Physiological and Biochemical Responses of Rainbow Trout (Oncorhynchus mykiss) of Different Body Sizes Under Varying Temperature and Salinity Conditions: An L9 Orthogonal Design

This study evaluated the endpoint biochemical responses and associated physiological status of rainbow trout (Oncorhynchus mykiss) of different body sizes during salinity acclimation under varying temperature and salinity conditions. An L9 (34) orthogonal design was applied using three nominal body-size levels (100, 250, and 500 g), three temperatures (10, 15, and 20 °C), and three salinities (10, 20, and 30‰) over 60 days. Hepatic superoxide dismutase (SOD), alkaline phosphatase (ALP), and acid phosphatase (ACP) activities and plasma malondialdehyde (MDA), urea nitrogen (BUN), ammonia, alanine aminotransferase (ALT), and aspartate aminotransferase (AST) were measured. For inferential analysis, the three fish sampled from each tank were averaged to obtain one tank-level value, yielding 18 tank-level observations. Tank-level main-effects analysis showed that body size was significantly associated with ALP, ACP, MDA, BUN, ammonia, ALT, and AST. Temperature was significantly associated with SOD, ACP, MDA, BUN, and AST, whereas salinity was significantly associated with MDA, ALT, and AST. MDA and AST were the only endpoints associated with all three factors. The biochemical responses were pathway-specific rather than uniformly directional across treatment combinations. Because interactions could not be independently estimated in the L9 design, the identified main effects should be interpreted as orthogonal screening associations. Overall, body-size class was associated with the broadest range of endpoints and should be incorporated into future experimental assessments of rainbow trout within the tested salinity range of 10–30‰.