Assemblages of small fish in patchy mangrove forests in Sydney Harbour

Mangroves are widely considered to be important nursery grounds because they contain large abundances of juvenile fish. Inadequate and confounded sampling designs have, however, limited interpretation of many previous studies. In addition, most studies have been done in relatively large patches of tropical mangrove forests. This study investigated the model that small patches of mangrove forests, which are exposed during low tide but flooded during high tide, in an urbanized harbour contain large numbers of small fish compared with nearby unvegetated habitats. Specific hypotheses about the abundance and diversity of fish in mangrove forests and on mudflats in urban regions of Sydney Harbour were tested using similar sampling methods in each habitat with spatial and temporal replication. Mean abundance, species richness and assemblages of fish were similar in these two habitats and between mudflats adjacent to mangroves and those more than 300 m away. Nevertheless, numbers of fish were greater in mangroves than on mudflats more often than expected by chance, due to large abundances of one species of goby. Abundances and species richness varied greatly and interactively among nets, sites, habitats and times of sampling. There was no evidence that fish moved out of the mangroves onto adjacent intertidal mudflats while the habitats were still flooded. A comparison of the use of fyke nets and seine nets indicated similar patterns using each fishing technique. These results indicate that these small patches of mangroves within Sydney Harbour may not be particularly important habitat for fish because similar species in similar abundances are found on adjacent mudflats.