This study investigated the effects of two key environmental parameters— temperature and pH, on the growth and physiological response of juvenile yellowtail kingfish in RAS operating under these same specific conditions. There appear to be no published data on the optimum temperature for growth of yellowtail kingfish. Yet given the ability to precisely control water temperature in RAS, this data is of critical importance for optimising fish performance and subsequently economic return. Although the pH of seawater typically ranges between 7.85 and 8.5, factors such as nitrification and carbon dioxide excretion cause pH in RAS to decline (Spotte, 1992). Although this decline is compensated for by the addition of a base to maintain both pH and alkalinity, pH values are usually maintained below typical seawater values for reasons including better biofilter performance and lower costs of buffering.