## 4 Discussion
This happens because some stages in fish life, such as breeding and migration, occur in certain months. By comparing the surface values to the mean of 100 to 300 m, it becomes clear that the latter falls behind. Conditions found below the surface are rarely accurately known [60], thus it is advised to use surface values. All feature categories from the signal processing area related to filters are the worst performing ones. The mean expanding (exponential window) and classical sta-Ita (Short Time Average over Long Time Average), which is used in seismic events, obtained very high importance. One would expect that relative minima and maxima and peaks would be good feature categories, like the minimums and maximums of the descriptive statistics. This is not true, as all the temporal environmental variables have high seasonality and the peaks are insignificant. After having investigated the features and their respective predictors that were considered as most valuable by the Reciprocal Ranking technique, in Figure 5 the top 10 predictors for each species are depicted in alphabetical order. The plots are qualitative and normalized so that the 10 th predictor would be at the center of the plot. Every decagon represents one unit in terms of Reciprocal Ranking importance. For example, anchovy zonal current is five times less important than distance from the coast. Longitude is among the top predictors for every species, in agreement with the western-to-eastern gradient in the trophic conditions of the Mediterranean Sea. This longitudinal gradient represents changes in water temperature, chlorophyll concentration and mixed layer depth over the epipelagic layer [61]. The Anchovy is mostly affected by the distance from the coast, the temperature and the euphotic depth; pilchard by water temperature, salinity, secchi disk depth and distance from the major rivers; round sardinella by salinity, temperature, distance from the coast and dissolved oxygen; atlantic club mackerel by salinity, temperature, latitude, nitrate and distance to from the coast; atlantic Mackerel by salinity, temperature and chlorophyll-\(\alpha\); picarel by temperature, zonal current, salinity and latitude; bluefin tuna by salinity, nitrate and latitude, and swordfish by salinity, temperature and latitude. Because of their fast life history strategy (rapid growth, early maturation, short lifespan), small pelagic fishes and especially their recruitment success [10], hence their distribution, abundance and fishery catches, are vulnerable to climate and environmental forcing [12; 13; 42]. However, the decline in landings of most small and medium pelagic species is only partially attributed to climate and environmental factors because, especially in the Mediterranean Sea, over-exploitation still remains the main driving force of their populations [9; 62]. Indeed, overfishing has been reported to modify the abundance, composition, and distribution of pelagic species, but also to induce drastic changes of state [63]. Following the above-described west-to-east gradient, the probability of occurrence of small pelagic fishes was higher in the western Mediterranean Sea and declined eastwards (Figure 6) with the exception of picarel, which was abundant throughout the basin. For medium and large-sized pelagic fishes, the probability of occurrence was higher in certain areas of the western and northern Mediterranean, while they were completely absent from the southeastern part of the basin, as a results of the raised temperature and salinity and the low chlorophyll levels at the surface layer (Figure 6). Small and medium pelagic species are generally concentrated in areas of high productivity because most of them are mainly plankton feeders [64]. These areas are associated with cooler and fresher nutrient rich water masses that could be either upwelling areas (e.g., west African coast: [63]), coastal areas affected by riverine input (e.g. NW Mediterranean: [65]) or areas affected by both riverine input and other water masses (e.g.