Aquatic weeds are an important component of any aquatic ecosystem, but when they are present in high densities and interfere with man's activities, they must be controlled. Chemical and mechanical techniques are usually costly and short-lived. Biological control techniques may be a satisfactory alternative. In southern California, (Tilaeia zillii) are being used for the biological control of aquatic weeds in irrigation canals. T. zillii seems to meet most criteria established for evaluation of herbivorous fish for aquatic weed control. In laboratory tests, it showed a definite avoidance for one weed species, but in field tests, it controlled all weeds. Numbers of associated fish species increased in spite of a dense T. zillii population. T. zillii cannot survive winter water temperatures in the irrigation canals, but they can be successfully reared in very large numbers. They are readily accepted by anglers. The conclusion reached is that T. zillii appears to be a good "biological herbicide," but will require annual reapplication just as any other herbicide.
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