Assessment of vessel strike risk and performance of the Canadian protection measures for North Atlantic right whales in the Gulf of St. Lawrence

Vessel strikes remain a major threat to cetaceans and a challenge for marine traffic management. Since 2017, the government of Canada has implemented mandatory and voluntary vessel slowdowns and restricted areas to mitigate this threat to North Atlantic right whales Eubalaena glacialis (NARWs) in the Gulf of St. Lawrence. Using a spatial-temporal model of vessel traffic and NARW probability of occurrence, we aimed to quantitatively assess the efficacy of the management measures on the reduction of vessel strike risk to NARWs. We examined 3 metrics in relation to NARW presence: the relative probability of an event, the relative average probability of lethality (accounting for vessel speed, size, and type), and the relative risk of lethal strike. We calculated the annual risk from 2015 to 2023, examined the contribution to risk of different vessel types, and assessed the performance of each management zone. Results showed there was no significant change in risk on an annual scale. Cargo ships, fishing vessels, and tankers contributed the most risk. The relative risk of lethal strike revealed that the small reduction in lethality achieved by speed limits (relative average probability of lethality) was outweighed by the increasing marine traffic in the Gulf (relative probability of event). Overall, the Restricted Area was the most effective measure to significantly reduce risk. Since speed limits cannot be a complete solution on their own, the most effective strategy for lowering vessel strike risk is to include measures aimed at reducing the co-occurrence of vessels and whales.