Established Nonindigenous Fish Species in Florida

Oscar (Astronotus ocellatus). The oscar is native to the Orinoco, Amazon, and La Plata river systems of South America. This popular aquarium species first appeared in canals in Miami, Dade County, Florida, in the late 1950s after a deliberate release from an aquarium fish farm (Courtenay et al. 1974). It is now established in Broward, Dade, Glades, Hendry (probably), and Palm Beach counties. Although the original release may have been from a fish farm, the spread of this species has been aided by anglers who stocked water bodies with the species to start new populations. The oscar is a popular sport and food fish throughout southern Florida and is sought by anglers (P. Shafland, Director, Non-native Fish Research Laboratory, Boca Raton, Florida, personal communication). Food-habit studies revealed the diet overlap of this species with native species and predation by this species on native species. The bulk of the material in the stomachs of 23 oscars was of animal origin and included insects, fishes, fish scales, amphibians, and some plant material, mainly unidentified seeds (Hogg 1974).

Peacock Cichlid (Cichla ocellaris). This species is sometimes called the butterfly peacock and belongs to the genus Cichla, which contains several species that are not well described. This creates confusion among professional fishery biologists. Eigenmann and Allen (1942) lumped several species as synonyms for Cichla ocellaris and C. temensis (Ogilvie 1966). The native range of the genus includes the rivers and lakes of the Amazon regions of Brazil and Bolivia, Peru, Venezuela, and Guyana. Ogilvie (1966a,b) summarized the general life-history information and the efforts to evaluate several species of peacock cichlids for introductions into Florida. Four seemingly different species were imported into Florida from the Orinoco River in Venezuela. All of the fishes were evaluated for introduction as game. Some isolated ponds were stocked with them, but none of the fishes survived, probably because of water temperatures in winter (Paul Shafland, Director, Non-native Fish Research Laboratory, Boca Raton, Florida, personal communication). All species were strictly pisciverous and not selective about size or species of forage fish. In their native range, some species grow to a maximum size of 13.5 kg, whereas other species average 1 to 2 kg. All species were potential sport fishes.

In the 1980's, fingerlings of Cichla ocellaris and C. temensis were imported from different geographical areas in South America (Brazil, Guyana, and Peru) into Florida and cross bred within species to maximize genetic species specific variability (Shafland 1984). Additional peacock cichlids, obtained from Texas, that had originally been obtained from Florida from the earlier stock, were bred with the recent imports. Resulting young were raised in Boca Raton, Florida. Between 1984 and 1986, Dade County canals were stocked with 20,000 peacock cichlid. Spawning began at 27 C. Shafland (1984) stated that studies with control temperatures conclusively revealed the inability of the peacock cichlid to survive water temperatures less than 15 C. After comparing the temperature tolerance found in the literature (Swingle 1966; Guest et al. 1979; Guest and Lyons 1980) with the results of temperature tolerance tests that he had completed earlier for other tropical fish species (Shafland and Piestrak 1982), Shafland (1984) concluded that peacock cichlid are considerably less tolerant of low water temperatures than any other currently established tropical fishes in Florida. Shafland (1984) later stated that the peacock cichlid, because of its lower lethal temperature tolerance of 15 C, could only survive in several major drainage canals in the most southern part of Florida. Temperatures in the North New River Canal near Ft. Lauderdale normally fall below 15.6 C (Anderson 1975). Water temperatures in winter during 1982-83 indicated that the species could survive in canals south of Miami but not in most natural freshwater habitats like the Everglades--not even during mild winters (Shafland 1984, 1989, 1990). Peacock cichlids cannot survive salinities that exceed 18 ppt.

The present established range of the peacock cichlid includes the coastal canals of eastern Dade and Broward counties within 50 km of the east coast of Florida and north to the Palm Beach-Broward county line (Morello 1993). The westward spread extends to the edge of the Everglades Conservation Area, 4.8 to 9.6 km south of the junction of U.S. Highway 27 and Interstate Highway 75 (Alligator Alley), where peacock cichlids have been caught by anglers. Although some fishery biologists are concerned that the peacock cichlid will become established in the Everglades National Park, Shafland (P. Shafland, Director, Non-native Fish Research Laboratory, Boca Raton, Florida, personal communication) believes that the existing range of the species will be reduced to the coastal canals of Dade County as soon as a colder-than-average winter occurs. This species is expanding its range because of its own ability to spread through interconnecting canals, the continued stocking of waters with the fish by the Game and Freshwater Fish Commission, and unofficial releases by anglers into favorite fishing areas. No studies have been conducted to determine the impact on largemouth bass populations that were in the canals before the introduction of the peacock cichlids.

The peacock cichlid, released in 1984 by the Florida Game and Freshwater Fish Commission, was the first legally introduced exotic species into the waters of Florida (Shafland 1993). It has overwintered and reproduced every year since it was introduced. Self-sustaining populations now exist in 530 km of canals, where they are heavily sought after by anglers. Bait and tackle stores are promoting the taking of the species, and special guide services have developed for this species (Tucker 1988). In 1989, a 12-month standard creel census revealed that people fished for this species. The estimated fishing pressure was 31,662 hours on 40 surface hectares of canal (792 h/ha). This provides an estimated 425,000 hours of recreational fishing year and an estimated annual economic worth of $1.4 million (Shafland 1993). Only 2 peacock cichlids/day can be taken, and only one can be more than 43.2 cm in total length. The current state record is a 3.4 kg fish caught by an angler in April 1992. The average size caught by anglers is in the range of 0.5-1.0 kg, but fishes as heavy as 4.5 kg may be taken.

