Interactions between White Sharks (Carcharodon carcharias) and artificial reefs along the east-coast of Australia

Abstract

Artificial reefs are deployed for a variety of reasons, often to enhance the productivity of commercial or recreational fisheries. Threatened, endangered or protected (TEP) species are generally not the target of artificial reef programs, but may interact with the structures, raising questions as to whether the reefs might provide resources, or represent a threat due to aggregated fishing pressure. A comprehensive acoustic tagging program of White Sharks (Carcharodon carcharias) has been running along the east-coast of Australia since 2015. At the same time, monitoring programs, including the installation of acoustic listening stations, were undertaken at seven artificial reefs. This provided the opportunity to monitor interactions between this TEP species and artificial reefs. We recorded 590 interactions from 199 different White Sharks over a four-year period. Most interactions were short, with a mean duration of 14 min and 90% lasting less than 40 min. When the duration of interactions was modelled against a suite of environmental and biological predictor variables, only Chl-a and shark length were influential, but resulted in only very small changes (< 10 min) in the duration of interactions across their range of values. There were temporal trends regarding the frequency of interactions, these were consistent with previously described migration patterns of the eastern Australasian population. Visits were more common at artificial reefs at southern latitudes during summer to autumn, and northern latitudes during winter. There was no evidence of any shark becoming resident at one of the structures, given the short duration of interactions, it is unlikely aggregated fishing pressure at artificial reefs pose a threat to White Sharks. The interactions are likely driven by their inquisitive nature and the timing related to broad scale migratory movements. This study highlights the usefulness of acoustic telemetry networks in monitoring interactions between artificial reefs and TEP species.