Preliminary observations on the movement ecology of a crested horn shark (Heterodontus galeatus).

The residency and movements of a single acoustically tagged female crested horn shark (Heterodontus galeatus) were monitored in Jervis Bay, Australia. The individual was intermittently detected by receivers throughout the 8-year study period and showed preference for particular rocky reefs in terms of its residency indices and duration of residency events. This individual exhibited lower residency and dissimilar movement patterns to that of the well-studied and sympatric Port Jackson shark (Heterodontus portusjacksoni), highlighting the need for research into the basic life history and movement ecology of H. galeatus.

Residency and movement patterns of adult Port Jackson sharks (Heterodontus portusjacksoni) at a breeding aggregation site.

Examining the movement ecology of mesopredators is fundamental to developing an understanding of their biology, ecology and behaviour, as well as the communities and ecosystems they influence. The limited research on the residency and movements of benthic marine mesopredators has primarily used visual tags, which do not allow for the efficient and accurate monitoring of individual space use. In this study, the authors investigated the residency and movement patterns of Port Jackson sharks Heterodontus portusjacksoni (Meyer 1793) at a breeding aggregation site in Jervis Bay, south-eastern Australia, using passive acoustic telemetry to further our understanding of the movement ecology of these important mesopredators. Between 2012 and 2014, individuals were tagged with acoustic transmitters, and their residency and movements within the bay were monitored for up to 4 years. H. portusjacksoni showed strong preferences for particular reefs within and between breeding seasons. Males had significantly higher residency indices at their favoured sites relative to females, suggesting that males may be engaging in territorial behaviour. Conversely, female H. portusjacksoni exhibited higher roaming indices relative to males indicating that females may move between sites to assess males. Finally, H. portusjacksoni showed temporal variation in movements between reefs with individuals typically visiting more reefs at night relative to the day, dusk and dawn corresponding with their nocturnal habits.

Does detection range matter for inferring social networks in a benthic shark using acoustic telemetry?

Accurately estimating contacts between animals can be critical in ecological studies such as examining social structure, predator-prey interactions or transmission of information and disease. While biotelemetry has been used successfully for such studies in terrestrial systems, it is still under development in the aquatic environment. Acoustic telemetry represents an attractive tool to investigate spatio-temporal behaviour of marine fish and has recently been suggested for monitoring underwater animal interactions. To evaluate the effectiveness of acoustic telemetry in recording interindividual contacts, we compared co-occurrence matrices deduced from three types of acoustic receivers varying in detection range in a benthic shark species. Our results demonstrate that (i) associations produced by acoustic receivers with a large detection range (i.e. Vemco VR2W) were significantly different from those produced by receivers with smaller ranges (i.e. Sonotronics miniSUR receivers and proximity loggers) and (ii) the position of individuals within their network, or centrality, also differed. These findings suggest that acoustic receivers with a large detection range may not be the best option to represent true social networks in the case of a benthic marine animal. While acoustic receivers are increasingly used by marine ecologists, we recommend users first evaluate the influence of detection range to depict accurate individual interactions before using these receivers for social or predator-prey studies. We also advocate for combining multiple receiver types depending on the ecological question being asked and the development of multi-sensor tags or testing of new automated proximity loggers, such as the Encounternet system, to improve the precision and accuracy of social and predator-prey interaction studies.