Summary

Seagrass beds in estuaries are important habitats and nursery grounds for many fishes, including many economically important species. The introduction of the invasive green alga Caulerpa taxifolia could potentially threaten the seagrasses of south-eastern Australia. This study examined the possible implications of the spread of Caulerpa for fishes by comparing the types and numbers of species found in native Posidonia and Zostera seagrass beds to those found in adjacent Caulerpa beds. Fishes were sampled using a small beam trawl at three sites in two Sydney estuaries on four occasions between September 2003 and February 2004.

Distinct differences were found in the types of fishes sampled in the three habitats, although the total number of fishes was similar among habitats. Fewer species were found in Caulerpa beds than in either of the seagrass habitats. Most notably, pipefishes (now protected in NSW) were not present in Caulerpa, and leatherjackets were rare in Caulerpa, but these species were sampled regularly in seagrasses. Conversely, species of gobies where often more abundant in Caulerpa than in seagrass beds. These preliminary results suggest that if Caulerpa replaces our native seagrasses, the resulting change in habitat could reduce the abundances of pipefishes and leatherjackets whilst increasing the abundances of gobies in estuaries.