Community structure and trophic relationships of the fish fauna of an estuarine Posidonia Australis seagrass habitat in port hacking, new South Wales

Abstract

The fish community present in a 400-m 2 area of seagrass meadow dominated by Posidonia australis Hook f. was quantitatively sampled during two springs and two autumns by enclosure and poisoning with rotenone. Thirty-nine species were collected and these were classified into permanent residents (∼ 50% of total numbers), temporary residents (∼ 30%) and transients (∼20%). Approximately 30% of the total numbers of fish collected belonged to species of some economic importance. Species diversity indices were calculated and compared between seasons, but no obvious differences were found. Shannon's H′ (heterogeneity index) ranged from 2.11 to 2.56, Gleason's d (richness index) ranged from 3.08 to 5.05, and scaled H (eveness index) ranged from 0.59 to 0.74. Stomach contents of all fish were analysed by an estimated volumetric method. Agglomerative polythetic classification to the average diets of all species collected indicated 7 basic feeding groups within the community. Many species preferred crustaceans as food (this latter group comprising approximately 50% of the overall community diet), and several species consumed seagrass (approximately 10% of the overall community diet). Upper water column dwellers were mainly microcrustacean feeders; canopy dwellers fed primarily on crustaceans, seagrass and algae; benthic dwellers consumed mainly crustaceans and polychaetes.