of control. The state of Queensland has generous expertise in this area, with the CSIRO Division of Entomology – Lands Department group in Brisbane boasting spectacular success against Salvinia and Eichhornia, and near the reservoir at James Cook University a USDA unit was involved in successes with the Tennessee Valley Authority (TVA) (see Chapter 12) using a range of stem-boring and leaf-mining insects (Balciunas et al. 1993). One might consider the herbivorous grass carp Ctenopharyngodon idella, originally from China, more as a harvester than a biological control agent. This fish grazes on submerged weeds such as Hydrilla, Myriophyllum, Chara, Potamogeton and Ceratophyllum, and at stocking rates of 75 fish/ha control is rapidly achieved. Some introductions in the USA have resulted in removal of all vegetation (Leslie et al. 1987), and in the Australian context the use of sterile (triploid) fish (Cassani and Canton 1985) could be the only consideration. However, in view of the damage already done by grass carp to some inland waterways in Australia, it is suspected that this option would be greeted with horror. Mechanical control involves the physical removal of weeds from a problem area and is useful in situations where the use of herbicides is not practical or poses risks to human health or the environment. Mobile harvesters sever, lift and carry plants to the shore. Most are intended for harvesting submerged plants, though some have been designed or adapted to harvest floating plants. Handling the harvested weed is a problem because of their enormous water content, therefore choppers are often incorporated into harvesting machinery design. However, many mechanical harvesters have a small capacity and the process of disposing of harvested plant material is time-consuming. Any material that remains may affect water quality during the decay process by depleting the water of oxygen. Furthermore, nutrients released by decay may cause algal blooms (Mitchell 1978). Another disadvantage of mechanical removal is that disturbance often promotes rapid new growth and germination of seed, and encourages the spread of weed by fragmentation. Some direct uses of macrophytes include the following: livestock food; protein extraction; manufacture of yeast; production of alcohol and other by-products; the formation of composts, mulches and fertilizers; and use for methane generation (Williams 1977). Herbicides either kill on contact, or after translocation through the plant. Some are residual and retain their toxicity for a period of time. Where herbicides are used for control of plants, some contamination of the water is inevitable (Bill 1977). The degree of contamination depends on the toxicity of the material, its fate and persistence in the water, the concentration used and the main purpose served by the water. After chemical defoliation of aquatic vegetation, the masses of decaying organic debris produced can interfere with fish production. Several factors must be taken into account when selecting and adapting herbicides for aquatic purposes, including: type of water use; toxicity of the herbicide to humans, fish, stock, and wildlife; rate of disappearance of residues, species affected and duration of control; concentration of herbicide; and cost (Bill 1977). The TVA has successfully used EPA-approved herbicides such as Endothall, Diquat, Fluridone and Komeen against Hydrilla (Burns et al. 1992), and a list of approved