Habitat choice and recruitment of tropical fishes on temperate coasts of Japan

Tropical reef corals are expanding on Japanese temperate coasts in response to rising sea surface temperatures, and many tropical fish juveniles have been observed routinely in these coral habitats. The present study explored how offshore tropical fish larvae locate coral habitat on the temperate coasts of Japan. Settlement-stage larvae were sampled between July and October 2009-2011 with light traps anchored on coral-replete and coral-free habitats (rocky habitats) at two-level distance (distance between each habitat type was 6 km and 500 m, respectively). Larval abundance was significantly higher on the coral-dominated habitat than that on the rocky habitat at both short and long distance sites, suggesting that coral habitats attract offshore tropical fish larvae. In underwater visual survey, Chaetodontidae and Pomacentridae juveniles were more abundant in coral habitats than in rocky habitats at both the sites, and a laboratory habitat choice experiment demonstrated that these larvae showed a preference for corals rather than rocks. In contrast, densities of juvenile Mullidae did not differ between the coral and rocky habitats, and the larvae did not show a substrate preference in the habitat choice experiment. These observations suggest that habitat choice at settlement possibly accounts for the differences in settlement patterns of tropical fishes between the two habitats. Taken together, our results showed that most tropical fish larvae colonize their settlement coast at a scale of similar to 0.5 km, and that they may locate coral habitats after reaching a reef. Moreover, the results suggest that coral habitat expansion on temperate coasts will lead to an increase in coral-associated tropical fishes and will change assemblage structures of fishes on temperate coasts.