The following paper is now available online at Inter-Research Marine Ecology Progress Series; full text is available at http://www.int-res.com/abstracts/meps/v373/p173-186/, or by emailing me (hsk@dhigroup.com): Resolving fine-scale spatio-temporal dynamics in the harbour porpoise Phocoena phocoena Henrik Skov and Frank Thomsen Abstract: With the development of human activities in offshore shelf waters, the harbour porpoise Phocoena phocoena has become a focal species in relation to management of human activities and marine habitat conservation efforts. In this study, acoustic and visual time series of harbour porpoise occurrence were analysed from the Horns Reef area, eastern North Sea, aiming at resolving periodicity, spatial scale of aggregation and habitat drivers in porpoises. A total of 51 fine-scale surveys and 474 d of hydrophone deployment at 2 stations were analysed by partial least squares regression and spatial modelling in relation to synoptic (spatio-temporal) data derived from a local, high-resolution hydrodynamic model. Small-scale changes in local currents reflecting upwelling driven by the interaction of the semi-diurnal tidal currents with the steep slopes of the bank were the main habitat driver identified from the acoustic as well as the survey data. Spatial modelling based on selected key predictor variables showed the distribution of harbour porpoises to alternate between 2 upwelling cells depending on the direction of tidal currents. The size of upwelling cells was less than 10 km, matching the small-scale spatial structure of survey data as revealed by geostatistical analysis. The findings may have great implications for the design of future protected areas for harbour porpoises, and they indicate that the overlap between areas of concentration of the species and areas targeted by fisheries and the offshore wind industry could be avoided without major changes to current management practices. Henrik Skov DHI Water & Environment Agern Alle 5 DK-2970 Hørsholm Denmark Telephone (45)45169220 Cell/Private (46)702478477 Fax (45)45169292 Skype henrikskov7249 www.dhigroup.com