The salmon-peloton: hydraulic habitat shifts of adult Atlantic salmon (Salmo salar) due to behaviour thermoregulation

Abstract In recent decades there has been an increase in conservation and restoration projects targeting Atlantic salmon ( Salmo salar – AS), as populations in eastern Canada decline. Missing however, is an understanding of thermo-hydraulic habitat use by adult AS during summer, and thus the actual benefits of altering in-river physical structures. Here, we illustrated how optical and thermal infrared (TIR) imagery acquired from a UAV can be used in concert with in-situ depth and velocity data to map adult AS and develop models of thermo-hydraulic habitats in the Miramichi River, New Brunswick. We found during optimal thermal conditions (< 19 °C) proximity to boulders and Froude numbers, a non-dimensional hydraulic metric, were key parameters that characterized adult AS habitat. However, during behavioural thermoregulation events (>19 °C), proximity to the cool thermal plume and Froude number, a non-dimensional hydraulic parameter, were critical controls on habitat use. We also observed AS formed a distinct geometric formation during behavioural thermoregulation events, and term this formation a ‘ thermal-peloton ’. The primary function of the peloton is undoubtedly to reduce thermally induced stressed; however, we conceptualize the geometry of the peloton attenuates hydraulic-drag, and reduces energetic expenditure of individuals practicing behavioural thermoregulation. These data provide an unrivaled viewpoint of thermo-hydraulic habitat selection by adult AS, and a blue print for restoration work. The use of UAV-based sensors has the potential to instigate a paradigm shift for river sciences. The age of applying hyper-resolution, remote sensing for river science and aquatic ecology is immensely exciting.