Global collision-risk hotspots of marine traffic and the world’s largest fish, the whale shark

Womersley, Freya C.; Humphries, Nicolas E. ; ...; Eguiluz, Víctor M., ...
Proceedings of the National Academy of Sciences 112, 1-10 (2022)

Marine traffic is increasing globally yet collisions with endangered megafauna such as
whales, sea turtles, and planktivorous sharks go largely undetected or unreported. Collisions leading to mortality can have population-level consequences for endangered species. Hence, identifying simultaneous space use of megafauna and shipping throughout
ranges may reveal as-yet-unknown spatial targets requiring conservation. However,
global studies tracking megafauna and shipping occurrences are lacking. Here we combine satellite-tracked movements of the whale shark, Rhincodon typus, and vessel activity to show that 92% of sharks’ horizontal space use and nearly 50% of vertical space
use overlap with persistent large vessel (>300 gross tons) traffic. Collision-risk estimates
correlated with reported whale shark mortality from ship strikes, indicating higher mortality in areas with greatest overlap. Hotspots of potential collision risk were evident in
all major oceans, predominantly from overlap with cargo and tanker vessels, and were
concentrated in gulf regions, where dense traffic co-occurred with seasonal shark movements. Nearly a third of whale shark hotspots overlapped with the highest collision-risk
areas, with the last known locations of tracked sharks coinciding with busier shipping
routes more often than expected. Depth-recording tags provided evidence for sinking,
likely dead, whale sharks, suggesting substantial “cryptic” lethal ship strikes are possible, which could explain why whale shark population declines continue despite international protection and low fishing-induced mortality. Mitigation measures to reduce
ship-strike risk should be considered to conserve this species and other ocean giants that
are likely experiencing similar impacts from growing global vessel traffic.

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