Circadian rhythms occur widely amongst living organisms, often in response to diel changes in environmental conditions. In aquatic animals, circadian activity is often synchronised with diel changes in the depths individuals occupy and may be related to predator–prey interactions, where the circadian rhythm is determined by ambient light levels, or have a thermoregulatory purpose, where the circadian rhythm is governed by temperature. Here, these two hypotheses are examined using animal-attached accelerometers in juvenile freshwater sawfish occupying a riverine environment displaying seasonal changes in thermal stratification. Across seasons, diel patterns of depth use (shallow at night and deep in the day) tended to occur only in the late dry seasons when the water was stratified, whereas individuals were primarily shallow in the early dry season which featured no thermal stratification. Activity was elevated during crepuscular and nocturnal periods compared to daytime, regardless of the thermal environment. Our observation of resting at cooler depths is consistent with behavioural thermoregulation to reduce energy expenditure, whereas activity appears linked to ambient light levels and predator–prey interactions. This suggests that circadian rhythms in activity and vertical migrations are decoupled in this species and respond to independent environmental drivers.
Bibliographical noteFunding Information:
We greatly acknowledge the involvement of the Nyikina-Mangala Rangers and the people of the Kimberley region of Western Australia. ACG is the recipient of an Australian Research Council Discovery Early Career Research Award (Project number 150100321), fieldwork was funded by the Fisheries Society of the British Isles, Australia Pacific Science Foundation, the Waitt Foundation, Western Australian Government State Natural Resource Management Program and Murdoch University. We would like to thank two anonymous referees (#2 in particular) whose crticisms greatly improved this paper.
© 2016, Springer International Publishing Switzerland.
- Behavioural thermoregulation
- Diel vertical migration