Persistent tidal mixing fronts as a key habitat feature for the harbour porpoise (Phocoena phocoena)

Despite being the most common cetacean species in European waters and the focal species in relation to management and marine habitat conservation efforts in these waters, a unified understanding of the distribution of harbour porpoise Phocoena phocoena in regions like the North Sea is still beyond reach. This situation has been created by fragmented and uneven survey data, as well as by the lack of dynamic 3-dimensional data on the oceanographic structures characterising the areas most used by the species. In an attempt to generalise distribution patterns in the North Sea on the basis of a large part of existing survey data a new approach has been tested involving dynamic species distribution modelling combined with post-processed data from detailed 3-D flow models. Distribution models (GAMs) were developed from 15-year survey databases to estimate harbour porpoise distributions in the German and British parts of the North Sea. Oceanographic conditions during porpoise surveys were estimated using a high-resolution hydrodynamic model. The stability of the water column described by temperature differences between surface and bottom and eddy activity were the most important dynamic determinants of the density of animals during summer in both parts of the North Sea. The spatial variability of the environmental envelope was estimated by classifying the optimal range of water column stability and eddy activity in each time step of the hydrodynamic model. The resulting time series depicts surprisingly stable regions of optimal oceanographic conditions coinciding with the areas of tidal mixing fronts. The time series of oceanographic envelopes were validated in-situ with positional data from satellite-tracked animals from the period 2004-2014. Our approach demonstrate that pelagic seascape metrics defined as a combination of 3-D oceanographic variables and their spatial gradients calculated at an appropriate spatial scale can resolve a comprehensive understanding of the key drivers behind the distribution of harbour porpoises and potentially other species of marine top predators.