Impacts of ship noise on harbour porpoise movements and population dynamics

Noise from commercial ships can affect cetaceans by scaring them away from their foraging grounds, which may ultimately affect populations negatively. As the number of ships increases globally, we urgently need new methods for assessing such impacts to guide management and conservation. Here we used data from GPS tracked harbour porpoises in Danish waters to assess to what extent they are deterred by vessel noise. The received noise levels were calculated based on the sound levels emitted by the ships, combined with knowledge of how sound propagates under different environmental conditions. Subsequently we incorporated the observed behavioural responses to individual ships in an agent-based model (ABM) where the individual animals’ movements were simulated to closely resemble those of real porpoises. The ABM also incorporated ships that moved along the same shipping routes as they use in the real world, while they emitted realistic amounts of noise. The modelled animals’ foraging efficiency was influenced by the extent to which they were scared away from their foraging grounds by ships, and their survival and reproduction depended on whether they were able to maintain a positive energy balance. Our results indicate that GPS tracked porpoises were deterred by ships 5–9% of the time even at distances >2 km. Model simulations for a re-routing of ship traffic which occurred in eastern Kattegat (between Denmark and Sweden) reproduced realistic temporal changes in soundscapes and porpoise presence. The results suggest that re-routing had no impact on abundance of porpoises in the region, but that further intensification of ship traffic could affect the porpoise population negatively. This illustrates how ABMs based on realistic animal movements and energetics can be valuable for guiding management and reducing negative impacts of disturbances.