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Biological Control


 

     Biological control refers to the control of algae or aquatic plants by increasing the abundance of herbivores. Although relatively new as a scientifically supported management option, biological control has, in fact, been practiced for centuries. Several ecosystems have been irreparably changed due to the purposeful release of one organism or another. It is now accepted among scientists that ecosystems are very complex systems and introduction of a species brings several (often-unanticipated) changes.
     For algae and plant control in ponds, there exist a few biological control techniques, including introduction of herbivorous fish (e.g., sterile grass carp, tilapia), and encouraging zooplankton populations. Placement of decaying barley straw in the pond is believed to curb algal growth by stimulating the release of inhibitory chemicals by bacteria. As control, if any, is likely due to chemicals released by the decomposing straw, it is discussed separately in the chemical treatments section.

  • Grass Carp 
  • Other fish 
  • Zooplankton
  •   1. Grass Carp (White Amur)

     

    grass carp Grass carp (Ctenopharyngodon idella ) a fish this size could be safely stocked into a pond with adult largemouth bass ( Micropterus salmoides)      The grass carp (Ctenopharyngodon idella ), also called White Amur, is native to China where it normally inhabits rivers. This species does quite well, however, in a pond setting. Only sterile (triploid) fish may be legally released in Pennsylvania. With three sets of chromosomes instead of the usual two, triploid fish are unable to reproduce should they escape into the wild. A permit from the PA Fish and Boat Commission is needed to sell, transport or possess Grass Carp. A fisheries scientist will review the application and make a stocking recommendation based on the information provided.

         As grass carp have been introduced in many localities for plant and algae control under a variety of conditions, there naturally exist both positive and negative anecdotal reports concerning them. Because each pond/lake is unique, a detailed knowledge of their biology should form the basis for deciding whether to introduce them into a pond or lake. Check laws in your state before purchasing or introducing any fish for aquatic weed control. There are several considerations to keep in mind when determining whether grass carp are appropriate for an introduction: gate for grass carp Grass carp are river fish in their native habitat and will readily seek out flowing water; grates must be installed to prevent fish from leaving ponds where they are introduced. 

    1.  Grass carp will consume any plant that is available to them, in the order of THEIR preference. The fish may not have a preference for the plant that is least desirable from the pondowner's perspective, and their presence may simply shift the balance of a plant community to more unpalatable types. Grass Carp may preferentially remove plants that are needed as habitat for desirable fish species in the pond. Grass Carp will eat filamentous algae, but it is not a preferred food item. If there are plenty of submersed plants that the fish find palatable, they may ignore filamentous algae completely.
    A partial list of plants preferred by grass carp*:
     1. Stonewort (Nitella
     2. Musk grass (Chara
     3. Pondweeds (Potamogeton
     4. Southern Naiad (Najas
     5. Elodeas (Elodea
     6. Hydrilla
     7. Duckweed & Watermeal (Lemna/Wolffia
     8. Coontail (Ceratophyllum
     9. Myriophyllum
     10.Bladderwort (Utricularia
    2.  Grass carp have extraordinarily inefficient digestive systems and only assimilate a small amount of what they take in. Material that the fish excrete returns to the sediments where it can contribute to further plant or algal growth.
    3. Grass carp are very shy fish; once introduced, removing them without draining the pond or harming other fish is difficult if not impossible.
    4. Stocking densities are difficult to calculate without sufficient data. If the fish are over-stocked they can remove all the vegetation in the pond; if understocked they will fail to provide adequate control.
    5. Grass carp are tolerant of highly turbid and almost anoxic (0.5mg/l oxygen) water1. They may be able to survive in water that is not healthy for other fish.
         The bottom line is that introduced grass carp should not be expected to be a cure-all. Combining other measures such as herbicides and colorants with grass carp may provide better control than any measure used alone.

    Pros Cons
    • low maintenance
    • good control of submersed weeds
    • long-lived fish that are tolerant of poor water quality
    • chemical free
    • may not control filamentous algae when other food is available
    • rapidly recycle nutrients
    • somewhat expensive to purchase
    • may reduce desirable vegetation
     * diet and preference information has been compiled from several sources and adapted to Pennsylvania. More information on Grass Carp is available from these sources:
    http://www.ecy.wa.gov/programs/wq/plants/management/aqua024.html
    http://www.ces.uga.edu/pubs/PDF/L418.pdf
    http://msucares.com/pubs/is1556.htm

      2. Other fish

         Other fish that have been introduced for control of vegetation in some localities with considerably less success include true algae-eating fish such as tilapia and gizzard shad. The gill rakers of these fish are spaced narrowly enough to facilitate removal of planktonic algae from the water column. Neither species is appropriate for release in Pennsylvania ponds.
         Tilapia (collective name for fish in the genera Sarotherodon, Oreochromis and Tilapia ) are tropical fish, common in aquariums.  Though the blue tilapia (Oreochromis aureus ) has survived in Florida and Texas, most other states are too cold for this fish to survive. It has been suggested that these fish become sluggish before dying, allowing them to be consumed by gamefish such as largemouth bass. If tilapia were to be stocked for algae control, they would need to be restocked yearly. Releasing non-native fish into PA waters may be illegal, even if the pond is privately owned and is not connected to other bodies of water. Always check with the PA Fish and Boat Commission before stocking your pond.
         Gizzard shad (Dorosoma cepedianum ) are also known to eat phytoplankton by filtering water as they swim. Unfortunately, they also filter out zooplankton effectively, much more so than juvenile bluegills. Gizzard shad may also grow too large to serve as forage fish for Bass, and muddy the pond bottom when feeding. As such, negative impacts on bluegill/largemouth Bass populations in the pond heavily outweigh their usefulness as an algae control agent.

    3. Zooplankton

      The zooplankton (a collective term for microscopic fauna found living in the water column), consists chiefly of protozoa, microcrustaceans and rotifers. Three types of microcrustaceans are depicted at left. Zooplankton grazers consume a wide range of algae, while some species consume other zooplankton. Larger species of zooplankton are important in the diets of many fish species, especially as fry. If fish predation on zooplankton is reduced, they may contribute to a reduction of phytoplankton and a desirable change in water clarity.
         Efforts to increase the role of zooplankton grazers as effective biological controls of algal growth often seek to reduce fish predation by creating refugia. Deepening the pond may be effective. Limnological studies have shown that some types of motile zooplankton exhibit a migration pattern vertically through the water column, coming up to feed at night when visibility is poor and returning to the depths during the day. Increased control of algae by zooplankton may thus result from dredging.
     
     

    Pros Cons
    • natural
    • ecosystem-based control
    • difficult to manage, especially in shallow ponds
     

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