Adfluvial Adult Bull Trout Depth Distribution and Diel Vertical Migration Across Multiple Seasons

For fish at temperate latitudes, lake depths offer a range of environmental conditions that can be accessed to optimize physiological processes such as metabolism and maximize species’ and life-stage specific fitness functions. The depths that fish occupy are ecologically relevant, however, only recently have detailed assessments of individual depth distribution been possible through the use of biotelemetry. In the current study, acoustic biotelemetry transmitters equipped with pressure sensors were used to provide a detailed ecological assessment of adfluvial adult bull trout depth distribution and vertical migration relative to several biotic and abiotic variables. The results showed that bull trout depth distribution fluctuated seasonally with a clear pattern in diel vertical movement. Overall, bull trout were deepest during the summer and shallowest in the spring. The reproductive period was characterized by moderate depths whereas the period associated with ice cover showed moderate to deep depths. The results of this study provide detailed ecological information on a threatened species and identify how cold-water stenotherms might access various depths on a daily and seasonal basis to theoretically maximize fitness.