Tigers without borders
Recent technological advances have opened a window on shark movements, including those of tiger sharks, and are helping researchers like Ryan Daly to understand how the sharks use their environment – and how best they can be protected.
Photo by Ryan Daly
Humans have recorded the migration patterns and seasonal presence of animals for millennia. Being able to observe large and conspicuous herds of animals, such as the wildebeest migration of the Serengeti, hunter-gatherer societies learned how to predict animal movements. As the seasons change, thousands of wildebeest still traverse the vast Serengeti plains in search of optimal areas for foraging and breeding. However, as migratory animals typically require enormous tracts of land that are free of barriers to movement, they are under increasing pressure from encroaching human development.
Today, the ocean is one of the last places on earth that remain free of barriers, and many species may roam over areas that encompass multiple countries and even continents and entire ocean basins. In the marine environment, though, the animals, unlike wildebeest herds, are not conspicuous and it is difficult to track their movements. This is especially true for species that cannot be observed from the surface or are highly mobile and cryptic. Only recently – within the past decade – have technological advancements enabled us to record the continuous movements of species such as tuna and sharks. This capability has completely transformed our understanding of the movements of marine animals and has provided insight into the global scale over which these animals travel.
Knowing how top marine predators such as sharks use their habitats is essential for predicting how and when they interact with their prey. This information is critical, as sharks, the top predators of the ocean, play an irreplaceable ecological role in an environment that is under increasing pressure. Additionally, as the number of people using the ocean for recreation grows, there is a need to understand how we can safely interact with potentially dangerous sharks. Finding the balance between keeping people safe and keeping shark populations healthy is critical both ecologically and economically. Shark tourism is becoming more and more popular around the world and contributes approximately US$314-million every year to the global economy. Iconic species, such as the tiger shark, are a crucial part of growing shark tourism, and learning to appreciate and interact with them is important.
The tiger shark is one of the largest of the shark species, growing to a length of 5.5 metres (18 feet), and it feeds on an exceptionally wide variety of prey, including birds, turtles, marine mammals and fish, throughout the world’s warm-temperate and tropical oceans. The species thus plays a key ecological role in shaping the population dynamics of its prey within and between marine ecosystems.
Although we know that tiger sharks occur in many different parts of the planet’s oceans, we still do not know when, why and how they travel between different areas. We have only recently discovered that adult tiger sharks can traverse an area of 6.7 million square kilometres (2.6 million square miles) in the Atlantic Ocean within a single year – an area about 223 times larger than the Serengeti, the location of the largest terrestrial migration on earth. This highlights the massive scale of tiger shark movements and suggests that our current understanding of their role within ocean ecosystems needs further exploration.
Considering that tiger sharks may cover vast distances, crossing oceans and navigating coastal regions, it is also important to take into account the wide range of threats that they may face. Currently tiger sharks are classified as Near Threatened on the IUCN’s Red List of Threatened SpeciesTM and they are killed throughout their range by commercial, artisanal and recreational fisheries. It is estimated that 100 million sharks are fished out of our oceans each year, primarily for the lucrative shark-fin trade, and tiger sharks are likely to be targeted for their large and valuable fins.
However, fishing is not the only threat to tiger sharks; habitat degradation, pollution and shark nets also contribute directly and indirectly to their mortality. As tiger sharks are relatively long-lived (up to 50 years) and reproduce slowly (gestating for 13–16 months), they are not adapted to deal with the current level of exploitation. Thus, the inevitable reduction in the size and numbers of tiger sharks could have negative consequences for marine ecosystems that depend on healthy populations of these top predators.
So, how do we find a balance for tiger sharks, humans and marine ecosystems? Marine protected areas (MPAs) can provide refuges free from fishing and may promote healthy ecosystems if managed properly. However, how do we know whether existing MPAs actually provide any benefit for tiger sharks? Typically, the delineation of an MPA does not take into account important tiger shark habitat that may be critical for mating, pupping and foraging. Furthermore, it is unlikely that MPAs will ever be large enough to encompass the entire range of an adult tiger shark. How can a patchwork of protected areas that only incorporates a small proportion of a shark’s coastal range provide relief to a species that undertakes migrations on an oceanic scale? To begin answering these complex questions we need to find out where and when adult tiger sharks move. This will enable us to estimate the level of exposure to risks that these sharks face and find out whether MPAs offer any protection to them.
