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

Abstract

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. KEY WORDS: Physical disturbance · Fish transport · Seagrass fishes · Post-settlement processesResale or republication not permitted without written consent of the publisher

Small-scale dynamics of secondary dispersal in a seagrass associated fish: a caging study

“…During calm weather conditions associated with offshore winds, recapture rates were lower, and numbers of recaptured fish showed no difference amongst cage sides. The net movement of fish offshore during rough weather conditions is consistent with previous studies conducted at this site, when fewer post-larval S. punctata were collected in the seagrass during high wave/ onshore wind conditions (Jenkins et al 1997, Moran et al 2003. From these results we suggest that episodic disturbance events operating over relatively short temporal scales, for example high-wave conditions generated by onshore winds, have the potential to reentrain post-larvae into the plankton, thereby facilitating secondary dispersal.…”

“…However, it was not possible to distinguish between dispersal through resuspension or mortality. Moran et al (2003) have recently suggested that secondary dispersal is a possible mechanism accounting for the significant reduction of post-larvae collected in seagrass beds during increased physical disturbance, although mortality through physical injury caused by wave action was also discussed. However, the present study has shown that mortality driven by physical injury cannot account for variability in post-larval abundance during physical disturbance associated with rough weather.…”

“…Recent studies in shallow seagrass beds suggest that wave disturbance generated by onshore winds may facilitate secondary dispersal for a number of seagrassassociated fishes (Jenkins et al 1997, Moran et al 2003. One species that may be particularly susceptible to secondary dispersal is the King George whiting Sillaginodes punctata.…”

“…One species that may be particularly susceptible to secondary dispersal is the King George whiting Sillaginodes punctata. High-frequency sampling has shown that numbers of post-larval S. punctata declined in seagrass beds during high wave disturbance compared to calm conditions (Moran et al 2003). Post-larvae of S. punctata enter Port Phillip Bay between September and November each year (Jenkins 1986).…”

Fish–habitat associations and the role of disturbance in surfgrass beds

“…Because of their close proximity to the shore, seagrass beds are extremely susceptible to natural (Holbrook et al 2002b, Moran et al 2003 and anthropogenic (Bell et al 2002) disturbances that result in thinning, fragmentation, and habitat loss. Surfgrasses are subject to high wave energy from storms and large swells (Holbrook et al 2002b, Bull et al 2004) that may result in their removal; in some instances, much of an entire bed may be lost (e.g.…”