The subtropicalization of temperate costal ecosystems in the East China Sea: Tropical rabbitfish and temperate kelp alga, Ecklonia cava

Abstract

Kelp forests mainly consisted in Ecklonia cava, a perennial endemic alga, serve ecologically diverse habitats and are economically important ecosystems in Korea and Japan. However, we confirmed the active feeding activity of tropical fish herbivores (Siganus canaliculatus) from the southern coast of the Korean peninsula in November 2023, and found E. cava with severe damage (i.e., bite marks and reduced lateral blade length) as a result. Sea surface temperature (SST) is a strong indicator of global climate change and an unusually higher SST in 2023 was observed in the northern East China Sea. The mean SST value in the study area from August to November 2023 were found to be 1.1°C higher than the SST anomaly averaged over the last two decades. Because S. fuscescens has been reported to have the highest feeding rate at water temperatures of 26-29°C, the critical damage of the E. cava population cause by the high activity and intensive grazing of rabbitfish is likely to be associated with the significant increase in SST in 2023. Furthermore, the timing of the year was been identified as the only annual reproductive peak for the temperate kelp species. Eventually, the decline of endemic E. cava in Korean waters is likely to be accelerated by the combined effects of global warming and invasive herbivory.