic (Biomos and Grobiotic A) or a metabolic modifier (Thermal Care). Diets were randomly assigned to triplicate tanks and fed twice daily to apparent satiation for 12 weeks. Fish were weighed every three weeks to assess growth and feed efficiency. At termination of the trial, three fish per tank were collected for whole-body proximate analysis and nutrient retention efficiencies. Blood from an additional three individuals was analyzed for acid-base balance and various metabolites. Five individuals per treatment were used to determine changes in maximum swimming speed. Data were compared using two-way ANOVA. When a significance level of p≤0.05 was found, Dunnet's contrast was used to compare treatments to fish fed the control diet at 15°C.

Elevated temperature had a significant effect on survival. Further, there was an interactive effect of temperature and diet on growth (gram gain per fish). Feed efficiency was higher and feed intake lower for individuals at 15°C than individuals at 18°C. There was a drop in plasma HCO₃^- from 15°C to 18°C and this effect was suppressed in fish fed Grobiotic A but not the other dietary additives. Other plasma metabolites also showed changes with temperature and diet. There was no change in maximum swimming speed between treatments; however, plasma PCO₂ was lower at exhaustion in 18°C fish fed a control diet than those at 15°C and this drop was suppressed in fish fed the dietary additives. Efficacy of these dietary constituents for physiological enhancement and implications for future productivity will be discussed.