Speckled Pavon (Cichla temensis). This species is sometimes called the speckled peacock. It was collected from the Orinoco River, Venezuela, during earlier efforts to determine the feasibility of introducing it into Florida (Ogilvie 1966a). It grows at a slower rate and may be less prolific than the peacock cichlid and is still protected from harvest in Florida (Chapman 1989). In its native waters, fishes as heavy as 13.6 kg have been taken. This species is not yet listed as established in Florida.

Black Acara (Cichlasoma bimaculatum). In earlier literature (Robins et al. 1991), this species was called the port cichlid (Aequidens portalegrensis). It was a popular aquarium fish during 1930-1950. Its native range is eastern Venezuela and Trinidad, Guyana, Surinam, French Guiana, Brazil, Ecuador (possibly), Bolivia, Paraguay, Uruguay, and northern Argentina (Regan 1905; Fowler 1954; Ringuelet et al. 1967). It is established in Broward, Collier, Dade, Glades (probably), Hendry, Monroe, and Palm Beach counties of Florida (Rivas 1965; Lachner et al. 1970; Kushlan 1972; Courtenay et al. 1974; Hogg 1976a,b). The first specimens in the open waters of Florida were found in the early 1960's (Rivas 1965) and probably escaped or were released from aquarium-fish farms (Courtnay et al. 1984). Survival and rapid rate of spread of this species can be attributed to several factors. It is a prolific breeder that spawns every month and guards its young well into the free-swimming stage (Taylor et al. 1984). Examination of 23 stomachs of this species indicated that plant material was a dominant food item and included filamentous algae and vascular plant fiber; stomach contents also included some chironomidae larvae and fish scales (Hogg 1974). This species has been collected in several new localities in southern Florida, indicating that it is continuing to expand its range (W. Loftus, Biologist, Everglades National Park, National Biological Survey, personal communication).

Midas Cichlid (Cichlasoma citrinellum). This species is native to the Atlantic slope of Nicaragua, including the Great Lakes basin south to Costa Rica (Miller 1966). In 1981, an established population of this species was discovered in the Black Creek Canal in southeastern Dade County, Florida (Anderson et al. 1984). The population extended approximately 10.5 km westward from the salinity structure. A pair of midas cichlids with young were seen in the C-102 canal in June 1982. An analysis of the contents of the intestines and stomachs of 16 specimens revealed plecypods, detritus, gastopods, and plant material. Barlow (1976) reported that midas cichlids eat young fishes including their own and shift to a more piscivorous diet as they grow larger. Anderson et al. (1984) found its lower lethal temperature was 11 C and predicted that this limits the species' distribution to south of a line connecting Tampa and Melbourne. The source of this i ntroduction is unknown, but it is generally believed that the fish was released from one of the fish farms in the area. Anderson et al. (1984) concluded that any harm from this species would be on fishes that already suffer from introductions of other nonindigenous species. Recent observations of this species indicated that it has persisted where it was established but does not seem to be expanding its range beyond extreme southern Florida.

Firemouth Cichlid (Cichlasoma meeki). The native distribution of this popular aquarium fish species is the Atlantic Slope drainages of South America from the Rio Tonala in Veracruz and Tabasco, Mexico, to southern Belize, including the Yucatan Peninsula and the upper Usumacinta basin in Guatemala (Courtenay et al. 1980).

This species shows stress when water temperatures drop below 17.9 C and dies at 10.3 C (Shafland and Pestrak 1983). Barlow (1974) classified this species as a substrate sifter, whereas Hogg (1976a) reported that it fed on nonfilamentous algae, filamentous chlorophyta, molluscan shell fragments, vascular plant fiber, fish eggs, and insect parts. Larger fishes tend to feed less on plant material.

Courtenay et al. (1986) discussed the early distribution in Dade and Palm Beach counties, Florida. It is established in vicinities of aquarium fish farms in the Tamiami and Snapper creek canals in Dade County. Page and Burr (1991) reported the species as established in Dade County, Florida. In 1993, personnel of the National Biological Survey (National Fisheries Research Center-Gainesville, Florida, personal communication) found the firemouth cichlid established in several small mosquito drainage ditches on Big Pine Key, Florida.

Rio Grande Cichlid (Cichlasoma cyanoguttatum). This species--sometimes called the Rio Grande perch or Texas cichlid--is native to southern Texas, the lower Rio Grande River and its lower tributaries, and southward into Mexico. It was probably first introduced into Florida from Texas stock in 1941 by a private individual (Courtenay et al. 1974). It was established in an abandoned phosphate pit near Mulberry, Polk County, where it was reported as numerous, in Dade County southwest of Miami, and in Hillsbough County. Some of the releases or escaped individuals were from fish farms. This species is locally established; its divided populations have not expanded their ranges.

