Summary

Objectives:

• To investigate patterns of dispersal, recruitment and growth of the invasive alga Caulerpa taxifolia and provide informationon spread within NSW estuaries.
• To investigate the vectors that may transfer C. taxifolia to new locations.
• To develop environmentally benign ways of removing C. taxifolia which might eventually lead to its elimination from whole sites or regions.

To date, Caulerpa taxifolia has been found in 9 estuaries or sheltered embayments in NSW in water depths of 0.5-10metres. C. taxifolia is capable of growing extremely quickly; stolons can extend by up to 13 mm per day. Vegetative growth isthe primary means by which the alga has invaded these NSW waterways, covering a total of 8 km2 by mid 2004. C. taxifolia reproducesthrough a process of fragmentation, dispersal and eventual anchoring of drifting fragments to the seafloor. Underwater surveys revealedlarge numbers of fragments within existing beds of C. taxifolia, and experiments showed that these were readily trapped by seagrassor other structures on the seafloor. Once trapped, even small fragments can attach to the seafloor and grow into new plants. Infestationsof C. taxifolia in NSW range from sparse distributions of scattered runners to dense beds 40 cm thick.

A boat-mounted mapping system was developed to record the extent and spread of C. taxifolia in NSW waterways. A procedurewhereby all known infestations are comprehensively mapped twice a year (mid-summer and mid-winter) has now been implemented. Thismapping has accurately documented the continued spread of C. taxifolia in affected NSW estuaries. Large-scale die-offs occurin shallow water (0.5-2 m) in most waterways during winter and this was particularly evident after heavy rainfall. This die-backmay be a consequence of decreased temperature, decreased salinity, increased turbidity or a combination of these.

Several natural vectors can aid the fragmentation and translocation of C. taxifolia; storms, and the increased wave action associated with them, were found to be particularly important. Many human leisure activities may also generate, trap and transport fragmentsof C. taxifolia, including swimming, diving, water skiing, anchoring or recreational fishing. Abundances of fragments werehigher in areas of human use, and experiments showed that boat anchors, in particular, were able to remove significant amountsof C. taxifolia. Additional experiments showed that fragments caught this way could survive for 1-2 days out of water in conditionsthat mimicked the anchor well on a small boat, meaning boats might constitute a major risk for transferal to other waterways.

Removing C. taxifolia by either hand-picking or using underwater suction devices worked for very small patches at shallowsites with sandy bottoms and good underwater visibility. However, many of the infested waterways in NSW are muddy and often turbid,making the detection of all plants difficult and increasing the risk of accidentally releasing fragments. Experiments with varioustypes of smothering materials, particularly jute matting, were also effective at killing most C. taxifolia in small-scaletrials. Their use for areas larger than a few hundred square metres, however, created more difficulties than they provided solutions.

The use of osmotic shock showed the most promise in preliminary trials. The addition of coarse salt directly onto the plants killed themwithin hours. Trials using salt delivered from a specially designed punt were very successful at scales of several hundred square metres,but results of larger scale salting were mixed. Salt rapidly dissolves in seawater and has little residual impact on the marine environment.Although salt may kill other marine organisms that are directly covered by it, experiments showed that Zostera marina (a seagrass)and invertebrate infauna, which often co-occur with C. taxifolia, recover after 6 months if salt is applied at 50 kgm-2. The use ofthis salting technique has now been adopted as a major component of the NSW Caulerpa Control Plan () which also addresses issues of risk assessment and vector management.