Abstract
Marine ecosystems worldwide are undergoing significant shifts due to the rapid warming of sea surface temperatures (SST), driving the tropicalization of marine communities. In the Canary Islands, fish assemblages are experiencing notable changes. From 2016 to 2021, reef fish communities across the archipelago were monitored to assess their spatio-temporal distribution patterns and anticipate potential future changes. Surveys were conducted twice a year at 20 sites across El Hierro, Tenerife and Lanzarote, recording species densities. Chromis limbata , Canthigaster capistrata , Similiparma lurida , Sparisoma cretense , and Thalassoma pavo were the most ubiquitous species. Fish communities exhibited a clear spatial distribution aligned with the longitudinal thermal gradient across the archipelago, alongside seasonal fluctuations. Classifying species into tropical, warm-temperate and temperate biogeographic affinity groups provided key insights into distribution patterns in response to SST variability. Notably, several marine heatwave events occurred during the study period, triggering significant and varied responses among fish communities to abrupt SST fluctuations. The findings suggest that ongoing climatic change is likely to drive the displacement of temperate fish while facilitating the expansion of tropical species in subtropical latitudes, leading to shifts in marine communities composition and potential alterations in local ecosystem structures. Continuous monitoring is essential to understanding these ecological transitions and informing effective conservation and management strategies.