Role of physical disturbance in structuring fish assemblages in seagrass beds in Port Phillip Bay, Australia

Disturbance in seagrass habitats may cause variation in the structure of fish assemblages and individual taxa. One important form of disturbance is wave action associated with strong winds. Total fish abundance and species richness from seagrass beds at 2 sites in Port Phillip Bay, Australia, were sampled during low and high wave disturbance. During high wave action (> 0.25 m) in seagrass beds, abundance of fish at one site decreased significantly, but species richness was unaffected at both sites. Plankton sampling conducted at the same time as seagrass sampling (directly 300 m offshore from the seagrass sites) found that species richness significantly increased during high wave conditions at both sites. Total fish abundance similarly increased in the plankton during high wave conditions at one site. We conclude that variation in assemblage structure during increased physical disturbance is related to variation in a small number of numerically dominant species within the assemblage. At the individual taxon level, numerically dominant species in the seagrass and plankton showed considerable variation in response to disturbance. In the seagrass assemblage, post-larval King George whiting Sillaginodes punctata (Cuvier and Valenciennes), adult weedfish Heteroclinus perspicillatus (Cuvier and Valenciennes), and pipefish juveniles and adults from the genus Stigmatopora decreased in abundance at either one or both sites during high wave conditions. Conversely, in the plankton, adult pipefish Hypelognathus rostratus (Waite and Hale), juvenile Stigmatopora and larval Gymnapistes marmoratus (Cuvier and Valenciennes) significantly increased in abundance during these same physical conditions. It appears that for some taxa physical disturbance may facilitate secondary planktonic dispersal.