Resolving fine-scale spatio-temporal dynamics in the harbour porpoise Phocoena phocoena

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 periodic- ity, 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 inter- action 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 predic- tor variables showed the distribution of harbour porpoises to alternate between 2 upwelling cells de- pending 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 indi- cate 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.