Reef sharks and inshore habitats: patterns of occurrence and implications for vulnerability

Abstract

Reef sharks play important roles in tropical reef environments, but population declines have occurred in various locations including the Great Barrier Reef (GBR). While many reef shark studies focus on coral reefs, some reef sharks have been found across a range of inshore and coastal habitats. This study analyses fisheries observer data across large spatial scales (19314 km2) to investigate the occurrence of reef sharks amongst a mosaic of inshore habitats in the GBR lagoon. Six reef shark species were recorded, but comprised a relatively small proportion (1.8% by number) of the elasmobranch catch. The blacktip reef shark dominated the catch of reef sharks (60.2% by number), with lemon sharks, zebra sharks, grey carpet sharks, whitetip reef sharks and grey reef sharks less frequently encountered. Reef sharks occur in several habitat types, and logistic regression models suggest that they are most likely to occur in reef habitats and shallow shore habitats. The presence of a reef within 2 km increased the encounter probability in all habitat types. While the use of these different habitats by reef sharks is not well understood, these habitat use patterns affect the exposure of these species to risks, including some risk factors not considered previously, and could also lead to cumulative impacts. These habitats may also perform important ecological functions in sustaining reef shark populations. Habitat use patterns should be considered in risk assessments and management and have important implications for method selection and survey design in field studies of reef sharks and other mobile reef species.