Responses of fish assemblage structures to annual and perennial life cycles of seagrass Zostera marina in Lake Hamana, central Japan

Abstract

The seagrass Zostera marina is characterized by either an annual phenotype, which flourishes during spring and dies off in other seasons, or a perennial phenotype, which lives for several years. To determine the influences of such differences in seagrass life cycles on fish assemblage structures, fish sampling and environmental measurements were conducted in annual and perennial seagrass beds, as well as on bare sand/mud flats, in Lake Hamana, central Japan. The perennial and annual seagrass beds harbored similar levels of fish species richness and abundances when the latter beds were flourishing, whereas the numbers of fish species and individuals showed a relative decrease in the annual beds and sand/mud flats during seasonal die-off. Such fish occurrence patterns were primarily determined by permanent seagrass habitat residents, seasonal residents and transients, which showed different occurrence patterns in each seagrass bed. Permanent residents preferred perennial seagrass beds, with the constant availability of seagrass vegetation structure likely being an essential habitat requirement. Seasonal residents showed different occurrence patterns, being abundant in the annual seagrass beds during periods of flourishing growth, sometimes exceeding levels in the perennial beds. Such fishes may use complex seagrass structures for only restricted periods. In contrast, transients did not show any elevated habitat use for a particular seagrass habitat structure, probably because of the lesser importance of the seagrass habitat as a feeding ground.