Rio Grande cichlids show stress when the water temperatures drop to 18.2 C and die at 5 C (Shafland and Pestrak 1982). Darnell (1962) found that the species is omnivorous but feeds mainly on vascular plant matter, filamentous fungi and algae, insect larvae, caterpillars, beetles, water mites, cladocera, and protozoans. Other researchers also found fish eggs and scales in stomach contents (Buchan 1971).

Jack Dempsey (Cichlasoma octofasciatum). This popular aquarium fish is native to the Atlantic Slope drainages from the Rio Chachalacas basin, Veracruz, Mexico, to the Rio Ulua basin in Honduras, including the Yucatan Peninsula (Miller 1966). It is established in four Florida counties: in ditches on the University of Florida campus, Gainesville, Alachua County (Shafland 1982; Courtenay et al. 1986; Jennings 1986); in canals near the Satellite Beach Civic Center and in other canals from Satellite Beach to Canova Beach, Brevard County (Dial and Wainright 1983); in Black Creek and Snapper creek canals, Dade County (Courtenay et al. 1974; Hogg 1976a,b); and in a roadside ditch in Ruskin, Hillsborough County (Courtenay et al. 1974). An established population of this fish was eradicated from a rockpit in Levy County by the Florida Game and Fresh Water Fish Commission (Levine et al. 1979). Another population was established in a canal near an aquarium-fish farm west of Lantana in Palm Beach County (Courtenay et al. 1974) but seems to have died out. This species has been collected near Micco, Brevard County, but does not seem to be established there (Courtenay et al. 1986). This species can survive a salinity of 2-8 ppt. Populations in Dade and Hillsborough counties seem to be escaped individuals or releases from aquarium-fish farms; those in Alachua and Brevard counties probably started with released aquarium fishes. A population of this species, which was studied by Jennings (1986), is still established in Gainesville, Alacuha County, Florida. Food-habit studies (ref) indicated that it is omnivorous and feeds opportunistically on invertebrates, insects, fishes, and vegetation. Hogg (1976a) found its stomach contained filamentous algae, crayfish exoskeletons, mollusk shell fragments, and various unidentified material. Levine et al. (1979) found a predominance of animal matter in its diet.

Temperature tolerance tests indicated that the lower lethal temperature for this species was 8 C (Shafland and Pestrak 1982). Nevertheless, Jennings (1986) collected it from a small creek (0.5-4.0-m width and 6-20-cm depth) in Gainesville, Florida, where it was able to survive an evening air temperature of -2.8 C. The water source was from an underground drainage system that produced an initial water temperature of about 25 C. Seemingly small heat sinks, such as existed in the above case, are sufficient for overwintering by cold-sensitive populations much farther north than anticipated. This species has the reputation of being a very aggressive fish. Local populations continue to exist but do no seem to be expanding. Populations started with escaped individuals from fish farms or with released aquariums fishes. This species is considered locally established.

Mayan Cichlid (Cichlasoma urophthalmus). The native range of this species includes the Atlantic Slope from the Rio Coatzacoalcos basin in Veracruz, Mexico, south to Nicaragua and the Yucatan Peninsula, Cozumel, and to Isla Mujeres (Miller 1966).

Specimens of this species were first collected from Florida's open waters in January 1983 in Snook Creek, a mangrove-lined, highly hyaline tributary of Joe Bay in the northeastern Florida Bay, Everglades National Park (Loftus 1987). Later in the year, Loftus collected more specimens from the Anhinga Trail ponds in the park. Both sites were in Taylor's Slough. Repeated visits revealed spawning by this species in both areas (Loftus 1987). This euryhaline cichlid is considered to be established by Loftus (1987) because it has survived adverse environmental conditions such as cold spells, droughts, and floods for 3 years. The source of this introduction is unknown, but it was probably a release into the two disjunct areas by an aquarium hobbyist (Loftus and Kushlan 1987; Loftus 1989). The Mayan cichlid has continued to expand its range and has been taken in several new locations in the Everglades (W. Loftus, Biologist, Everglades National Park, National Biological Survey, personal communication). Stomach contents of eight fishes from Snook Creek consisted of Cerithium, other snails, and Chara.

Convict Cichlid (Cichlasoma nigrofasciatum). The convict cichlid is native to the Pacific Slope drainages from Guatemala to Costa Rica and to the Atlantic Slope drainage of Costa Rica (Miller 1966). It has been reported as established in Nevada, Arizona, and Florida (Courtenay et al. 1986), however, the population in Florida, reported by Rivas (1965) in a rockpit in northwest Miami, Dade County, has not been observed in recent years (Courtenay et al. 1986). This species is no longer considered as established in Florida.

