Multi-Channel Data-Logging: Towards Determination of Behaviour and Metabolic Rate in Free-Swimming Sharks

Elucidation of the behavioural ecology of elasmobranchs is essential to further Our understanding of the role of this order of vertebrates in marine ecosystems and therefore facilitate their conservation. Methodologies employed so far often rely on untested assumptions due to technological problems in resolving location, and orientation in three-dimensional space. This highlights the importance developing systems to resolve elasmobranch behaviour in the field. To that end semi-captive trials were conducted on two lemon sharks (Negaprion brevirostris) at Bimini Bahamas, where these individuals were equipped with archival-tags measuring 13 parameters (tri-axial acceleration, tri-axial magnetic field strength and a suite of environmental factors). Using tri-axial acceleration and tri-axial magnetic field strength, four behaviours and their intensities could be distinguished; resting, initiation of swimming after rest, steady-swimming, and fast-start swimming. Each behaviour could be characterised by the parameters measured (frequency and amplitude of acceleration peaks, changes in compass orientation). During steady swimming, animals displayed a wide range of tail-beat frequencies (0.4-1.2 Hz) and tail-beat acceleration amplitudes (0.002-0.16g). Overall dynamic body acceleration (ODBA), which is correlated with activity-specific metabolic-rate in terrestrial animals, showed a positive linear relationship with tail-beat frequency, indicating its potential as a proxy for energy expenditure in sharks. Comparison of ODBA for four distinct behaviours revealed it to be highest during fast-start swimming, and lowest during resting phases.