Abstract
Seasonal changes in temperature and photoperiod shape immunity across vertebrates, yet their interactive effects on adaptive immune responses in ectotherms remain poorly understood. In teleost fish, temperature is a well-established regulator of adaptive immunity, but the contribution of photoperiod—and its interaction with temperature—remains unclear. This knowledge gap has important implications for predicting immune responses under climate change and optimizing environmental conditions in aquaculture. Using a fully factorial experimental design, rainbow trout (Oncorhynchus mykiss) were acclimated to two temperatures (10°C and 15°C) and three photoperiods (8L:16D, 12L:12D, and 16L:8D) before immunization with a defined antigen. Temperature emerged as the primary driver of adaptive immunity, with fish held at 15°C exhibiting faster and stronger specific B-cell responses, higher plasma IgM concentrations, and greater antigen-specific antibody titers than fish maintained at 10°C. In contrast, photoperiod exerted more modest and context-dependent effects, with short-day conditions enhancing specific antibody production primarily in immunized fish. These findings demonstrate that adaptive immunity in rainbow trout is influenced more strongly by temperature than photoperiod, suggesting that warming water temperatures may alter immune performance even when seasonal photoperiod cues remain unchanged. Long-day photoperiods (16L:8D) did not significantly suppress adaptive immune responses relative to shorter photoperiods, indicating that extended lighting regimes commonly used in aquaculture may enhance fish production without compromising immune function. By clarifying how temperature and photoperiod interact to shape adaptive immunity, this study provides insight into disease ecology in changing aquatic environments and offers practical guidance for optimizing trout husbandry and vaccination strategies.