Yellowbelly Cichlid (Cichlasoma salvini). This cichlid is native to southern Mexico, Guatemala, Yucatan, and Honduras and from the Atlantic drainage of the Rio Papaloapam, Veracrux, Mexico, to the Sulphur River near Puerto Barrios, Guatemala. This species is considered a snappish and quarrelsome species in aquaria, and, although it is a very attractive fish, it is not a popular aquarium species. It can survive in water temperatures of slightly less than 20 C (Sterba 1966). A well-established, reproducing population of this species was found in a borrow pit in an abandoned amusement park in Dania, Broward County, in March 1980. Established C. meeki were also found in the same borrow pit. Because the yellowbelly cichlid population was able to survive unusual and extremely cold temperatures in February 1981, it was eradicated by the Florida Game and Fresh Water Fish Commission later that year. However, in 1992 and 1993, the yellowbelly cichlid was found in a canal in Davie, Broward County, by personnel of the National Biological Survey (National Fisheries Research Center-Gainesville, Florida, personal communication). The presence of different size classes suggested multiple spawnings. The species is considered locally established in Florida.

Redstriped Eartheater (Geophagus surinamensis). The native range of this species includes the Guianas and the Amazon river basins of Bolivia, Brazil, Colombia, and Peru (Gosse 1975). In 1982, a reproducing population of this species was found by personnel of the Non-Native Fish Research Laboratory, Florida Game and Fresh Water Fish Commission, in the Snapper Creek Canal, Dade County, Florida (Metzger and Shafland 1984). Its presence there may be the result of escaped individuals from a fish farm or from a dumped aquarium. As other locally established populations of cichlids in southern Florida, the redstriped eartheater has the potential to expand its range. It was recently collected by National Biological Survey personnel (National Fisheries Research Center-Gainesville, Florida, personal communication) from the Snapper Creek Canal in the same general area where the first specimens were collected. Because the population shows no signs of spreading, it is considered locally established. This popular aquarium fish (Axelrod et al. 1980) grows to a maximum total length of 30 cm (Puyo 1949) and is a bottom feeder.

African Jewelfish (Hemichromis bimaculatus; now identified as H. letourneauxi by Smith-Vaniz). This species is native to rivers and lakes throughout western Africa, in the Chad Basin, in the Nile River, and south to the Congo River (Daget and Iltis 1965). In Dade County, Florida, it is established in the Hialeah Canal and in connecting canals to the west and south of the Miami International Airport and in the Comfort Canal, the channelized South Fork of the Miami River (Rivas 1965; Courtenay and Robins 1973; Courtenay et al. 1974; Hogg 1976a,b). Courtenay et al. (1984) reported its probable establishment in a canal east of Goulds, Florida, and in the Snapper Creek, north of the Tamiami Canal. It has been collected in a canal near an aquarium-fish farm near Micco, Brevard County, Florida, but there is no evidence of its establishment. This species seems to be increasing its range recently because it has been collected in several new localities (Bill Loftus, Biologist, Everglades National Park, National Biology Survey, personal communication).

The sources for the introduction of this fish in Dade County are unknown. Its distribution is largely centered in canals around the Miami International Airport, which is a major port of entry for aquarium fishes. Possibly, the jewelfish was released near the airport or from aquarium-fish farms northward along the Hialeah Canal.

Examinations of 26 stomachs of the African jewelfish revealed filamentous algae, its own young, assorted insects parts, and predominantly plant material (Hogg 1976a).

Blue Tilapia (Tilapia aurea = Oreochromis aureus). This species is native to the Senegal River, the middle Niger River as far south as Bussa (not recorded from the lower Niger or from the Volta River), Lake Chad, pools and lagoons of the lower Chari and Logone rivers, the lower Nile from near Cairo to the Delta lakes (but seemingly only in freshwater), the Jordan River system, the Na'aman and Yarkon rivers in Israel, and the Asraq marshes and hot pools at Ein Fashkha, Jordan (Trewavas 1966).

Sources and reasons for the introduction of this species into the United States and the establishment of wild populations are varied and in some cases only suggested. Shelton and Smitherman (1984) summarized the suitability of this species for aquaculture. Introductions in Alabama were made by the Auburn University for research (Smith-Vaniz 1968). To determine the general biology of the species, McBay (1962) conducted laboratory studies at the Auburn University from April 1959 to May 1960 and misidentified the fish as Tilapia nilotica. This study included investigations of spawning behavior and early life history, food habits, and lower lethal temperature. The species was introduced into Florida initially by the Florida Game and Fresh Water Fish Commission for research (mostly biological control) and subsequently by individuals (Crittenden 1962; Buntz and Manooch 1969a; Courtenay and Robins 1973, 1975; Courtenay et al. 1974; Harris 1978).

In 1961, 3,000 Nile tilapia (Tilapia nilotica), later correctly identified as Tilapia aurea (Smith-Vaniz 1968), were imported into Florida by the Florida Game and Freshwater Fish Commission for experimental studies. The purpose was to determine the fish's use as a biological control agent of nuisance aquatic plants and as a sport fish (Crittenden 1965). Phosphate pits at the Pleasant Grove Research Station near Tampa were stocked (Ware and Fish 1971; Ware 1973). The species proved to be a failure as a control agent and as a sport fish; however, public relation had already promoted it as a superfish. Before the study could be completed and the results published, the public gained access to the fish and began spreading it throughout the state. The species may have also unofficially been given to friends for private ponds. By 1964, Barkuloo (1964) strongly recommended that this species not be introduced into Florida's freshwater because experimental studies revealed it provided little recreation, could survive Florida winters, could become very abundant, and probably could interfere with the native forage base in some water bodies.

