UC Weed Research & Information Center

Aquatic information

Biological control

In California, two type of biological control agents have been used in limited situations to control aquatic weeds.

  • The triploid grass carp (Ctenophyllum idealla) will consume filamentous algae and most submersed aquatic plants. The triploid grass carp are functionally sterile and will not reproduce. However, because it is a non-native species, its use is currently restricted to three counties in southern California. Contact the California Department of Fish & Game for permitting requirements.
  • In the early 1980s, a weevil was introduced into the Sacramento-San Joaquin Delta for control of water hyacinths. However, there is little evidence that the weevils have had much impact, apparently due to difficulty with surviving the low winter temperatures which may occur in this region.

Habitat manipulation

In California, the lakes or rivers may be modified to reduce growth of aquatic weeds:

  • Line the shoreline with rocks to reduce space available for rooting.
  • Use various types of fabrics or plastics to cover the sediments.
  • Apply non-toxic light absorbing dyes to reduce light penetration into the water column.
  • Reduce the water level during critical stages of plant growth.
  • Planktonic algae may be reduced using aeration devices to increase water circulation and prevent anoxic conditions from developing in the deep water layer (hypolimnion).
  • Reduce nutrient inputs into the lake using grassy buffer zones around the lake or by reducing fertilizer applications within 20 feet of the lake.
  • Eliminate shallow areas either at the time of pond construction, or later by dredging older lakes or ponds.

Mechanical harvesters

In California, mechanical harvesting machines are used in rivers and lakes. They are equipped with cutter bars that cut submersed plants from 3-6 feet below the water's surface. The material is usually collected and removed from the lake or river. Another type of mechanical harvesting device is a dragline. Draglines are usually chains or cables connected between tractors on either side of an irrigation canal. As the line drags along the canal bottom it uproots aquatic weeds which are collected downstream. Collection is usually by manual labor at the next trash rack downstream from the dragline operation.

Mechanical harvesters usually remove filamentous algae as well as submersed plants. There are no use restrictions on the water during or following harvesting operations. The harvested material is removed from the lake reducing the likelihood of oxygen depletion problems and nutrient release by decaying plants. Harvesters can be used to cut lanes from boat docks to open water or around swimming areas.

Harvesting is similar to mowing a lawn and repeated harvests may be required. Mechanical harvesters produce many plant fragments which are not collected, and it may enhance spread of the weedy plant throughout the lake. Many small fish and invertebrates which live on or around the harvested plants may be trapped and removed from the lake.