Abstract
Despite some evidence of within-population phenotypic variation in fish thermal behaviour, the occurrence of alternative tactics of this behaviour is rarely explicitly considered when studying natural populations. Brook charr provide an example of within-population variability in behavioural thermoregulation as revealed by a recent study on a lacustrine population of this species. The objectives of the present study were (i) to determine the influence of natural variability in the lake's thermal profiles on the expression of thermoregulatory tactics, and (ii) to determine the vertical and horizontal movements of individuals at different periods of the day to better understand the spatio-temporal behaviour associated with each thermoregulatory tactic. During summer 2010, 30 adult brook charr were equipped with thermo-sensitive radio transmitters to monitor their selected temperatures and daily movements. These individuals exhibited the same four behavioural thermoregulatory tactics observed in 2003 and 2005, but the expression of two of these was weaker in 2010. This result was associated with lake warming, which constrained the expression of two thermoregulatory tactics: brook charr significantly decreased their selected temperatures and daily movements when the mean daily epilimnion temperature was above 22.4°C. This study shows for the first time that the expression of behavioural thermoregulatory tactics is related to the lake's thermal regime and that the tactics are plastic through time.
Highlights
In ectotherms, body temperature is largely controlled by environmental temperature, and many mobile organisms have developed thermoregulatory mechanisms to survive and optimize temperature-dependent physiological processes [1,2,3]
This behaviour is observed in brook charr (Salvelinus fontinalis) when water temperatures reach 20 to 23uC [12,13]
Thermal tactics Lake Ledoux was stratified during the whole study period (5 July to 29 August 2010; Fig S1)
Summary
Body temperature is largely controlled by environmental temperature, and many mobile organisms have developed thermoregulatory mechanisms to survive and optimize temperature-dependent physiological processes [1,2,3]. Like rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar), stop feeding and defending their territory to seek cool-water sites when the temperature is over a specific threshold [10,11]. This behaviour is observed in brook charr (Salvelinus fontinalis) when water temperatures reach 20 to 23uC [12,13].
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