Influence of a major tsunami event on larval and juvenile fish assemblage composition associated with a seagrass habitat in Okirai Bay, Japan

On 11 March 2011, the Pacific coast in northeastern Japan was affected by the tsunami and land subsidence caused by the great east Japan earthquake. The disaster brought about various major environmental changes to the region. In our sampling site, examined over the period 2007-2015, the Zostera seagrass bed covered an area of approximately 3000 m(2), but almost all of the shoots of the seagrasses were washed away by the tsunami. However, the seagrasses started to grow again in the summer of 2011, and the seagrass bed area expanded to approximately 3400 m(2) by August 2014. Underwater observations revealed that the process of expansion resulted from regrowth from survived rhizomes and from seeds which were produced after 2012. A total of 28,969 fish larvae and juveniles, representing at least 65 species, were collected from 2012 to 2015, which showed a change in the composition of the dominant species. However, based on Jaccard's coefficient of community, the species composition observed in 2015 returned to almost the same composition as observed before the tsunami. This implies that the seagrass formed a key habitat for the fish faunal recovery. Furthermore, it is implied that the occurrence of some transient species and casual (intermittent occurrence) species in the seagrass bed were associated with increase of these fishes in areas near to the seagrass bed. On the other hand, some species which occurred intermittently before the tsunami were not sampled or decreased after the tsunami.