Abstract
Large whales are likely the marine megafauna most at risk from vessel collisions. However, studies often identify areas of greatest risk by calculating two-dimensional overlap metrics, overlooking vertical overlap in the three-dimensional water column. This study quantifies collision risk for fin whales in the north-east Atlantic by including dynamic parameters in collision risk models and simulates potential mitigation measures. Dive profiles of 21 fin whales instrumented with time-depth recorders in the Azores were used to estimate the proportion of time spent within reach of vessel draughts across the north-east Atlantic. Fin whales were at greater vertical risk at night, with a median dive depth of 5 m compared to 12 m during daylight hours. When dive depths (vertical risk) was integrated into spatio-temporal variation of vessel characteristics and animal distribution, 972 (796–1200 when incorporating uncertainty in animal density estimates) collisions were estimated to occur annually, of which 641 (525–791) were likely to be fatal – when assuming no avoidance response. This is well above the estimated sustainable potential biological removal level of 131 individuals a year in this region. Simulation of potential mitigation measures indicated that re-routeing shipping lanes may be largely infeasible, whereas vessel slowdowns may be more practical and effective to minimize population-level impacts to a vulnerable species. However, how such measures would be accepted by the shipping industry remains to be seen. These findings provide a quantitative assessment of ship-strike risk and inform mitigation methods, providing an invaluable basis for discussions with environmental managers and maritime authorities.