The influence of habitat structure on nearshore fish assemblages in a southern Australian embayment: colonisation and turnover rate of fishes associated with artificial macrophyte beds of varying physical structure

Abstract

Experiments were undertaken to investigate (1) whether nearshore fish assemblages were mainly influenced by the presence or absence of macrophyte beds, irrespective of their structure, or whether the particular structural characteristics of the macrophyte beds were important, and (2) whether fishes associated with macrophytes can be considered to `settle and stay' or whether assemblages are relatively mobile. In the first experiment, replicate artificial macrophyte beds of equal leaf area to bottom ratio, but with narrow and wide leaf morphologies, were placed at two sites in Port Phillip Bay, Australia. Artificial macrophyte beds were sampled with a small seine net on two occasions. Species richness was not significantly different between the two macrophyte treatments, but these treatments were significantly higher in species richness than unvegetated sand. Abundances were much higher in the narrow-leaf treatment compared with the wide-leaf treatment, that was in turn higher than unvegetated sand. Thus, the physical structure of the habitat did influence fish assemblages beyond the simple presence or absence of structure. In the second experiment, artificial beds of the two leaf morphologies were sampled either frequently (1–2 days), or only at the end of the sampling period of 9 days, with the small seine net. Most fishes were juveniles rather than recently settled post-larvae, and abundances were again higher in the narrow-leaf treatment. The cumulative abundance of fishes in the frequently sampled treatment was much higher than in the once sampled treatment, suggesting high turnover of post-settlement fishes in the macrophyte beds. Physical data suggested that wave action may have been an important factor in the high turnover of fishes in the beds.