The influence of habitat structure on nearshore fish assemblages in a southern Australian embayment: Comparison of shallow seagrass, reef-algal and unvegetated sand habitats, with emphasis on their importance to recruitment

Abstract

Assemblage structure and juvenile recruitment of fishes was compared amongst three habitats: seagrass, Heterozostera tasmanica (Martens ex Aschers.) den Hartog; reef-algal; and unvegetated sand. Sampling was conducted monthly from October 1993 to March 1994 at three locations in Port Phillip Bay, southern Australia. A fine-mesh seine net was used to sample subtidally at a depth of approximately 0.5 m. Fish assemblages differed primarily between structured habitats and unvegetated sand, and a number of species previously reported to occur in seagrass habitat were also found to utilise reef-algal habitat. Species richness was highest in seagrass and lowest in unvegetated sand with significant differences amongst all habitats. Total abundance varied between habitats depending on location and month examined, but the most common pattern was highest abundance in seagrass and lowest in unvegetated sand, with a significant difference only between seagrass and unvegetated sand. At the level of individual taxa, pipefishes of the genus Stigmatopora showed a strong preference for seagrass habitat from an early juvenile stage. The King George whiting, Sillaginodes punctata (Cuvier and Valenciennes), showed a complex relationship with habitat, occurring on both seagrass and reef-algae immediately after settlement, but, with growth, showing an increasing preference for reef-algae before finally shifting to unvegetated sand approximately 4 months after settlement. Other species previously found as juveniles on seagrass beds also recruited to reef-algal habitats. Strong locality effects were also found, particularly for King George whiting. This variation was unlikely to be related to habitat structure, because macrophyte biomass showed much greater variation within locations than amongst locations. We conclude that while the presence of structure per se is sufficient for the recruitment of many species, some taxa will discriminate amongst habitats based on structural characteristics.

Introduction

There is wide acceptance that seagrass beds are important `nursery' areas for fishes (Pollard, 1984, Bell and Pollard, 1989), although there are exceptions where seagrasses do not appear to act as nursery areas (Bell and Harmelin-Vivien, 1982, Heck Jr and Thoman, 1984). Reefs and associated algae, and unvegetated sediments, have also been reported as important nursery areas, albeit much less frequently than is the case with seagrass. In the Mediterranean, juvenile fish were mainly associated with shallow, sublittoral reefs, rather than deeper seagrass (Posidonia) beds (Bell and Harmelin-Vivien, 1982). Macrophyte (algae and seagrass) debris in surf zones apparently forms an important nursery habitat for fishes in Western Australia (Robertson and Lenanton, 1984). Sogard and Able (1991)found that seagrass, Zostera, and algae, Ulva, were superior habitats (based on abundances) compared with adjacent unvegetated habitat. Moderately exposed beaches in South Africa may provide a nursery habitat as important as estuaries (Bennett, 1989). Finally, unvegetated sediment adjacent to seagrass, in contrast to the same habitat some distance (greater than 100 m) from seagrass, was an important habitat for juveniles of a number of commercial species in New South Wales (Ferrell and Bell, 1991). Thus, it is possible that the importance of seagrasses may have been over-emphasised relative to other habitats in terms of their nursery function.

The emphasis on the importance of seagrass over other habitats may partly relate to the dominance of this habitat in areas of larval settlement. Bell et al. (1987)conducted experiments on settlement to artificial seagrass that included cage treatments. Results showed high settlement to cage controls with no associated artificial seagrass, leading to the suggestion that shelter per se was critical, rather than the particular type of shelter. In New South Wales, seagrasses usually occur in the middle reaches of estuaries, while rocky reef dominates at the entrance and mangroves dominate in the upper brackish reaches (Bell and Pollard, 1989). Thus, emphasis on the importance of seagrass for certain species of juvenile fish may simply reflect a tendency for larvae of these species to settle in the middle reaches of estuaries (Bell and Pollard, 1989).

The present study compares the diversity and abundance of fishes, including the recruits and juveniles of commercial species, amongst shallow (<1 m) reef-algal, seagrass and unvegetated sand habitats in Port Phillip Bay, Australia. In Port Phillip Bay, reef, seagrass and sand habitats can be found interspersed at the same location, allowing a test of whether the presence of structure per se is more important than the specific structural characteristics. The study was conducted at locations known to span a wide range of fish-recruitment levels, and over the time period of highest fish recruitment (Jenkins et al., 1993, Jenkins et al., 1997b).