No. 140: Disturbance Effects on the Harbour Porpoise Population in the North Sea (DEPONS): Status report on model development

van Beest, F.M, Nabe-Nielsen, J., Carstensen, J., Teilmann, J. & Tougaard, J. 2015: Disturbance Effects on the Harbour Porpoise Population in the North Sea (DEPONS): Status report on model development. Aarhus University, DCE – Danish Centre for Environment and Energy, 43 pp. Scientific Report from DCE – Danish Centre for Environment and Energy No. 140

Summary

There is mounting scientific evidence that anthropogenic disturbances, such as offshore wind farm construction, affect the behaviour of harbour porpoises (Phocoena phocoena) and other marine mammals. The impact of noise generated through human activities on porpoise population dynamics has, so far, only been evaluated for the inner Danish waters (IDW) population. As part of the Disturbance Effects on the Harbour Porpoise Population in the North Sea (DEPONS) project, the main goal of this report is to demonstrate how the individual-based model (IBM) that was developed to simulate the cumulative impacts of different kinds of disturbances on the porpoise population in the IDW can be extended for evaluating the impact of pile-driving noise from offshore wind farm construction on the porpoise population in the North Sea (DEPONS model). The focus of this status report is on model parameterization and the types of processes included in the DEPONS model. The presented results are only indicative of the kinds of effects to be expected in the final model simulations. Although the results based partly on parameters for the IDW can give a first indication of the impact of different wind farm construction scenarios on the porpoise population in the North Sea, the DEPONS model output will become more accurate as the behaviour of the simulated porpoises is refined to resemble North Sea porpoises more closely. Porpoise deterrence behaviour in relation to pile-driving noise was parameterised based on empirical data showing reductions in porpoise density at different distances from the construction site before and during pile-driving events as collected around the DanTysk wind farm in the North Sea. We considered five different porpoise movement and dispersal strategies, which were either derived from theoretical movement models or from behaviour observed in the IDW. We used the movement/dispersal models in combination with three hypothetical pile-driving scenarios ranging from no noise to a realistic but worst case piling scenario. We compared the average simulated porpoise population sizes and dynamics across movement/dispersal models and pile-driving scenarios. Although the results should be considered preliminary, the patterns generated by the current version of the DEPONS model did not suggest any clear, long-lasting effects of pile-driving noise on the average porpoise population size and dynamics in the North Sea. The movement/dispersal strategy employed by the simulated porpoises had a far greater impact on the population size and whether population dynamics stabilized over time. To improve the DEPONS model further and enhance model inference, we highlight and discuss several model components that need to be investigated in more detail so as to replace certain underlying assumptions in the model with a direct data driven parameterization approach. Most importantly, empirical data on porpoise movement behaviour and dispersal strategies from the North Sea are required to better parameterize the model. Most of the data deficiencies in the current parameterization will be addressed during the remainder of the DEPONS project. The preliminary results presented here do indicate that the DEPONS model can ultimately become a valuable and powerful modelling platform for use in informing offshore wind farm construction planning.