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.

Philopatry and Regional Connectivity of the Great Hammerhead Shark, Sphyrna mokarran in the U.S. and Bahamas

“…Preliminary analyses indicated that like other coastal species (Kessel et al, 2014a;Kajiura and Tellman, 2016; e.g., lemon and blacktip sharks) water temperature was a major driver of seasonal occurrence, however, future research should include other abiotic and biotic factors to better understand the causal relationship between environmental factors responsible for migratory behavior in this species (Schlaff et al, 2014). Like other coastal shark species (e.g., bull sharks, Espinoza et al, 2016), FIGURE 6 | Regional movements and migrations of great hammerhead sharks, Sphyrna mokarran tracked using satellite telemetry and photo ID (insert), main map acoustic telemetry and tracks of great hammerhead # 12 (black dashed line) and great hammerhead # 13 (green dashed line) and multiple individuals (blue dashed line). Numbers below symbols denote # directed movements between locations.…”

“…The idea that a portion of a population remains resident while the other migrate, known as partial migration, is a common phenomenon across the animal kingdom . It has recently been suggested for some highly mobile shark species including tiger and bull sharks (Carcharhinus leucas) (Papastamatiou et al, 2013;Lea et al, 2015;Espinoza et al, 2016) with important implications for management and conservation. Further, Grubbs and Kraus (2010) demonstrated differential migration in the highly migratory and highly exploited sandbar shark, Carcharhinus plumbeus where partial migration is sex, age and maturity dependent.…”

Acoustic telemetry and fisheries management

“…Acoustic telemetry has the potential to identify the relative proportion of a fish stock that remain resident vs. migrate, and provide clues as to why it might do so (e.g., Espinoza et al. ). When attempting to manage populations that undertake cross‐jurisdictional movements, it is important to know timing of these events, and the spatial scales over which they occur.…”

Bioecology and movements of bull sharks, Carcharhinus leucas , caught in a long-term longline survey off northeastern Brazil

“…Bull sharks have been reported to migrate towards warmer latitudes during the austral winter in the western Indian Ocean (Daly et al, 2014), and evidence of partial migrations in C. leucas tagged at the Great Barrier Reef, Australia, suggests a reproductive response (e.g. females migrating potentially to give birth) as the main driver (Espinoza et al, 2016). Although the shark tagged in this study has moved southwards during the austral winter, the temperature regime in this equatorial region might be sufficiently warm to inhibit a temperaturedriven latitudinal migration.…”

“…Adult C. leucas sharks make use of shallow nearshore habitats (Brunnschweiler et al, 2010;Hammerschlag et al, 2012;Heupel et al, 2015;Graham et al, 2016), particularly in areas of high freshwater inflow, associated with an elevated productivity, where they usually exhibit some level of site fidelity (Carlson et al, 2010;Daly et al, 2014). Nevertheless, this species also undertakes coastal migrations regulated by environmental, foraging and reproductive drivers (Daly et al, 2014;Espinoza et al, 2016). Open ocean migrations have also been observed, including one pregnant individual, which is known to have traversed the Indian Ocean from the Seychelles to Madagascar (Lea et al, 2015).…”