The spread of the blue tilapia has been rapid. During a routine survey in 1966, reproducing tilapia were found in Lake Morton, Polk County (Ware 1973). Eradication was immediately started. The estimated standing crop in the lake was 163 kg/ha; and blue tilapia were the second-most abundant species in the lake. The size distribution revealed three year classes, indicating that the species had reached that population level in only 3 years. When a reproducing population was found in Lake Parker, an 810 ha lake, which is in the headwaters of the Peace River watershed, all efforts to stop its spread were abandoned (Ware 1973). In 1968-1969, only 2 years after the introduction, blue tilapia comprised 5% of the composition by weight in samples from Lake Parker (Horel 1969). By 1972, this fiqure jumped to 68% of the composition by weight (Babcock 1974). The tilapia population remained at a high level until an apparent die-off from cold weather. Some tilapia survived or the lake was again stocked with them because they are still present there. The existence of a power plant on the lake almost guarantees survival of the species in even cold years.

In 1976, blue tilapia were first collected in Lake Tohopekaliga, Florida. During 1979-1981, the juvenile tilapia population increased by approximately 500%/year (Hulon and Williams 1983). The lake is eutrophic because it receives in excess of 91 million liters/day of secondarily treated sewage effluent from the greater Orlando metropolitan area.

The rapid spread of the tilapia across the state can be attributed to its releases by humans, flooding, natural migrations, and piscivorous birds (Foote 1977). The existence of highly eutrophic water bodies also assisted in its survival and spread. Establishment of this species may harm some Florida ecosystems; however, it created a commercial fishery in some lakes that would otherwise have been ecosystems with low yields (Langford et al. 1978). Wattendorf et al. (1980) studied the interspecific interactions between blue tilapia and other native fishes in Florida.

In Florida this species is now established in 18 counties: Alachua, Brevard, Dade, DeSoto, Hardee, Hernando, Hillsborough, Lake, Manatee, Marion, Orange, Osceola, Palm Beach, Pinellas, Polk, Sarasota, Seminole, and Volusia. Foote (1977) recorded the species from Broward, Charlotte, Glades, and Pasco counties, but we know of no specimens from these counties. This species is also reproducing in the saline waters of Tampa Bay.

Zale (1984) summarized the literature on the lower lethal-temperature tolerance of the blue tilapia. It varied from 5 to 12 C, depending on the circumstances or the type of test (Yashou 1960; McBay 1962; Crittenden 1965; Germany and Noble 1978; Lee 1979; Shafland and Prestrak 1982). The fishes exhibited some form of stress as the temperature dropped below 12 C and died at 5 C. If the species is preconditioned for warmer temperatures, the fish is already stressed, the period of decreasing temperatures is prolonged, or the drop in temperature is rapid, death can occur at higher than 5 C. In the wild, the northern expansion of the blue tilapia population in the United States is limited to the southern part of the southern tier of states unless a source of heated water such as a power plant or a warmwater spring is available. During cold weather, all culture of this species in the colder climates depends on a heated water source or overwintering of the species indoors. Zale (1987) discussed the behavior of the blue tilapia that seasonally migrate into stenothermal spring runs in north-central Florida to escape colder waters of the St. Johns River, Florida, in winter. The species can probably survive in larger water bodies in Florida without seeking refuge.

The main component of the blue tilapia diet is phytoplankton, a small percentage of which is insects such as chironomid larvae (McBay 1962; Manooch 1972; Stickney 1976). McBay (1962) found that the smaller, less-than-50-mm tilapia used zooplankton such as rotifers, copepodes, cladocerns, and ostracodes (Foote 1977).

The impact of blue tilapia on receiving ecosystems can vary by ecosystem and by the relative abundance of the fishes to native species. In some water bodies, the tilapia population does not explode but remains in balance with native predators that control it. In fact, in some water bodies, the tilapia may be an important prey species of native predators (Shafland and Pestrak 1981). In other water bodies--probably because native predators were inadequate to control the tilapia, the system was already stressed, or turbidity or vegetation cover interfered with predation on the tilapia (Schramm and Zale 1985)--the tilapia population exploded. Noble et al. (1975) found that, when the tilapia population reached a high biomass near 1,121 kg/ha, the blue tilapia spatially displaced native species and interfered with their reproduction. Competition for spawning sites between native centrarchids and mouth-brooding tilapia in Florida have repeatedly been cited (Buntz and Manooch 1969a; Babcock and Chapman 1973; Courtenay et al. 1974; Harris 1978 and Zale 1984). Noble et al. (1975) concluded that if blue tilapia could be managed at an intermediate level where they did not interfere with the reproduction of largemouth bass, they would not only be a valuable forage for bass but could provide a sizeable, harvestable food fish. A large population of blue tilapia in a body of water can suppress recruitment in largemouth bass. Swingle (1956), Chew (1973), and Shafland and Pestrak (1983) discussed the possible mechanisms: behavioral interactions (harassment) during reproduction; predation by normally non-piscivorous species because of crowding; and competition for food between basses and tilapia young-of-year or chemical suppressive factors (Swingle 1956; Chew 1973). Blue tilapia build nests on or close to preferred spawning sites of largemouth bass and bluegills. Because they usually begin to build nests when largemouth bass and bluegills spawn, male tilapia may interfere with the spawning of the two species. Zale (1984) found that male tilapia occupied the spawning beds in Silver Glenn Springs several months before they were ready to spawn. When nest building by tilapia is most active in 1-2 m of clear water, the shoreline looks as if it had been bombed with large 30-cm-wide depressions--some are as deep as 45 cm. No nests or spawning by native species is usually evident in or near tilapia nesting areas while the tilapia are spawning (personal observation). Our recent visits to Silver Glenn Springs in mid-February 1993 revealed that the tilapia population was not as large as during the early 1980's (Zale 1984), and many nests were abandoned. We also observed bow fishing from recreational boats in the clear water run from the spring. Interviews with hunters in three boats indicated that this sport was popular in this area and conducted almost year round; however, it was concentrated in the colder months when the fishes sought refuge in the warmer water. This hunting effort removed large numbers of fishes and probably helped to manage the tilapia population. By 10:00 h on the day of the observations, more than 35 adult fishes with an estimated weight of more than 2.25 kg had been removed prior to our interview of the group. Discussions with the hunters indicated that hunting had been slow that day. No information was available on the total number of annually removed fishes or the total number of removed fishes on that day.

