Cool runnings: behavioural plasticity and the realised thermal niche of basking sharks

Abstract

Long-distance migrations by marine vertebrates are often triggered by pronounced environmental cues. For the endangered basking shark (Cetorhinus maximus), seasonal changes in water temperature are frequently proposed as a cue for aggregation within (and dispersal from) coastal hotspots. The inference is that such movements reflect year-round occupancy within a given thermal ‘envelope’. However, the marked variance in timing, direction and depth of dispersal movements hint at a more nuanced explanation for basking sharks. Here, using data from pop-off archival transmitters deployed on individuals in Irish waters, we explored whether autumnal decreases in water temperature triggered departure from coastal habitats and how depth and location shaped the sharks’ realised thermal environment over time. Temperature was not an apparent driver of dispersal from coastal seas, and variance in daily temperature ranges reflected occupancy of different habitats; coastal mixed/stratified and offshore subtropical/tropical waters. Furthermore, individuals that moved offshore and into more southern latitudes off Africa, exhibited a distinct daily cycle of deep dives (00:00–12:00, 200 m–700 m; 12:00–00:00, 0–300 m), experiencing a more extreme range of temperatures (6.8–27.4 °C), including cooler minimum temperatures, than those remaining in European coastal habitat (9.2–17.6 °C). Collectively, these findings challenge the supposition that temperature serves as a universal driver of seasonal dispersal from coastal seas and prompts further studies of deep-water forays in offshore areas.

Highly variable basking shark (Cetorhinus maximus) diving behavior in the lower Bay of Fundy, Canada

“…Sharks in 2012 stand out amongst the rest of the deployments in that no dive shape dominated. While we cannot speak to the survival of these sharks beyond the deployment period, this does provide empirical evidence of behavioral adjustment by basking sharks to localized climate change effects, as has been suggested in the NE Atlantic by Johnston et al (2022). Additionally, the threshold density of stage V C. finmarchicus required to support a basking shark is likely significantly lower than that for a right whale mitigating some of the drastic changes in right whale distributions in the western North Atlantic (Davies et al, 2019;Record et al, 2019;Sorochan et al, 2019;Meyer-Gutbrod et al, 2021b).…”

“…However, recent work by Nakamura et al (2020) demonstrates gigantothermy from the very large body size of the whale shark serves to buffer changes in internal muscle temperature, even during vertical movements to far colder water temperatures. In basking sharks, Johnston et al (2022) found that temperature was not a factor affecting timing of long-term movement patterns in the NE Atlantic, although these authors state that their findings might only apply locally. We hypothesize that without some metric of foraging behavior on a given subsurface movement, perhaps derived from accelerometers (Gleiss et al, 2011b), it will be difficult to connect time at a given water temperature to the vertical movements of basking shark movements in the Bay of Fundy.…”