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       Three African cichlid fish species introduced into the Southwestern United States during the 1960-1980 period continue to flourish and to provide excellent biological control of chironomid midges, mosquitoes and aquatic weeds while also constituting a significant game fishery. The fish, Tilapia zillii (Gervais), Oreochromis mossambica (Peters, and Oreochromis hornorum Trewazas, were introduced originally to improve the game fishery and subsequently as biological controls of Aquatic Weeds, Mosquitoes and Chironomid midges  (Garcia & Legner 1999, Hallock & Ziebell 1970, Hauser et al. 1976, 1977, Legner & Medved 1973l; Legner et al. 1973, 1975, St. Amant 1966).




       In parts of Arizona and southern California, they thrive without management in irrigation channels and drains, in golf course lakes, sewage treatment lagoons, flood control channels, the Salton Sea of southeastern California and the lower Colorado River drainages of Arizona and California.

       A dynamic process of hybridization among the three species appears to occur in some areas, especially the Salton Sea. Although Tilapia zillii, a non mouth-brooder, is particularly effective in suppressing noxious aquatic weeds in the irrigation system, it is competitively disadvantaged in the presence of mouth-brooding Oreochromis whose fry are thus protected from predation. This behavior has made their rearing and dissemination too tedious for most irrigation districts that continue to rely on mechanical means of weed control (Legner 1979,m Legner & Fisher 1980, Legner & Murray 1981, Legner 1979). Therefore, these tropical fish is largely confined to those water delivery channels and drains in which they can overwinter. Many of the waterways in the lower Colorado Desert have thermal springs that sustain feral populations. However, these fish are for the most part unable to persist in the colder waters of the main irrigation canals, such as the All American and Coachella canals. Along the coastal areas of southwestern California they survive the winter primarily around warm water effluent given off by several electric power plants.


Left = Nesting sites of Oreochromis along the Colorado River; Right = Golf course lagoon

 cleaned by Oreochromis

       Within the Salton Sea and Colorado River backwaters downstream from Parker, Arizona, these chchlids are now acknowledged as one of the principal game fishes. There are 1.5 lb, 10-inch long fish commonly being caught by anglers the year round. A significant role of these herbivore/insectivores as a food source for increasing the densities of larger predatory game fish is evident from stomach analyses. A very large population of Oreochromis hybrids is now firmly adapted to the flood control channels in the Los Angeles Basin, extending inland along the Santa Ana River to Riverside. Population densities frequently exceed 20 adult fish per square yard by mid summer, and they regularly constitute over one-third of the catch by anglers. Migratory populations of these fish colonize other channels of the flood control system by gaining access from the littoral zone of the Pacific Ocean. Their biological control impact against chironomid midges of the genus Chironomus is especially pronounced, and sufficient to eliminate the need for insecticide applications (Legner & Pelsue 1980, Legner et al. 1980, Garcia & Legner 2000). They may serve as a significant food source for immature Pacific Ocean game species such as bass, red snapper and corvina.


Garcia, R. & E. F. Legner. 1999. Biological control of medical and veterinary pests. In: T. W. Fisher & T. S. Bellows, Jr. (eds.), Chapter 15, p. 935-953, Handbook of Biological Control: Principles and Applications. Academic Press, San Diego, CA

Hallock, R. J. & C. D. Ziebell. 1970,. Feasibility of a sport-fishery in tertiary treated wastewater. J. Water Pollution Control Fed. 42(9): 1656-1665.

Hauser, W. J., E. F. Legner, R. A. Medved 7 S. Platt. 1976. Tilapia-- a management tool for biological control of aquatic weeds and insects. Fisheries (a Bulletin of American Fisheries Soc.) 1(2): 15-16.

Hauser, W. J., E. F. Legner & F. E. Robinson. 1977. Biological control of aquatic weeds by fish in irrigation channels. Proc. Water Management for Irrigation & Drainage. ASCE/Reno, Nevada/ Jul 20-22: 139-145.

Legner, E. F. Considerations in the management of Tilapia for biological aquatic weed control. Proc. Calif. Mosq. & Vector Control Assoc. 47: 44-45.

Legner, E. F. & T. W. Fisher. 1980. Impact of Tilapia zillii (Gervais) on Potamogeton pectinatus L., Myriophyllum spicatum var. exalbescens Jepson, and mosquito reproduction in lower Colorado Desert irrigation canals. Acta Oecologica, Oecol. Applic 1(1): 3-14.

Legner, E. F. & R. A. Medved. 1973a. Influence of Tilapia mossambica (Peters), T. zillii (Gervais (Cichlidae) and Mollienesia latipinna Le Sueur (Poecillidae) on pond populations of Culex mosquitoes and chironomid midges. Mosquito News 41(2): 241-50.

Legner, E. F. & R. A. Medved. 1973b. Predation of mosquitoes and chironomid midges in ponds by Tilapia zillii (Gervais), and T. mossambica (Peters) (Teleosteii: Cichlidae). Proc. Calif. Mosq. Control Assoc. 41: 119-121.

Legner, E. F. & C. A Murray. 1981. Feeding rates and growth of the fish Tilapia zillii (Cichlidae) on Hydrilla verticillata, Potamogeton pectinatus and Myriophyllum spicatum var. exalbescens and interactions in irrigation canals of southeastern California. J. Amer. Moswq. Control Assoc. (Mosquito News) 41(2): 241-250.

Legner, E. F. & F. W. Pelsue, Jr. 1980. Bioconversion: Tilapia fish turn insects and weeds into edible protein. Calif. Agric. 34(11 & 12): 13-14.

Legner, E. F., T. W. Fisher & R. A. Medved. 1973. Biological control of aquatic weeds in the lower Colorado River basin. Proc. Calif. Mosq. Control Assoc. 41: 115-117.

Legner, E. F., W. J. hauser, T. W. Fisher & R. A. Medved. 1975. Biological aquatic weed control by fish in the lower Sonoran Desert of California. Calif. Agric. 29(11): 8-10.

Legner, E. F., R. A. Medved & F. Pelsue, Jr. 1980. Changes in chironomid breeding patterns in a paved river channel following adaptation of cichlids of the Tilapia mossambica-hornorum complex. Ann. Entomol. Soc. Amer. 73(3): 293-299.

Legner, E. F., R. A. Medved & F. Pelsue, Jr. 1980. Changes in chironomid breeding patterns in a paved river channel following adaptation of cichlids of the Tilapia mossambica-hornorum complex. Ann. Entomol. Soc. Amer. 73(3): 293-299.

                St. Amant, J. A.  1966.  Progress report of the culture of Tilapia mossambica (Peters) hybrids in southern California.  The Resources Agency,

                 California Dept. of Fish & Game, Inland Fisheries Admin. Rept. No. 66-9.  25 p.