The world’s largest fish suffered a mysterious decline. Researchers embarked on a global tracking project to find out why.
Reference: Womersley, F. C., Humphries, N. E., Queiroz, N., Vedor, M., da Costa, I., Furtado, M., Tyminski, J. P., Abrantes, K., Araujo, G., Bach, S. S., Barnett, A., Berumen, M. L., Bessudo Lion, S., Braun, C. D., Clingham, E., Cochran, J. E. M., de la Parra, R., Diamant, S., Dove, A. D. M., … Sims, D. W. (2022). Global collision-risk hotspots of marine traffic and the world’s largest fish, the whale shark. Proceedings of the National Academy of Sciences, 119(20), e2117440119. https://doi.org/10.1073/pnas.2117440119
The whale shark is an iconic fish—it’s captured our imagination in popular culture, from being featured on stamps and currency, to starring in movies. However, their population has suffered an unexplained decline over the last few years, decreasing by over 50% over the last 75 years. Researchers have proposed that a likely culprit may be industrialized shipping—over the last 25 years, the number of large merchant vessels (>100 gross tons) has increased by over 50 times worldwide. Trade routes frequented by these ships often overlap with the shallow coastal waters where whale sharks gather in large numbers, potentially leaving these massive, slow swimmers vulnerable to lethal collisions. But researchers were still left with a challenging question: how should they monitor and quantify the impact of industrialized shipping on whale sharks?
A titanic task
Led by researchers from the Marine Biology Association and the University of Southampton, scientists from over 50 research institutions mapped the movements of both whale sharks and large vessels across the globe. The extensive endeavor involved using satellite data to track nearly 350 whale sharks— no small feat considering whale sharks are known to cover thousands of kilometers in trans-oceanic migrations. By submitting the data to the Global Shark Movement Project, the researchers were able to piece together data from different locations to create a global map of whale shark movements.
When compared to marine traffic data across the world, whale shark movements overlapped with shipping routes over 85% of the time. Researchers also confirmed that areas with a high density of whale sharks co-occurred with areas of high collision risk. Busy ports in semi-enclosed gulfs, such as the Arabian Gulf, proved perilous for whale sharks, as encroaching landmasses crowded whale sharks and ships together. Notably, signals from whale shark tracking tags were cut off more frequently in high traffic shipping lanes.
A striking conclusion
Beyond assessing the potential risk of large ships, researchers also found that tracking data revealed the “smoking gun” of their investigation. Tracking data showed whale sharks moving into the path of a shipping route, then recorded slowly descending depth readings— a strong indication that the whale shark was struck and killed, and its body sinking to the sea floor. Short of witnessing a collision firsthand, this data was the closest to direct evidence for the lethality of ship collisions.
By demonstrating that industrialized shipping poses a real threat to whale sharks, researchers hope that their data will be used to inform regulatory and conservation policy. Mitigating and managing the risks of industrial shipping, such as reducing vessel speeds in collision hotspots, could substantially decrease the risk of lethal collisions. More widespread monitoring and regulation is likely to benefit not only on whale sharks, but other large creatures such as sea turtles and ocean sunfish—hopefully, this study will result in positive (and less literal) impacts on marine life as a whole.
I’m a Ph.D. candidate in Biology at Caltech. I study animal regeneration across phylogeny— my interests include the evolution and development of weird animals and obscure science history.