Tiger sharks (Galeocerdo cuvier) are one of the top predators in tropical and subtropical waters throughout the world. These large sharks are known to occupy both coastal and open seas, and can travel over both short and long distances. Recent studies have put greater focus on understanding their movement patterns, with the hope of providing more accurate information on their migration routes and vulnerability to commercial fisheries.
However, much of their diving behaviour and movements to different depths remains a mystery. But a new study published in Ecology and Evolution has attempted to reveal more to this secret side of tiger sharks.
The researchers used satellite tagging techniques to track the movements of nine tiger sharks in the northern Caribbean Sea between 2008 and 2009. Data from the tags revealed that most of the sharks exhibited a similar type of diving behaviour. Most individuals made ‘yo-yo’ dives (repeated oscillatory dives) in the upper 50 m of the water column. Although they generally spent most of their time near the surface (in the upper 5 m of water), they also frequently made dives to depths of over 200 m. One shark was even recorded at a depth of 828 m.
However, of greater interest was the differences that were observed in the frequency of these diving patterns. Some individuals spent most of their time in the upper 5 m of water, while others tended to remain at a depth of 20-50 m. In addition, some sharks preferred to make their deep dives at night, but others only dove during daylight hours. Several other inconsistencies in diving behaviour were observed, none of which could be linked to obvious differences in sex, age, or physical environment.
One possible explanation is that the tiger sharks use deep dives as a method of orientation, and move to greater depths whenever they travel off or on to their home range. Another possibility concerns foraging behaviour. The low abundance of prey items in open ocean areas may force the sharks to vary their diving behaviour, thereby enabling them to target alternative prey such as deep water crabs and cephalopods.
Regardless, it appears that these erratic diving patterns may actually be helping this species to avoid the worst effects of the fishing industry. The high variability means that only a subset of the tiger shark population is ever likely to be at risk from fishing gear on a local scale. It may even partly explain why tiger shark population declines are typically not as great as for other shark species exposed to the same fisheries.
Reference: Vaudo, J., Wetherbee, B., Harvey, G., Nemeth, R., Aming, C., Burnie, N., Howey-Jordan, L., & Shivji, M. (2014). Intraspecific variation in vertical habitat use by tiger sharks (Galeocerdo cuvier) in the western North Atlantic Ecology and Evolution DOI: 10.1002/ece3.1053
Image: Albert Kok