Population organisation in reef sharks: new variations in coastal habitat use by mobile marine predators

Abstract

Coastal habitats provide important functions for many species and may serve as nursery grounds for teleost fishes and sharks and rays. However, the importance of these habitats in sustaining marine species at seascape scales is debated, and their significance to reef shark populations is poorly understood. The blacktip reef shark Carcharhinus melanopterus is a widely distributed mobile marine predator; we explored its use of shallow, turbid coastal habitats in the Great Barrier Reef using tagging and acoustic telemetry. Residency and movement patterns of 23 individuals were monitored for up to 28 mo, revealing different patterns between sexes and sizes. Neonate/juveniles were short-term residents; adult females were long-term residents and preliminary data suggest that adult males were vagrants. Adult females and neonate/juveniles had small activity spaces (50% kernel utilisation distribution, KUD < 5 km²) and shared the same habitats and locations while adult males (when present) had larger activity spaces (50% KUD up to 14 km²). Population organisation, movement patterns and biological data suggest that blacktip reef sharks may use turbid coastal habitats for reproduction. This study reveals a new variation in coastal habitat use by sharks with habitat use patterns differing from those observed in other coastal sharks, and from conspecifics on coral reefs. These patterns do not conform to the characteristics of classical shark nurseries, and highlight the species' ecological flexibility. The study also demonstrates that shark behaviour and habitat use patterns can affect their vulnerability to fishing, habitat loss and climate change, and can affect the efficacy of marine protected areas.