Vessel speed increases collision risk for the green turtle Chelonia mydas

Abstract

Vessel collisions contribute to the anthropogenic mortality of several threatened marine species including turtles, manatees, dugongs and whales, but scant data exist to inform the design of optimal mitigation measures. We conducted a field experiment to evaluate behavioural responses of green turtles Chelonia mydas to a research vessel approaching at slow, moderate or fast speed (4, 11 and 19 km h–1, respectively). Data were recorded for 1890 encounters with turtles sighted within 10 m of the research vessel’s track. The proportion of turtles that fled to avoid the vessel decreased significantly as vessel speed increased, and turtles that fled from moderate and fast approaches did so at significantly shorter distances from the vessel than turtles that fled from slow approaches. Our results imply that vessel operators cannot rely on turtles to actively avoid being struck by the vessel if it exceeds 4 km h–1. As most vessels travel much faster than 4 km h–1 in open waters, we infer that mandatory speed restrictions will be necessary to reduce the cumulative risk of vessel strike to green turtles in key habitats subject to frequent vessel traffic.