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.