Diel patterns in swimming behavior of a vertically migrating deepwater shark, the bluntnose sixgill (Hexanchus griseus)

Abstract

Diel vertical migration is a widespread behavioral phenomenon where organisms migrate through the water column and may modify behavior relative to changing environmental conditions based on physiological tolerances. Here, we combined a novel suite of biologging technologies to examine the thermal physiology (intramuscular temperature), fine-scale swimming behavior and activity (overall dynamic body acceleration as a proxy for energy expenditure) of bluntnose sixgill sharks (Hexanchus griseus) in response to environmental changes (depth, water temperature, dissolved oxygen) experienced during diel vertical migrations. In the subtropical waters off Hawai'i, sixgill sharks undertook pronounced diel vertical migrations and spent considerable amounts of time in cold (5-7˚C), low oxygen conditions (10-25% saturation) during their deeper daytime distribution. Further, sixgill sharks spent the majority of their deeper daytime distribution with intramuscular temperatures warmer than ambient water temperatures, thereby providing them with a significant thermal advantage over non-vertically migrating and smaller-sized prey. Sixgill sharks exhibited relatively high rates of activity during both shallow (night) and deep (day) phases and contrary to our predictions, did not reduce activity levels during their deeper daytime distribution while experiencing low temperature and dissolved oxygen levels. This demonstrates an ability to tolerate the low oxygen conditions occurring within the local oxygen minimum zone. The novel combination of biologging technologies used here enabled innovative in situ deep-sea natural experiments and provided significant insight into the behavioral and physiological ecology of an ecologically important deepwater species.

Behavioural thermoregulation linked to foraging in blue sharks

“…Nonphylogenetic models provided a better fit than phylogenetic models, indicating that little phylogenetic signals are present in mass-adjusted heat transfer coefficients. Our result of negative allometry agrees with previous intraand inter-specific comparisons of k value in fishes; however, previous studies had much narrower body size range than the present study Fry 1970, 1974;Fechhelm and Neill 1982;Weller et al 1984;Kubo et al 2008;Coffey et al 2020) or analysed only k cool (Nakamura et al 2020). Heat is transferred via conduction across body surface and via convection through the gill (Carey and Gibson 1987;Boye et al 2009).…”

Environmental cycles and individual variation in the vertical movements of a benthic elasmobranch

“…For example, reports from recreational anglers of higher catches around slack tide suggest that tidal cycles may be associated with fine-scale changes in foraging behaviour. Alongside evidence for tidal influences on the movements of other elasmobranchs (Ackerman et al 2000;Whitty et al 2009;Campbell et al 2012) and benthic flatfish (Metcalfe et al 1990;Scott et al 2016), this suggests that the influence of the tides would benefit from further investigation, particularly with accelerometry (Gleiss et al 2013;Coffey et al 2020).…”

“…However, if higher activity is associated with movement off the seabed, the vulnerability of skate to trawling in autumn and winter may actually be lower, as noted for other species such as spurdog (Squalus acanthias) (Ellis et al 2015). This highlights the need for high-resolution data on the vertical movements of skate in relation to the seabed, which could be provided by accelerometers and sonar tags (Gleiss et al 2013;Lawson et al 2015;Coffey et al 2020). This research is essential for the effective implementation of fisheries management measures and spatial management approaches, such as marine protected areas, designed to achieve reductions in bycatch and improve skate conservation (Siskey et al 2019).…”

Climate-driven deoxygenation elevates fishing vulnerability for the ocean's widest ranging shark

“…Data-logging tags capable of measuring DO and swimming depths have been developed for recording DO levels that are directly encountered by free-ranging marine predators ( Bailleul et al, 2015 ) including large sharks ( Coffey and Holland, 2015 ). Oxygen saturations as low as 9.4% of normoxia (~0.8 ml O 2 l −1 at 5°C) were recorded from tags on bluntnose sixgill sharks ( Hexanchus griseus ) off Hawaii, Pacific Ocean, during dives to nearly 700 m depth ( Coffey and Holland, 2015 ; Coffey et al, 2020 ). DO sensor tag results to date confirm that very low DO concentrations are encountered and can be tolerated by large sharks during normal vertical movement.…”