Effects of Shark Removal on Spatial Partitioning Among Large Mesopredators on Coral Reefs

Coral reefs host high densities of large mesopredators occupying upper trophic levels. Understanding how these species partition space is essential to reveal underlying ecological dynamics and inform conservation. We hypothesised that red bass ( Lutjanus bohar ) and grey reef sharks ( Carcharhinus amblyrhynchos ) exhibit spatial and/or temporal partitioning influenced by species-specific behaviour and historical fishing pressure. To test this, we used acoustic telemetry to investigate habitat partitioning between these two large mesopredators at the Rowley Shoals and the Scott Reefs, Western Australia. These analyses were based on acoustic detections collected between 2007 and 2016 at the Rowley Shoals (17°20’S, 119°10’E) and the Scott Reefs (14°3’S, 121°46’E) on the north-western Australian continental shelf. Analysis of 95% kernel utilisation distributions (KUDs) showed broad spatial overlap, with both species frequently occupying habitats near reef fronts and channels. However, 50% KUDs revealed finer-scale partitioning: at the Rowley Shoals, core space use overlapped by less than 20%, whereas at the Scott Reefs, overlap exceeded 60%. These differences likely reflect historical fishing pressure, particularly the depletion of adult reef sharks at the Scott Reefs. There was little evidence of temporal partitioning. Both species were most active in the evening and highly resident in the same habitats throughout the year. Red bass exhibited wider-ranging movements from March to June, likely associated with spawning activity while grey reef sharks exhibited consistent presence and high site fidelity year-round. These findings underscore the influence of mesopredator size structure and abundance on spatial behaviour and highlight the conservation value of no-take marine reserves.