Behavioural thermoregulation linked to foraging in blue sharks

Abstract

Large pelagic fishes often dive and surface repeatedly as if they were airbreathers, raising a question about the functions of these movements. Some species (e.g., bigeye tuna, ocean sunfish) apparently alternate foraging in deep cold waters and rewarming in shallow warm waters. However, it is unclear how prevalent this pattern is among species. Blue sharks are the widest-ranging pelagic shark with expanded vertical niches, providing a model for studying foraging-thermoregulation associations. We used electronic tags, including video cameras, to record the diving behaviour, muscle temperature, and foraging events of two blue sharks. During repeated deep dives (max. 422 m), muscle temperature changed more slowly than ambient water temperature. Sharks shifted between descents and ascents before muscle temperature reached ambient temperature, leading to a narrower range (8 °C) of muscle temperature than ambient temperature (20 °C). 2.5-h video footage showed a shark catching a squid, during which a burst swimming event was recorded. Similar swimming events, detected from the entire tag data (20 − 22 h), occurred over a wide depth range (5 − 293 m). We conclude that, instead of alternating foraging and rewarming, blue sharks at our study site forage and thermoregulate continuously in the water column. Furthermore, our comparative analyses showed that the heat exchange rates of blue sharks during the warming and cooling process were not exceptional among fishes for their body size. Thus, behavioural thermoregulation linked to foraging, rather than enhanced abilities to control heat exchange rates, is likely key to the expanded thermal niches of this ectothermic species.

Global-Scale Environmental Niche and Habitat of Blue Shark (Prionace glauca) by Size and Sex: A Pivotal Step to Improving Stock Management

“…Furthermore, blue sharks swiftly end their deep dives when muscle cools to 15°C, with a range of water temperature met during the dives spanning from 7 to 26°C (Carey et al, 1990). While finding the same minimum muscle temperature, Watanabe, Nakamura, and Chiang (Watanabe et al, 2021) concluded that the regular deep-diving behavior of this species can be parsimoniously explained by their motivation for maintaining body temperature within a narrow range while foraging in the stratified water columns. Overall, blue shark classes have variable tolerance to minimum temperature (LJF < SJ < [LJM, AF] < AM), which is valuable habitat information for identifying bycatch mitigation measures targeting the most vulnerable classes.…”

PILOT tags, new non-invasive towed tags for free ranging deep-diving megafauna, advantages and challenges

“…However, these may be overestimates because our model assumed that sharks were stationary relative to the water flow, which can be 300% more hydrodynamically efficient than a swimming fish undulating body (23). In addition, Watanabe et al (24) recently reported that gliding behaviour accounted for only 10% and 20% of total descent duration of tagged blue sharks, equating to less than 5 to 10% of the time during the typical yoyo-diving behaviour (25). The G-Pilot tags produce significantly lower drag penalties for blue sharks, close to the 5% threshold for animals larger than 2.5 m and are thus more suited to investigate the ecology of mature blue sharks and other slender-bodied sharks.…”

“…5 ms -1 , fig. 10,(24,25). The towed G-Pilot tag successfully quantified swimming performance, depth and temperature, and yielded precise geolocations of tagged sharks and mobulas, and thus is a valuable tool for short-term studies where high-resolution data is required and fixed tags are not an option.…”

Capture heats up sharks

“…Acute body temperature elevations have been known to cause a number of physiological, ecological and behavioural consequences; in terms of physiology, rapid elevations of body temperature can speed up digestion rates ( Carey et al, 1984 ), increase the expression of heat shock proteins ( Renshaw et al, 2012 ) and affect biological rates, such as metabolism ( Morley et al, 2019 ), potentially contributing to a deficit in the sharks’ daily energy budget, as has been shown before ( Bouyoucos et al, 2017 , 2018 ). Regarding behavioural changes, elevations of body temperature can result in distributional changes as sharks are known to adjust their position in the water column as a means to behaviourally thermoregulate ( Sims et al, 2006 ; Nakamura et al, 2020 ; Watanabe et al, 2021 ), or may expand their horizontal range if topographically limited (e.g. the tiger sharks in the Bahamas are depth limited and therefore may travel further offshore to seek out colder, deep waters) ( Gallagher et al, 2021 ).…”