23 June 2022
Whale sharks face significant threat from ship strikes
Published online 17 May 2022
Marine traffic may be responsible for a larger number of whale shark deaths than previously considered.
Fatal collisions of large ships with whale sharks may have been occurring unchecked for many years without sufficient research or management to prevent them, according to a new study.
An international team of scientists, including researchers in Qatar and Saudi Arabia, combined satellite-tracked movements of whale sharks and vessel activity to identify areas of risk and possible collisions. Data from 348 whale sharks showed that 92% of their horizontal space use and nearly 50% of vertical space overlap with large vessel traffic.
The study identified shark hotspots in the Atlantic Ocean (Gulf of Mexico, St Helena), Indian Ocean (Arabian Gulf, the Red Sea, northwest Madagascar, western Australia) and the Pacific Ocean (Gulf of California, Gulf of Panama, Panama Basin, New Guinea, the Philippines). About one third of whale shark hotspots overlapped with the highest collision-risk areas.
The team found that whale shark tag transmissions ended more often than expected in the busiest vessel areas, such as marine highways and port entrances. Using depth-sensing tags, the team inferred that loss of transmissions was due to whale sharks being hit, killed and sinking to the seafloor.
“Assessing ship strike risk at relevant space and time scales is crucial to understand the significance of risk for marine megafauna,” says Hannah Blondin at Hopkins Marine Station, Stanford University, US, who was not involved in the study. “This study advances how risk is calculated by incorporating individual whale shark tracks and all three dimensions of movement (horizontal and at depth) to show how ship strike can be detrimental for a vulnerable species like whale sharks.”
The results indicate high levels of undetected or unreported ship strikes, which may explain why whale shark populations continue to decline despite international protection. With populations declining by more than 50% over the last 75 years, the whale shark is currently listed as endangered by the International Union for the Conservation of Nature (IUCN) Red List.
“The implications of our findings are wide-reaching for whale sharks and other at-risk marine megafauna such as whales, and suggest that we need to improve our capacity for ship-wildlife collision monitoring as well as collision protection,” says Freya Womersley of the University of Southampton, UK, who led the study as part of the Global Shark Movement Project. “This will involve stakeholder engagement from private shipping companies and international regulatory bodies such as the International Maritime Organisation.”
She adds that the study provides the first blueprint of mapped high-risk areas that can be used to identify where to undertake future research or implement new management. “Speed limits may help prevent collisions, as well as providing wider benefits such as reduced emissions and lower noise levels, but we need more research focusing on the implications of reducing ship speed around whale shark areas to support this,” she says. “Hopefully our work, along with an increase in public awareness, will facilitate these activities and protect whale sharks from population declines in future.”
Womersley, F. C. et al. Global collision-risk hotspots of marine traffic and the world’s largest fish, the whale shark. PNAS https://doi.org/10.1073/pnas.2117440119 (2022).