Testing the efficacy of ship strike mitigation for whales in the Hauraki Gulf, New Zealand

Evidence-based policy, with proven efficacy, is key to the mitigation of human-wildlife conflict. Voluntary agreements to implement conservation actions can take less time to implement than mandatory policies. However, their efficacy is questionable as they rely on the awareness of the measures, cooperation, and goodwill of affected stakeholders. The aim of this study was to examine the efficacy of voluntary ship strike mitigation measures implemented in 2013 to protect a threatened, resident population of Bryde's whales (Balaenoptera edeni brydei) in the Hauraki Gulf, New Zealand. Automatic Identification System (AIS) and whale sightings data were used to determine the distribution and density of ships and whales throughout the Gulf to quantify and assess compliance to the voluntary 10 kt speed limit, and subsequently the risk of lethal ship strike to whales. From October 2014–September 2016, the location of 189 groups of whales and shipping routes of 703 vessels ≥70 m in length were plotted using ArcGIS. Hotspots of preferred whale habitat differed from 2010 to 2013, but shipping traffic density did not. Probability of lethal ship strike to whales in the Gulf has nearly halved (26%) since 2013 (51%) due to a reduction in average vessel speed from 13.2 kt to 10 kt. Observation and reporting systems developed to reduce potential ship strike events, were analysed at a fine scale using reporting rates and AIS data to measure the reactions of ships to real-time reports. Few whale sightings were reported to Harbor Control (n = 37; 8% shipping, 68% whale watch vessel, 24% other vessel types), and no ships diverted course to avoid reported whale locations. This study highlights the importance of testing the efficacy of conservation mitigation measures, as effective voluntary actions may provide a means to reduce the threat of ship strike to whales.