Summary

Caulerpa taxifolia is a marine green seaweed that is native to tropical and sub-tropical regions around the world. The seaweed became a public issue in 1984 when an invasive strain was discovered in the cooler waters of the Mediterranean Sea. C. taxifolia had not been reported in NSW, Australia, prior to April 2000 when it was discovered in Port Hacking. Since then, outbreaks of the seaweed have been reported in 7 other waterways in NSW. Infestations, which range from sparse distributions of scattered runners to dense beds 40 cm thick, covered approximately 4.32 km2 of seafloor in NSW by late 2003. In this paper, we outline the invasion of C. taxifolia in NSW and describe the methods used to control the seaweed.

Various methods for eradication and control of C. taxifolia in NSW were investigated, including physical removal using a suction pump or by hand (for small patches), smothering with rubber conveyer belts and jute matting (a natural fibre, or hessian), and osmotic shock (with sea salt). The application of coarse sea salt at a concentration of 50 kg/m2 was found to be an effective method of control as it rapidly killed the seaweed, had relatively minor effects on native biota (seagrass and invertebrates living in the sediment) and was relatively cheap. In small-scale trials, the average number of fronds (i.e. leaves) of C. taxifolia decreased by 70 - 95% one week after salting and showed no signs of recovery after 6 months. Seagrass and invertebrates were also affected by salt, but their numbers had generally recovered 6 months after salting. The effectiveness of salting at larger scales depends on the method of application used and salting appears to be most successful in the cooler months when C. taxifolia dies back to a certain level naturally. Results of large-scale salting trials have been mixed. In one waterway (Lake Macquarie), a single application of salt has resulted in the apparent removal of almost 5200 m2 of C. taxifolia; in another (Careel Bay, Pittwater), repeated salting of a 3000 m2 infestation has led to a considerable reduction in the density of the seaweed, but no overall change in the boundaries of the infestation. Eradication of C. taxifolia from NSW is unlikely, but local control measures, extensive monitoring and experimentation are continuing in an attempt to limit the impact and spread of this invader.