O’Sullivan, A. M., Linnansaari, T., Leavitt, J., Samways, K. M., Kurylyk, B. L., & Curry, R. A. (2022). The salmon-peloton: Hydraulic habitat shifts of adult Atlantic salmon (Salmo salar) due to behavioural thermoregulation. River Research and Applications, 38( 1), 107– 118. https://doi.org/10.1002/rra.3872

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 normal temperature conditions (<19
C)
boulder proximity, depth, velocity, and Froude number, 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 were critical controls on habitat use. We also observed AS forming
a distinct geometric formation during behavioural thermoregulation events, and
term this formation a “thermal-peloton”; in reference to competitive cycling in which
groups of cyclists pack together. The primary function of the peloton is undoubtedly
to reduce thermally induced stressed; however, we conceptualize that the geometry
of the peloton attenuates hydraulic-drag, and reduces the energetic expenditure of
individuals practicing behavioural thermoregulation. These data provide a blue print
for Atlantic salmon restoration work. The use of UAV-based sensors has the potential
to initiate a paradigm shift for river sciences.

KEYWORDS
Atlantic salmon, behavioural thermoregulation, optical bathymetry, Random Forest, thermal
infrared, thermal refuge, UAV

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