About the Tropicalization of the Spanish Mediterranean Waters: Effects on Fish Communities

Recent studies have compiled all records of new fish species discovered between 2017 and 2025 in two regions of Spanish Mediterranean waters: the Strait of Gibraltar–Alboran Sea (ESAL) and the Levantine–Balearic Sea (LEBA). These studies compared the mean temperatures and thermal ranges (minimum and maximum temperatures) at which these newly recorded species lived in their original distribution zones with the preferred temperature ranges of native species present in Spanish Mediterranean waters prior to 2017. In order to characterize the thermal preferences of Mediterranean native species, previous works used the temperature of waters corresponding to the complete geographical distribution of such species. This comparison identified a tropicalization process in the ESAL region, but not in the LEBA. To further explore and refine these findings, the present work provides a new estimation of the thermal ranges of Mediterranean species occurring in these two regions. This estimation is based on existing temperature climatologies from both international and local databases. However, in the present work, we only use the temperature of waters within the ESAL and LEBA regions, instead of using those of the complete geographical distribution of Mediterranean native fish species. This new comparison offers a more accurate characterization of the thermal conditions under which Mediterranean species live. This updated comparison reveals a surprising result: the Mediterranean species recorded in the LEBA up to 2017 exhibit higher thermal preferences than those reported after this year, a difference that exceeds what had been observed in previous analyses. In contrast, in the ESAL region, the newly recorded species still display higher thermal affinities than native ones, although the differences are smaller than those found in earlier studies. A more detailed analysis, considering both the depth ranges and geographical origins of the new species, reveals a complex scenario. While some species are indeed of tropical origin, contributing to the ongoing tropicalization of the Western Mediterranean, others originate from more northern latitudes or even Mediterranean regions. This suggests that the arrival of invasive species in this part of the Mediterranean is not solely, (or even primarily) driven by the intense warming of the basin, but also by other anthropogenic factors.