Potential ship strikes and density of humpback whales in the Abrolhos Bank breeding ground, Brazil

Ship strikes are one of the major threats to large whales worldwide. The potential impact from increasing vessel traffic therefore is a concern for the future of the Brazilian humpback whale (Megaptera novaeangliae) population. In order to evaluate the risk of collision between large vessels and humpback whales along coastal shipping routes in the region of the Abrolhos Bank – the most important breeding ground for the species in the south-western Atlantic Ocean – commercial vessels were used as platforms of opportunity to monitor the coastal shipping routes. Humpback whale density along coastal routes was estimated through multiple covariate line-transect ‘distance sampling’. The number of potential collisions per year was estimated using a model based on vessel size and speed, track lengths, population density and the surfacing behaviour of whales. During the peak of the 2011 breeding season, whale density on the coastal route between Belmonte and Caravelas was estimated to be 0.085 whales km-2 and between Caravelas and Barra do Riacho, 0.023 whales km-2. The three commercial vessels operating in coastal waters between Belmonte and Barra do Riacho had the potential to collide with 25 humpback whales in total, and kill 17 of these, during the 2011 breeding season. As vessel traffic increases in the Abrolhos Bank region and humpback whale population grows, the likelihood of a vessel collision will increase. A simple and effective framework to study how changes in whale density will affect their vulnerability to ship strikes, and ensure the suitability of alternative shipping routes is presented, while evaluating whether additional mitigation measures are necessary, such as speed limits in areas or periods with higher densities of whales. Copyright © 2014 John Wiley & Sons, Ltd.