Fish response to expanding tropical Sargassum beds on the temperate coasts of Japan

The invasion of tropical Sargassum species and extinction of temperate Sargassum species on the temperate coasts of several countries is associated with global warming. However, little is known about how these shifts in habitat-forming species impact local ecosystems and fisheries. To better understand the effects of tropical Sargassum colonization on temperate coastal fishes, we investigated differences in the habitat use patterns of fishes in temperate and tropical Sargassum species beds over 2 seasons on the temperate coast of Japan. The tropical Sargassum species were characterized by lower thallus height and a shorter vegetative period compared with temperate species; the thalli of the former were absent during autumn and winter. Peak of recruitment of juvenile fishes matched the biomass peak of both algae from late spring to early summer. Moreover, assemblage structures and food habits of fishes as well as the density of commonly consumed invertebrates did not differ significantly between the temperate and tropical algal beds during these seasons, suggesting that the habitat function (living/refuge space and foraging availability) of tropical algae was similar to that of temperate algae for most fishes. However, although most fishes displayed low specificity to host algae, some species, including important fishery species, recruited only during the middle of winter, when tropical algae were unavailable. This suggests that declining temperate algae may negatively affect some fish populations and fisheries within the next few decades.