## 2 Materials and Methods
### Environmental Variables  
Species Distribution Modelling involves the merging of species occurrence data with environmental layers collected from other sources. Each longitude-latitude occurrence pair must be matched with the environmental conditions taking place in that exact or closest location. Environmental variables are known in the bibliography with various names such as variables, parameters, indicators, predictors, and features. In order to choose the most appropriate parameters, a domain knowledge from experts is needed, and depending on the individual problem to solve, researchers select the most appropriate set of variables for their experiments. Each environmental problem is unique and requires to be treated differently in terms of parameter selection. However, some variables are extensively used in most related works, such as sea surface temperature, salinity, bathymetry, and distance to coast. In this work, fifteen initial predictors were selected based on the variables that have been reported to affect fish distribution. The use of these particular environmental variables is based on data availability and ecological degree of relevance for fish species. There are several online resources that give access to marine environmental variables. Most species distribution models use low resolutions for their models. Having high resolution, spatially explicit, continuous data is rare in marine environments [21]. However, for the Mediterranean Sea there are two platforms that provide high resolution envelopes for environmental predictors describing marine conditions. These are the Copernicus Marine Environmental Service (CMEMS) database [33], which was used for the extraction of variables changing in time, and the European Marine Observation and Data Network (EMODnet) [34], which was used for the bathymetry and substrate datasets. Variables chosen for this work are divided into two categories, temporal and static. The temporal include temperature, salinity, dissolved oxygen, meridional current, zonal current, chlorophyll-\(\alpha\), euphotic depth, secchi disk depth, wave height, nitrate, and phosphate. Static include distance to coast, distance to major river mouths, bathymetry, and substrate. Temporal variables are extended into time and vertical space. In particular, monthly-mean data with a timespan of 10 years, from January 2008 to December 2017 were collected resulting in 120 values for each variable at each location. Also, because the selected species are pelagic, 2 depth levels were considered. These include values for sea surface and 100 to 300 m mean. Table 2 summarizes the environmental variables used. More information about such variables can be found in Reference [27]. \begin{table}
\begin{tabular}{l c c l l} \hline \hline
**Variable** & **Depths** & **Spatial Resolution** & **Data Product** & **Data Type** & **Data Provider** \\ \hline Temperature (\({}^{\circ}\)C) & 2 & 0.0625\({}^{\circ}\)\(\times\) 0.0625\({}^{\circ}\) & \begin{tabular}{l} MEDESA\_REANALYSIS \\ PHYS\_006\_004 \\ \end{tabular}  & Sea Physics Reanalysis & CMES \\ Salinity (psu) & 2 & 0.0625\({}^{\circ}\)\(\times\) 0.0625\({}^{\circ}\) & \begin{tabular}{l} MEDESA\_REANALYSIS \\ PHYS\_006\_004 \\ \end{tabular}  & Sea Physics Reanalysis & CMES \\ Diss. Oxygen (mmol/m\({}^{3}\)) & 2 & 0.06\({}^{\circ}\)\(\times\) 0.06\({}^{\circ}\) & \begin{tabular}{l} MEDESA\_REANALYSIS \\ BDO\_006\_08 \\ \end{tabular}  & Beamalysis & CMES \\ Meridional Current (m/s) & 2 & 0.0625\({}^{\circ}\)\(\times\) 0.0625\({}^{\circ}\) & \begin{tabular}{l} MEDESA\_REANALYSIS \\ PHYS\_006\_004 \\ \end{tabular}  & Sea Physics Reanalysis & CMES \\ Zonal Current (m/s) & 2 & 0.0625\({}^{\circ}\)\(\times\) 0.0625\({}^{\circ}\) & \ \begin{tabular}{l} MEDESA\_REANALYSIS \\ PHYS\_006\_004 \\ \end{tabular}  & Sea Physics Reanalysis & CMES \\ Chlorophyll-\(\alpha\) (mg/m\({}^{3}\)) & 1 & 0.009\({}^{\circ}\)\(\times\) 0.009\({}^{\circ}\) & \begin{tabular}{l} MEDESA\_REANALYSIS \\ BDO\_006\_08 \\ \end{tabular}