Heat-seeking sharks: support for behavioural thermoregulation in reef sharks

Abstract

Most fish are ectotherms and for this reason, water temperature is thought to be one of the main physical determinants of behaviour. We tested the hypothesis that behavioural thermoregulation occurs in 4 species of reef sharks (n = 58), where water temperature influences patterns of long-term (>1 yr) movement, by analysing acoustic monitoring and environmental datasets. We also used biotelemetry to measure the body temperature of 5 adult female blacktip reef sharks Carcharhinus melanopterus in response to changes in water temperature, to test whether they participate in inshore aggregations potentially to increase metabolic rates. We found that water temperature had the greatest influence on shark presence at each of our study sites (deviance explained = 3.7−54.6%), when compared to other variables. On average, body temperatures of female blacktip reef sharks were consistently warmer than average (±SE) water temperature by 1.3 ± 0.57°C, providing support for behavioural thermoregulation. The maximum body temperature of blacktip reef sharks while at the aggregation site corresponded to peak daily water temperatures. The average hourly body temperature varied little among individuals. Our models showed that the relationship between water temperature and the presence of female blacktip reef sharks was strongest at the aggregation site relative to other sites. These results suggest that reef shark movements are influenced by water temperature and provide additional support for the behavioural thermoregulation hypothesis. Such data are necessary for predicting how sharks might be affected by climate change and other human modifications to water temperature patterns.