Back to the wild: movements of a juvenile tiger shark released from a public aquarium.

Sharks are an important attraction for aquaria; however, larger species can rarely be kept indefinitely. To date, there has been little work tracking shark movements post-release to the wild. The authors used high-resolution biologgers to monitor a sub-adult tiger shark's pre- and post-release fine-scale movements following 2 years of captivity in an aquarium. They also compared its movement with that of a wild shark tagged nearby. Despite the differences in movement between the two sharks, with vertical oscillations notably absent and greater levels of turning seen from the released shark, the captive shark survived the release. These biologgers improve insight into post-release movements of captive sharks.

Links between the three-dimensional movements of whale sharks (Rhincodon typus) and the bio-physical environment off a coral reef.

BACKGROUND: Measuring coastal-pelagic prey fields at scales relevant to the movements of marine predators is challenging due to the dynamic and ephemeral nature of these environments. Whale sharks (Rhincodon typus) are thought to aggregate in nearshore tropical waters due to seasonally enhanced foraging opportunities. This implies that the three-dimensional movements of these animals may be associated with bio-physical properties that enhance prey availability. To date, few studies have tested this hypothesis. METHODS: Here, we conducted ship-based acoustic surveys, net tows and water column profiling (salinity, temperature, chlorophyll fluorescence) to determine the volumetric density, distribution and community composition of mesozooplankton (predominantly euphausiids and copepods) and oceanographic properties of the water column in the vicinity of whale sharks that were tracked simultaneously using satellite-linked tags at Ningaloo Reef, Western Australia. Generalised linear mixed effect models were used to explore relationships between the 3-dimensional movement behaviours of tracked sharks and surrounding prey fields at a spatial scale of ~ 1 km. RESULTS: We identified prey density as a significant driver of horizontal space use, with sharks occupying areas along the reef edge where densities were highest. These areas were characterised by complex bathymetry such as reef gutters and pinnacles. Temperature and salinity profiles revealed a well-mixed water column above the height of the bathymetry (top 40 m of the water column). Regions of stronger stratification were associated with reef gutters and pinnacles that concentrated prey near the seabed, and entrained productivity at local scales (~ 1 km). We found no quantitative relationship between the depth use of sharks and vertical distributions of horizontally averaged prey density. Whale sharks repeatedly dove to depths where spatially averaged prey concentration was highest but did not extend the time spent at these depth layers. CONCLUSIONS: Our work reveals previously unrecognized complexity in interactions between whale sharks and their zooplankton prey.