Seasonal Plasticity in Diel Depth Distribution and Nocturnal Activity Patterns of Burbot: Exploring the Function of Diel Vertical Migrations of a Freshwater Benthic Predator

Diel vertical migration (DVM) is a strategy common amongst freshwater organisms and is thought to be driven by either thermal habitat selection for bioenergetic advantages, foraging habitat optimisation or predator avoidance. While a larval DVM has been reported for burbot, evidence supporting adult burbot DVMs and nocturnal activity patterns are scarce and the function of these behaviours remains unclear. In this study we utilized a continuous monitoring telemetry array featuring high frequency temperature and depth measures to model the seasonal and diel depth distribution and thermal habitat use of 30 burbot in Kinbasket Reservoir, BC, Canada. Burbot exhibited a clear DVM and nocturnal vertical activity pattern, except during their pre-spawning/spawning period, when DVM ceased and high night-time activity rates continued through the day. We argue that, the high night-time activity levels found during the DVMs, the continuation of DVMs when no thermal advantage was available and the increase in DVM amplitude among larger individuals together suggest that for a chemosensory feeder adapted to hunt under low light conditions, the increased foraging optimisation strategy rather than the thermal bioenergetics optimisation or competition/predation avoidance strategies may largely be driving both burbot DVMs and the seasonal shifts in DVM observed.