Repeated long-range migrations of adult males in a common Indo-Pacific reef shark

The grey reef shark, Carcharhinus amblyrhynchos, is one of the most abundant coral reef sharks throughout the Indo-Pacific. However, this species has been critically impacted across its range, with well-documented population declines of > 90% attributed to human activities. A key knowledge gap in the successful implementation of grey reef shark conservation plans is the understanding of large-scale movement patterns, along with the associated biological and ecological drivers. To address this shortfall, we acoustically monitored 147 adult and juvenile grey reef sharks of all sexes for more than 2 yr across the New Caledonian archipelago, West Pacific. Here, we document multiple adult males undertaking return journeys of up to nearly 700 km in consecutive years. This constitutes the first evidence of repeated long-range migrations for this species. Although only a limited number of adult males were definitively tracked undertaking migrations, similar timing in changes in the detection patterns of a further 13 animals, mostly adult males, suggests this behavior may be more common than previously thought. The paucity of evidence for juvenile migrations and timing of adult movements suggest that mating is the motivation behind these migrations. Our results have important implications for management, given the potential of mature individuals to recurrently travel outside managed or protected areas. Future management of this species clearly needs to consider the importance of large-scale migratory behaviors when developing management plans.