Tropicalization of the Mediterranean Sea Reflected in Fish Diversity Changes: A Case Study from Spanish Waters
Davinia Torreblanca, José Carlos Báez
Abstract
The Mediterranean Sea, a semi-enclosed basin at mid-latitudes, is experiencing significant environmental changes driven by global warming. This study examines recent shifts in fish species composition within Spanish Mediterranean waters, focusing on the potential tropicalization of marine communities. Using an updated dataset derived from the Spanish marine fishes checklist, we analyzed newly recorded species across two Spanish demarcations: the Levantine-Balearic (LEBA) and the Strait of Gibraltar and Alboran Sea (ESAL). A total of 25 new records (including 23 new species) were reported, with 15 new records in LEBA and 10 new records in ESAL and also including 2 new species recorded occurring in both demarcations. To assess changes in species’ thermal preferences, we compared the mean temperature of newly recorded species with that of previously established species in each demarcation using the Mann–Whitney U test. While no significant differences were found in LEBA, a marked increase of up to 6.08 °C in thermal preference was observed in ESAL. These findings suggest that tropicalization is occurring unevenly across the Spanish Mediterranean, with the Alboran Sea and Strait of Gibraltar being particularly affected. The complex oceanography of the Alboran Sea, coupled with extreme weather events and biological invasions, may exacerbate these shifts.