Like most other tilapia species, the blue tilapia has a tendency to overpopulate waters where native predators are stressed or already low in abundance or where turbidity impedes efficient feeding by predators (Forest Ware, Florida Game and Fresh Water Fish Commission, Tallahassee personnel commnuication). Highly eutrophic water bodies with heavy algae blooms tend to become overpopulated with tilapia. Shafland and Pestrak (1983) speculated that an abundance of this species in any water body may also adversely affect the forage base of the community. In Texas and Florida, increases in densities of blue tilapia in eutrophic water bodies negatively related to sharp declines in native shad populations (Horel 1969; Babcock and Chapman 1973; Ware 1973; Shafland et al. 1980; Wattendorf 1981, 1982). Food-habitat studies by McBay (1962) revealed overlapping diets with native species.

Rapid spread and high standing crops of this species have been reported many times (Buntz and Manooch 1969a; Courtenay and Robins 1973; Ware 1973; Courtenay et al. 1974). Horel (1969) and Babcock and Chapman (1973) documented an explosion of the tilapia population in Lake Parker, Polk County, Florida. Ware (1973) concluded that the abundance of blue tilapia depends on primary productivity of the water body. High densities develop in eutrophic waters. Denzer (1966) believes that the ability of blue tilapia to withstand periods of low dissolved-oxygen concentrations may allow it to survive and flourish in hyper-eutrophic systems in which many native species cannot survive.

Overpopulation and stunting of tilapia in Florida adversely affect spawning of native species (Shafland and Pestrak 1983). In March 1973, in Lake Effie near the city of Lake Wales, almost the entire population of tilapia died from water pollution (Chapman and Young 1973). The dead blue tilapia weighed more than 2,242 kg/ha. Lake Effie receives waste water from two citrus processing plants, and the water is hyper-eutrophic and sometimes anoxic. Constant aeration is now required to prevent the lake from going anoxic. Zale (1984) found indications that the spawning of blue tilapia in Silver Glenn Springs, which discharges directly into the St. Johns River, may interfere with the spawning of largemouth bass.

In 1970, blue tilapia were first collected from Lake Lena near Auburndale in Polk County. From 1978 to 1989, fish populations were sampled and plankton and water chemistry data were collected to evaluate changes that took place with establishment and increase of the tilapia population (P. L. Shafland, Director, Non-native Fish Research Laboratory, Boca Raton, Florida, personal communication). The data tentatively showed that some populations of native species were affected as the tilapia populations expanded. Analysis of the data is presently underway.

Commercial fishing for tilapia by haul seines is legal in several Florida lakes (lakes Banana, Cannon, Conine, Effie, Hancock, Hollingsworth, Howard, Lulu, Parker, and Shipp) with a permit from the Florida Game and Fresh Water Fish Commission (Langford 1976). The lakes are shallow and enriched with heavy blue-green algae blooms and muck bottoms. Banana and Lulu lakes are enriched with inflows of domestic sewage, and heated water from power plant is discharged into Lake Parker (Langford et al. 1978). The lakes have highly urbanized shorelines and receive storm-water drainage. Commercial fishery landing of tilapia from each lake were made every year during 1972-1977 (Langford et al. 1978). The highest yield, 1,973.2 kg/ha, was from the 41.2-ha Lake Effie in 1975 (Langford et al. 1978). Some of the lakes still support a small commercial haul-seine or castnet fishery.

Sale of tilapia produced in the United States must compete with foreign imports. From July 1992 to June 1993, tilapia imports into the United States totaled 8.3 million kg at a value of $14.7 million. The American Tilapia Association estimated that the 1992 U.S. production reached 4.08 million kg (Zajicek 1993). The volume and value of imports reduced profits from tilapia by Florida's aquaculture industry (Florida Game and Fresh Water Fish Commission, Gainesville, Fl., unpublished records). Two years ago, fresh, ice packed tilapia fillets sold for approximately $7.85/kg. In 1993, imported frozen tilapia fillets sold for $1.32-$5.48/kg, depending on the country of origin (Zajicek 1993). The farm-gate prices for live tilapia in 1994 are about $2.75 at ethnic Asian markets of Chicago, Philadelphia, New York, and Atlanta.

