Occurrence and reproduction of tropical fishes in ocean warming hotspots of Japanese temperate reefs

Abstract

Reproduction of tropical species beyond their geographic range associated with ocean warming is regarded as the key indicator of a range shift. However, the lack of historical breeding records poses challenges for detecting distinct range shifts of tropical fishes. To obtain baseline data of the current status of the occurrence and breeding activity of tropical pomacentrid and apogonid fishes in ocean warming hotspots of temperate reefs (Kochi and Wakayama, 33°N) of Japan, we conducted a two-year underwater visual survey and synthesized those data with recently published information. By combining data from the present as well as past studies, the results confirmed the occurrence of 52 pomacentrid and 34 apogonid species, whereas breeding activity was confirmed for 19 and 16 species, respectively. Species richness and abundance of recruitment periphery and breeding active species were high at the warmer site adjacent to the Kuroshio Current. Most observed species were found beyond their known geographic range. Some species showing active breeding were widespread tropical fishes (e.g., Amphiprion clarkii, Pomacentrus coelestis and Apogon notatus) and probably have established breeding populations irrespective of recent global warming. The winter sea water temperature around the study sites will continue to rise, increasing by >2 °C by the end of the century; therefore, our results are highly relevant and represent the first step to elucidate the potential range extension of tropical fishes into temperate reefs with climate change.