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

Abstract

Background - Biologging technologies have yielded new insights into the ecology and behaviour of elasmobranchs, but to date most studies involve animal capture and restraint to attach tags. Capturing animals usually results in a period of atypical behaviour after release and is undesirable or simply not possible for large and vulnerable elasmobranchs such as mobulas and whale-sharks. We developed and tested two non-invasive multisensor towed-tags designed to avoid animal capture and restraint. The use of towed packages creates additional data analysis challenges relative to fixed packages because towed device wobble independently of animal movements. We present five examples, two mobulas (reef manta and sicklefin devil ray) and three sharks (blue, tiger and whale-shark), to illustrate the advantages and challenges of this approach. We used animal-borne video to validate behavioural data derived from accelerometers and conducted an animal-attached versus towed-tag experiment to compare accelerometer data from both methods. Results - We used fluid dynamic models to calculate the added drag of towed devices on target species. We found that drag impact is acceptable for short-term tagging of large mobulas but the drag penalty associated with the current camera tag design is greater than 5% for most mature blue sharks. Despite wobble effects, swimming behaviour (tail-beat and wing-stoke frequency) captured by towed accelerometers was consistent with concurrent data from animal-attached tags and animal-born video. GPS sensors recorded up to 28 and 9 geolocations per hour of surface swimming by sicklefin devil ray and blue sharks, respectively. Conclusions - Towed tags with non-invasive attachments provide an effective low-impact alternative for acquiring high resolution behaviour and environmental data without capturing and handling animals. This tool yields great potential to advance current knowledge of mobula ecology and behaviour without capture or invasive tagging.