A computer list, compiled in 1993 by the Florida Game and Fresh Water Fish Commission, Tallahassee, Florida, showed that there were 56 permit holders registered for possessing tilapia. A total of 32 permits allowed the culture or sale of tilapia. Only three producers were selling tilapia in 1993. In 1991, 16 tilapia growers reported Florida's tilapia sales totalled $572 thousand. Seventy-seven hectares were used in production, mostly in private phosphorus pits. The sale of tilapia from aquaculture operations in Florida is in competition with the capture of wild tilapia with haul seines and cast nets. Because the records and sales of cultured and wild tilapia are not reported separately, the size of the catch of the wild tilapia cannot be determined (R. Freie, State Statistician, Florida Aquaculture Statistics Service, Orlando, Florida, personal communication).

The potential of tilapia for aquaculture in the United States is discussed by Suffern (1980). He summarized that tilapia can be grown outdoors year round only in the deep south or in areas with a supply of supplemental industrial, solar, or geothermal heat. In some years, the fishes survive a mild winter, such as the exceptionally warm winter of 1971-72 in Luverne, Alabama (Habel 1975). In this case, the overwintering of the fishes resulted in the removal of almost 14.2 metric tons of harvestably sized tilapia, 15 cm or longer, from this 21.4-ha public fishing lake in 1972-73, showing the capability of this species to increase its biomass in a short time under favorable conditions.

Nile Tilapia (Tilapia nilotica = Oreochromis niloticus). The Nile tilapia is native to the Nile River and to coastal rivers of west-central Africa. It is a relatively large tilapia that attains a total length of 50 cm. It is superficially like the blue tilapia, Tilapia aureas. Trewavas (1966) distinguished the Nile tilapia from the blue tilapia and subsequently (Trewavas 1983) contrasted the Nile tilapia with other tilapia species. Trewavas (1983) also recognized four subspecies of Oreochromis niloticus.

The only Florida record of the Nile Tilapia a locally established population in Lake Seminole, Jackson County. Based on a report in 1993 from the manager of a fishing camp on Lake Seminole, this tilapia has been caught in the lake since 1988 or 1990. He provided photographs and specimens of Nile tilapia taken during 1991-93. Survival of this species during winter is probably linked to availability of warmwater discharges from numerous springs in the lake. The pathway of this introduction is unknown but is probably associated with private aquaculture in the area.

Spotted Tilapia (Tilapia mariae). Tilapia mariae is native to coastal lowlands in freshwater from the middle Ivory Coast to southwestern Ghana and from southeastern Benin to southwestern Cameroon (Thys van den Audenaerde 1966; Trewavas 1974). In its native habitat, this species prefers brackish water. It is established in Dade County and Broward County (Hogg 1974, 1976a,b; Courtenay and Hensley 1979, 1980; W. R. Courtenay Jr., Professor, Florida Atlantic University, Boca Raton, Florida, personal communication). It has also been reported from a pond south of Copeland and now occupies much of southern Collier County (W. R. Courtenay Jr., Professor, Florida Atlantic University, Boca Raton, Florida, personal communication). A population was established near Micco, Brevard County, in 1979 but subsequently died out. Continuing its rapid expansion in southeastern Florida, this species now occurs throughout the New River Canal system in central Broward County, the Tamiami Canal in western Dade County, in the Aerojet Canal system in Dade County, and throughout much of the Everglades National Park (Loftus 1989). Based on recent collections of National Biological Survey personnel (National Fisheries Research Center-Gainsville, Florida, personal communication), it is the most common nonindigenous species in canals in southern Florida. It has been found below salinity dams in three Dade County canals leading into Biscayne Bay, and monitoring of movements by this euryhaline species is underway. Hogg (1974, 1976a,b) suggested that Florida populations originated with escaped individuals or releases from aquarium-fish farms. Courtenay and Hensley (1980) suggested a possible purposeful release of this fish near Miami.

Adults grow to a length of approximately 23 cm. It is not a mouth brooder but protect its eggs and fry (Courtenay and Hensley 1979b). The species is aggressive toward native and toward other nonindigenous species. This species could replace many other species because it is expanding its population. The success of this species is attributed to its prolific reproduction, spawning during all months of the year, and guarding of its young into the free-swimming stage (Taylor et al. 1984). The contents of 58 stomachs of this species contained filamentous algae, non-filamentous algae, vascular plant fibers, and bryozoa. Plant material was five times more important than animal material. The smaller individuals are more omnivorous, but larger individuals were herbivorous. The species seems to do best in canal systems with steep sides and limited shallow areas.

