Ontogenetic variation in phototactic responses of juvenile chum salmon (Oncorhynchus keta) under different light intensities

Abstract Phototactic behavior plays an important role in habitat selection, predator avoidance, and migration in juvenile fishes, yet ontogenetic variation in responses to light intensity remains poorly understood. I examined the phototactic responses of chum salmon ( Oncorhynchus keta ) fry to white LED illumination under controlled laboratory conditions and compared the results with previously published data for smolts obtained using an identical experimental protocol. Fry (mean standard length 40.8 mm) were exposed to six light treatments ranging from 7 to 4,217 lx in an indoor raceway tank. Phototactic response was quantified as the cumulative number of fish observed near the light source and analyzed using generalized linear mixed models with a negative binomial distribution. A model including a quadratic effect of light intensity provided a substantially better fit than a linear model (ΔAIC = 22.3). Phototactic response increased with increasing light intensity at low irradiances but declined under high-intensity illumination, indicating a significant nonlinear relationship. Both the linear and quadratic effects of light intensity were significant, whereas life stage was not retained as a significant predictor in the final model. Nevertheless, fry tended to exhibit lower response values than smolts across most light conditions. At illuminances exceeding approximately 1,000 lx, both fry and smolts showed reduced occurrence near the light source, suggesting avoidance of strongly illuminated environments. The similar response patterns observed before and after smoltification suggest behavioral continuity during early ontogeny in O. keta . These findings indicate that strong illumination may be avoided by juvenile chum salmon and provide new insights into the ecological significance of phototactic behavior during their freshwater life stage.