Residency Patterns and Migration Dynamics of Adult Bull Sharks (Carcharhinus leucas) on the East Coast of Southern Africa

Abstract

Bull sharks (Carcharhinus leucas) are globally distributed top predators that play an important ecological role within coastal marine communities. However, little is known about the spatial and temporal scales of their habitat use and associated ecological role. In this study, we employed passive acoustic telemetry to investigate the residency patterns and migration dynamics of 18 adult bull sharks (195–283 cm total length) tagged in southern Mozambique for a period of between 10 and 22 months. The majority of sharks (n = 16) exhibited temporally and spatially variable residency patterns interspersed with migration events. Ten individuals undertook coastal migrations that ranged between 433 and 709 km (mean = 533 km) with eight of these sharks returning to the study site. During migration, individuals exhibited rates of movement between 2 and 59 km.d−1 (mean = 17.58 km.d−1) and were recorded travelling annual distances of between 450 and 3760 km (mean = 1163 km). Migration towards lower latitudes primarily took place in austral spring and winter and there was a significant negative correlation between residency and mean monthly sea temperature at the study site. This suggested that seasonal change is the primary driver behind migration events but further investigation is required to assess how foraging and reproductive activity may influence residency patterns and migration. Results from this study highlight the need for further understanding of bull shark migration dynamics and suggest that effective conservation strategies for this vulnerable species necessitate the incorporation of congruent trans-boundary policies over large spatial scales.

Conservation challenges of sharks with continental scale migrations

“…However, an absence of neonates in Sydney Harbor suggests this estuary was not functioning as an area for pupping, but could possibly be a location used for mating (Smoothey, unpublished data). This result differs from that of Daly et al (2014) who reported distinct temporal movements in adult C. leucas, presumed to be driven by seasonal temperature changes.…”

“…Recent research has indicated segregation by size may occur in C. leucas populations with smallest individuals using river habitats and largest individuals inhabiting marine regions (Simpfendorfer et al, 2005;Brunnschweiler and Barnett, 2013). Movements and habitat use by juvenile bull sharks has been well-documented in several studies (Heupel and Simpfendorfer, 2008;Heithaus et al, 2009;Heupel et al, 2010;Curtis et al, 2013), but data on adult movements are limited (Daly et al, 2014). In one of the few explorations of adult movement satellite tagged C. leucas were tracked moving distances of 2-1506 km (mean 143.6 km) over periods up to 85 days in the Gulf of Mexico spending the majority of their time in warm, shallow coastal habitats (Carlson et al, 2010).…”

“…In one of the few explorations of adult movement satellite tagged C. leucas were tracked moving distances of 2-1506 km (mean 143.6 km) over periods up to 85 days in the Gulf of Mexico spending the majority of their time in warm, shallow coastal habitats (Carlson et al, 2010). Acoustic tracking in South Africa also recorded movements ranging from 433 to 709 km (mean 533 km) over periods of 10-22 months (Daly et al, 2014).…”

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

“…Even though there is no evidence that this species copulates in freshwater, parturition likely occurs in warm-water estuaries and rivers (Montoya, Thorson, 1982;Compagno et al, 2005;Pillans et al, 2006). 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).…”

“…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).…”