Evidence of Partial Migration in a Large Coastal Predator: Opportunistic Foraging and Reproduction as Key Drivers?

Abstract

Understanding animal movement decisions that involve migration is critical for evaluating population connectivity, and thus persistence. Recent work on sharks has shown that often only a portion of the adult population will undertake migrations, while the rest may be resident in an area for long periods. Defining the extent to which adult sharks use specific habitats and their migratory behaviour is essential for assessing their risk of exposure to threats such as fishing and habitat degradation. The present study used acoustic telemetry to examine residency patterns and migratory behaviour of adult bull sharks (Carcharhinus leucas) along the East coast of Australia. Fifty-six VR2W acoustic receivers were used to monitor the movements of 33 bull sharks in the central Great Barrier Reef (GBR). Both males and females were detected year-round, but their abundance and residency peaked between September and December across years (2012-2014). High individual variability in reef use patterns was apparent, with some individuals leaving the array for long periods, whereas others (36%) exhibited medium (0.20-0.40) or high residency (> 0.50). A large portion of the population (51%) undertook migrations of up to 1,400 km to other coral reefs and/or inshore coastal habitats in Queensland and New South Wales. Most of these individuals (76%) were mature females, and the timing of migrations coincided with the austral summer (Dec-Feb). All migrating individuals (except one) returned to the central GBR, highlighting its importance as a potential foraging ground. Our findings suggest that adult bull sharks appear to be highly dependent on coral reef resources and provide evidence of partial migration, where only a portion of the female population undertook seasonal migrations potentially to give birth. Given that estuarine habitats face constant anthropogenic pressures, understanding partial migration and habitat connectivity of large coastal predators should be a priority for their management.

Fig 1. Location of acoustic receiver arrays (black dots) along the East coast of Australia.

Receiver arrays: LOI—Low Is.; ORI—Orpheus Is.; TSV—Townsville Reefs; CB—Cleveland Bay; HI—Heron/One Tree Is.; LEI—Lady Elliot Is.; FI—Fraser Is.; SC—Sunshine Coast; MB—Moreton Bay; CR—Clarence River. Inset on the right top corner shows a close-up view of the TSV Reefs: Bra—Bramble; Rib—Rib; Bre—Brewer; Lod—Lodestone; Kee—Keeper; Whe—Wheeler; Broadhurst—Bhu; Dav—Davies; Pin—Pinnacle; Kel—Kelso; Hel—Helix; Gru—Grub; Cen—Centipede; Glo—Glow; Yan—Yankee; Cot—Cotton; Arc—Arc.

Fig 2. Presence plot of bull sharks (Carcharhinus leucas) monitored along the East coast of Australia.

The tag number, sex (M—males; F—females) and size (fork length) of individual sharks are provided.

Fig 3. Dendrogram showing individual bull sharks (Carcharhinus leucas) grouped based on their residency to the Townsville Reefs.

Fig 4. Pattern of occurrences of bull sharks (Carcharhinus leucas) monitored in the central Great Barrier Reef.

Panels show: (A) mean number of sharks detected (± SE) and cumulative number of sharks tagged during the monitoring period; (B) mean residency index (± 95% confidence intervals—shaded polygons) and temperature fluctuations recorded during the monitoring period.

Fig 5. Connectivity plot of bull sharks (Carcharhinus leucas) monitored in the central Great Barrier Reef.

Panels A and B show the number of male and female bull sharks moving between monitoring reefs. Each colour indicates a unique reef and arrows represent the number of individual sharks that move between reefs. Panel C shows the frequency of moves (%) within and between inner and outer midshelf reefs.

Fig 6. Proportion of bull sharks (Carcharhinus leucas) that: (i) moved away from TSV and was detected in other arrays; (ii) stayed and/or was only detected within TSV; and (iii) returned to TSV.

Fig 7. Migratory patterns and broad scale connectivity of bull sharks (Carcharhinus leucas) in Queensland (QLD) and New South Wales (NSW), East coast of Australia.

Acoustic receiver arrays are indicated by red dots and dashed lines represent movement path between arrays. Each colour from the connectivity plot indicates a unique array and arrows represent the number of individual sharks that move between arrays. Receiver arrays: LOI—Low Is.; ORI—Orpheus Is.; TSV—Townsville Reefs; CB—Cleveland Bay; HI—Heron/One Tree Is.; LEI—Lady Elliot Is.; FI—Fraser Is.; SC—Sunshine Coast; MB—Moreton Bay; CR—Clarence River.

Fig 8. Number of bull sharks (Carcharhinus leucas) detected in different acoustic receiver arrays along the East Coast of Australia (grey bars—males; white bars—females).