Environmental information is needed as a basis for species distribution models. For this project, we rely heavily on a recent compilation of North Sea wide environmental information by van der Reijden et al. (2018). These authors have compiled their datasets on bathymetry, grain size distribution, temperature and salinity from diverse literature sources. They have made their data available in the form of geo-tiff files, that we have downloaded for use in the present project. In the files, there is also information on bottom shear stress, but this is based on a rather coarse model. We have replaced it with results of the Deltares DCSM-FM model for the greater North Sea. The datasets used are listed in Table II. Sources of the data are van der Reijden et al. (2018) for fisheries and calculations of 'Bathymetric Position Index' values based on bathymetry, Stephens (2015) for grain size data, Copernicus marine services (www.marine.copernicus.eu) for salinity and temperature, EMODnet bathymetry (http://portal.emodnet-bathymetry.eu/)) for basic bathymetry, Deltares for bottom shear stress calculated with DCSM-FM.
The 'BPI' (Bathymetric position index) calculates for each point, the difference of the depth of the point with the average depth of the surrounding area, where the surrounding area is a circle with a fixed radius. BPI5 uses 5 km as a radius for the surroundings, and similar for the other BPI variables. van der Reijden et al. (2018) also define a weighted average BPI, but we did not use that in our analysis.
Temperature difference is a measure for the change in temperature between 2008 and 2013. This is not distributed homogeneously over the North Sea. Atlantic water has warmed very little, whereas the North Sea has been warming considerably over the past decades. Consequently, the largest temperature differences are seen in the eastern and north-eastern parts of the North Sea.
No temporal (e.g. seasonal) variance of salinity and temperature has been used in the present study. It is known that variation of these variables is often very important in estuarine conditions. However, in the North Sea the ranges are much more limited. It is unlikely that any of these parameters would fall outside of the tolerance of the species, with the probable exception of temperature for the boreal species Modiolus modiolus. However, also mean temperature appeared to be a very useful variable in predicting the range of this species, and obviously there is a tight correlation between mean temperature and yearly temperature range in the North Sea.
Due to fishing and other human activities, reef forming species have almost completely disappeared over roughly the past century. They are important structures that accommodate juvenile fish and other small organisms. For protection of areas where such reefs could possibly be reintroduced, it is important to define areas that are suitable habitats. This product aims to classify areas in the North Sea based on current occurrence in combination with environmental variables that are particularly suitable for these organisms.