Unpredictable extreme cold events: a threat to range-shifting tropical reef fishes in temperate waters

Abstract

Persistent ocean warming has caused the rapid poleward shift of various tropical marine organisms. However, extreme cold events have been reported to have greater impacts on corals, yet no reports have elaborated how such cold events affect range-shifting tropical reef fishes in temperate waters. This study assessed benthic cover and assemblage structure of tropical reef fishes (Pomacentridae, Chaetodontidae, Scaridae, and Acanthuridae) on two reefs, dominated by tabulate Acropora corals, in Tosa Bay (33°N, 133°E), southwestern Japan. The study was conducted during winter and summer within 2 years (2017–2018), fortuitously covering periods before, during, and after the occurrence of an extreme cold event in early 2018. The event resulted in an approximately 2-month extremely low sea surface temperature (SST) of 90% corals. About 80% of the fish species richness and > 80% of their abundance declined during the winter of 2018, with a slow increase in their populations during the summer of 2018 as a consequence of the coral loss. The extremely low SST negatively affected non-established and breeding fish populations, while the massive coral loss severely affected corallivorous fishes. This study demonstrates the potential impact of extreme cold events on the persistent establishment of tropical reef fishes in temperate waters. Thus, our study highlights the potential instability of temperate waters as a refuge for range-shifting tropical reef fishes since extreme climatic events are predicted to increase amidst climate change. This may potentially impair benefits from the tropicalized temperate reef ecosystem services (e.g., local tourism and fishery industries).