One-fifth of whale sharks in Ningaloo Reef Marine Park in Western Australia have major scarring or fin amputations, likely due to boat collisions. The team was tipped off by the whale shark tourism industry, which expressed concerns they were seeing more whale sharks with scarring in recent years.
"We wanted to determine whether there was an actual increase in the percentage of individual sharks with scarring from collisions with boats, or whether the tourism operators were encountering the same scarred individuals on multiple occasions," Emily Lester, a PhD candidate at the University of Western Australia, told IFLScience.
However, doing so is easier said than done. Our world is a web of movements, from the global shipping of goods to people going about their days to spiders catching prey. Untangling this network and understanding its repercussions is, at times, a difficult task, particularly if the creature in question spends much of their lives migrating long distances beneath the surface of the ocean. To make it even more challenging, whale sharks are "negatively buoyant," meaning they sink to the seafloor when they die and their injuries often go unnoticed and unreported.
To begin the study, more than 900 images of whale sharks were snapped by tour operators between 2008 and 2013, with only those of the left-hand side of the creatures used in the final analysis in order to eliminate double counting. Of those imaged, 16 percent had some kind of serious injury. The number of those injured had almost doubled during 2012 and 2013 compared to 2011, according to the study in Marine Ecology Progress Series.
Since sharks are able to travel well beyond the boundaries of Ningaloo Marine Park, they team could not determine where the injuries occurred. Given the creatures are protected inside Ningaloo Marine Park and the whale shark tourism industry is well-managed by the Department of Biodiversity of Conservation and Attractions, Lester said she would be surprised if these injuries were only occurring inside Ningaloo Marine Park.
"It is likely that we are underestimating the number of whale shark deaths because the scarred whale sharks that were included in our study are the animals that obtained an injury, survived the ordeal and returned to Ningaloo Marine Park to be photographed by tourism operators. This means that sharks that obtained severe injuries that quickly resulted in mortality were not accounted for."
The team also made note of bite marks from predators and those caused by boats. A whale shark attack will leave semi-circular wounds on the whale sharks, noted Lester. If the injury happened recently, there may even be some teeth left behind in the flesh. On the other hand, a propellor from boat collisions will leave a series of parallel lacerations on the body.
"By searching the literature we found that this threat is not unique to Western Australia, and whale sharks have been found with scarring from boat collisions throughout their distribution, which spans the world's tropical and subtropical oceans. This is important because it shows that even the world’s biggest fish is vulnerable to human activities, even if they are not directly targeting the sharks, on a global scale."