Seagrasses sometimes construct dense colonies (seagrass beds) in shallow coastal water. Eelgrass Zostera marina has two phenotypes, i.e. annual and perennial forms. In Japan, the former flourishes during spring and dies off in other seasons, whereas the latter lives for plural years. It is well known that perennial eelgrass beds often support large number of fishes, while to date little information on fish assemblage structures in annual eelgrass beds has been obtained. We therefore clarified fish assemblage structures in annual beds for a year and compared them with those in perennial beds in Lake Hamana, Japan.

In the period of annual eelgrass dying off, the fish species composition differed between annual and perennial beds, and species richness and abundances were lower in the former. In the period of annual eelgrass flourishing, fish species composition still differed between them, although species richness and abundances did not differ. Such a difference is mainly due to the occurrence patterns of permanent resident fishes which strongly associate with eelgrass. Permanent residents may have to leave annual beds when eelgrass dies off, and thus not prefer annual to perennial beds, resulting in relatively fewer occurrences in the former. Life cycles of eelgrass may therefore strongly influence fish assemblage structures.