Acoustic accelerometry reveals diel activity patterns in premigratory Port Jackson sharks

Abstract

Distinguishing the factors that influence activity within a species advances understanding of their behavior and ecology. Continuous observation in the marine environment is not feasible but biotelemetry devices provide an opportunity for detailed analysis of movements and activity patterns. This study investigated the detail that calibration of accelerometers measuring root mean square (RMS) acceleration with video footage can add to understanding the activity patterns of male and female Port Jackson sharks (Heterodontus portusjacksoni) in a captive environment. Linear regression was used to relate RMS acceleration output to time‐matched behavior captured on video to quantify diel activity patterns. To validate captive data, diel patterns from captive sharks were compared with diel movement data from free‐ranging sharks using passive acoustic tracking. The RMS acceleration data showed captive sharks exhibited nocturnal diel patterns peaking during the late evening before midnight and decreasing before sunrise. Correlation analysis revealed that captive animals displayed similar activity patterns to free‐ranging sharks. The timing of wild shark departures for migration in the late breeding season corresponded with elevated diel activity at night within the captive individuals, suggesting a form of migratory restlessness in captivity. By directly relating RMS acceleration output to activity level, we show that sex, time of day, and sex‐specific seasonal behavior all influenced activity levels. This study contributes to a growing body of evidence that RMS acceleration data are a promising method to determine activity patterns of cryptic marine animals and can provide more detailed information when validated in captivity.

Diverse Activity Rhythms in Sharks (Elasmobranchii)

“…In the Port Jackson sharks, there was a conspicuous day/night difference in activity, with swimming commencing with the onset of night. This is consistent with activity patterns observed from wild and captive animals of the same species (Kadar et al, 2019). During the 9-day protocol, a number of conclusions were drawn.…”

“…Many benthic sharks rely on the sensory advantages they have over prey in low-light conditions by foraging at night (Kempster et al, 2015; Hammerschlag et al, 2017). The decreased activity during the day following prolonged activity at night observed in wild Port Jackson sharks (Kadar et al, 2019) may therefore arise from the digestive processes of sharks after nocturnal feeding events.…”

“…Visual observations from captive and wild animals (O’Gower, 1995; O’Gower and Nash, 1978; Powter and Gladstone, 2008, 2009), as well as historic catch records (McLaughlin and O’Gower, 1971), have highlighted the movements and migratory patterns of Port Jackson sharks. Acoustic tracking data of wild Port Jackson sharks have shown this species to be nocturnally active and travel greater distances at night (Powter and Gladstone, 2009; Kadar et al, 2019). This is consistent with data obtained from video and accelerometry data from captive animals, which exhibited late-evening activity peaks and daytime troughs.…”

Assessment of Machine Learning Models to Identify Port Jackson Shark Behaviours Using Tri-Axial Accelerometers

“…The ethogram consisted of resting and swimming (broader temporal scale; >2 s-mins) and vertical swimming and chewing as a proxy for foraging (finer temporal scale; <2 s). Port Jackson sharks are a benthic, nocturnal, non-obligate ram-ventilating species that spend the majority of their time resting on the benthos [ 18 ]. They aggregate in large numbers during winter breeding season, displaying high site fidelity [ 19 ], and it is probable that they exert large effects on the surrounding kelp forests through regulating echinoderm species that consume kelp habitats [ 20 , 21 ].…”

“…This context is ideal because it reduces the likelihood of other signatures in the accelerometry data (e.g., tailbeats) that may occur in synchrony with feeding masking the signature of this fine-scale behaviour. Resting and swimming represent periods of inactivity and activity, and sex, time of day and sex-specific seasonal activity have all been shown to influence activity in this species [ 18 ]. Accelerometers are a potential candidate for assessing the impacts of environmental variables upon activity and, in some cases, require validation to address these unknowns in free-ranging sharks.…”