Cetacean response to summer maritime traffic in the Western Mediterranean Sea

- We investigated the relationship between summer naval traffic and cetaceans sightings.
- We recorded the number of vessels in the presence and absence of cetacean sightings.
- All cetacean species were generally observed in locations with lower vessel abundance.
- Species response depends upon a combination of biological and environmental factors.
- Overlapping shipping lanes and cetacean hotspots doesn't adequately define risk areas.

Maritime traffic is one of many anthropogenic pressures threatening the marine environment. This study was specifically designed to investigate the relationship between vessels presence and cetacean sightings in the high sea areas of the Western Mediterranean Sea region. We recorded and compared the total number of vessels in the presence and absence of cetacean sightings using data gathered during the summer season (2009-2013) along six fixed transects repeatedly surveyed. In locations with cetacean sightings (N = 2667), nautical traffic was significantly lower, by 20%, compared to random locations where no sightings occurred (N = 1226): all cetacean species, except bottlenose dolphin, were generally observed in locations with lower vessel abundance. In different areas the species showed variable results likely influenced by a combination of biological and local environmental factors. The approach of this research helped create, for the first time, a wide vision of the different responses of animals towards a common pressure.