Abstract
Large-bodied pelagic ectotherms such as sharks need to maintain internal temperatures within a favourable range in order to maximise performance and be cost-efficient foragers. This implies that behavioural thermoregulation should be a key feature of the movements of these animals, although field evidence is limited. We used depth and temperature archives from pop-up satellite tags to investigate the role of temperature in driving vertical movements of 16 oceanic whitetip sharks, Carcharhinus longimanus, (OWTs). Spectral analysis, linear mixed modelling, segmented regression and multivariate techniques were used to examine the effect of mean sea surface temperature (SST) and mixed layer depth on vertical movements. OWTs continually oscillated throughout the upper 200 m of the water column. In summer when the water column was stratified with high SSTs, oscillations increased in amplitude and cycle length and sharks reduced the time spent in the upper 50 m. In winter when the water column was cooler and well-mixed, oscillations decreased in amplitude and cycle length and sharks frequently occupied the upper 50 m. SSTs of 28 oC marked a distinct change in vertical movements and the onset of thermoregulation strategies. Our results have implications for the ecology of these animals in a warming ocean.
Highlights
Knowledge of how animals respond to the physical environment is vital for management strategies and prediction of how climate change will impact their ecology[1]
Over the year the mixed layer depth (MLD) increased from a depth of 28.07 ± 10.11 m in summer (21st June – 22nd September) to 64.85 ± 24.93 m in winter (21st December – 20th March) as the water column became increasingly well-mixed
Theories of foraging ecology and thermal physiology suggest that large animals found in tropical environments should maintain an optimal and stable body temperature range[4,14,15]
Summary
Knowledge of how animals respond to the physical environment is vital for management strategies and prediction of how climate change will impact their ecology[1]. A global analysis has shown that the primary thermal challenge is to avoid overheating, especially in tropical and desert areas[6] For this reason, description of behavioural strategies used as a response to temperature is important in the context of warming global climate[6]. The detailed patterns of vertical movement necessary to reconstruct relationships between temperature and depth generally require physical recovery of the tag, in order to access the high-frequency data stored in the tag’s archive This is a rare event[9,10], due to the unpredictability of movements post-tagging and the open ocean environments inhabited by pelagic sharks, such tags are occasionally recovered and provide access to data that is sampled at sufficient temporal scales to investigate hypotheses concerning the drivers of movement patterns[11]
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