Blackchin Tilapia (Tilapia melanotheron = Sarotherodon melanotheron). This species is native to river-delta lagoons from middle Liberia to southern Cameroon (Thys van den Audenaerde 1971). It is established in Florida in Hillsborough County from Lithia Springs to the mouth of the Alafia River, and southward along the eastern shore of Tampa Bay to Cockroach Bay, Manatee County (Springer and Finucane 1963; Finucane and Rinckey 1964; Buntz and Manooch 1969b; Lachner et al. 1970; Courtenay et al. 1974).

It is also established in Brevard County in canals near Satellite Beach and in the Indian and Banana rivers from Merritt Island southward to below Canova Beach, a distance of 27 km (Dial and Wainright 1983). Springer and Finucane (1963) suggested that this species either escaped or was released from an aquarium-fish farm on the eastern shore of Tampa Bay. The Brevard County population may have resulted from aquarium-fish releases into the reflecting pool at the Satellite Beach Civic Center or by anglers attempting to establish a sport or commercial fishery on the east coast similar to the one on the west coast of Florida. This species is now taken in commercial catches by netting in estuarine waters and by hook and line in freshwater (Dial and Wainright 1983). Finucane and Rinckey (1964) and Anderson (1981) summarized the results of food-habit studies that revealed the diet overlap between this species and native species.

On the east coast of Florida, this species spread southward to the Jupiter Inlet, Palm Beach County, by August 1990. The northern limit of its range is the Ponce de Leon Inlet near New Smyrna Beach, Volusia County (Jennings and Williams 1992). They found the species in a variety of habitats including canals, drainage ditches, lagoonal areas at the mouth of creeks and canals, open water areas, and freshwater tributaries. The species prefers quiet backwaters with aquatic vegetation and a mucky, organic substrate. There is no recent information of its status on the west coast of Florida in the Tampa Bay tributaries except that it is still present in some of the same streams where it was previously found. Its population is considered to be expanding at least on the east coast of Florida.

Mozambique Tilapia (Tilapia mossambica = Oreochromis mossambicus). The native range of this species is the eastward-flowing rivers of Africa from the lower Zambezi and Shire river systems in Mozambique southward in coastal drainages to Algoa Bay, South Africa (Jubb 1967; Thys van den Audenaerde 1968). It is established in Florida in the saline Banana River near Cocoa Beach, Brevard County (Dial and Wainright 1983), and throughout Dade County (Courtenay et al. 1984; W. R. Courtenay Jr., Professor, Florida Atlantic University, Boca Raton, Florida, personal communication). Dial and Wainright (1983) found this species in brackish water with salinity as high as 8 ppt in the Satellite Beach area. Populations in Brevard County seem to have originated from stocks of aquarium fishes released at Satellite Beach (Courtenay et al. 1974; Dial and Wainright 1983). Most of those in Dade County probably escaped from aquarium-fish farms. At one locality, their establishment resulted from an introduction for aquatic plant control by a developer (Courtenay et al. 1984). The Mozambique tilapia seems to have maintained populations in canals where it was originally found in Dade County, Florida. It seemingly has not spread over any appreciable distance in recent years in southern Florida. No Mozambique tilapia were found during the surveys of nonindigenous fishes in the Indian River system by Jennings and Williams (1992).

Lower lethal-temperature tests indicated that this fish stops feeding at 15.6 C; individuals began to die at 14.4 C, and all individuala were dead at 8.3 - 9.4 C (Kelly 1955). Kelly (1955) concluded that this species could not survive winter conditions throughout most of the Southeast.

Kelly (1955) conducted food studies that revealed diet overlap of this species with native species. Studies in its native range of South Africa indicated that young of this species fed mainly on detritus and neuston in eulittoral pools and in shallow margins along shore. Adults fed mainly on detritus, periphyton, and insects in shallow water (Bruton and Boltt 1975). Kelly (1955) reported that the diet of this species was mainly composed of planktonic forms of plants and animals. Wieland et al. (1982) compiled a complete biological synopsis on this species.

Hybrid crosses of this species with other tilapia species have been used in Florida and elsewhere in the United States for aquaculture (Wieland et al. 1985). Most culture of this species in Florida is used for brood stock to establish and supply aquaculture facilities in foreign countries. This species has a tendency toward overpopulating waters and toward having stunted growth, thus making it a problem in culture and in the open waters (Wieland et al. 1985). Swingle (1958) discussed the use of this species for culture in ponds. Most commercially sold tilapia in Florida are blue tilapia, not Mozambique tilapia.

The Mozambique tilapia was introduced into open waters as a weed-control agent in Arizona (Minckley 1973) and as control agents of aquatic plants, mosquitos, and chironomids in California (Knaggs 1977; Legner and Pelsue 1977) and elsewhere in the world. Its ability to control nuisance aquatic plants is now considered questionable because its well being seems to depend on the availability of other food. The destruction of higher plants seems to be the result of the tilapia scraping the periphyton from the leaves and from the stalks of the plants. In doing so, the Mozambique tilapia injure the plants enough to control their growth (Wieland et al. 1985). Released individuals of these species to control weeds in California created established, overpopulating populations that are replacing native species, especially in estuaries (Knaggs 1977; Legner et al. 1980).

Redbelly Tilapia (Tilapia zilli). This species of tilapia occurs in Africa in the Bandama, Benue, Chari, Ituri, Niger, Sassandra, Senegal, Ubangi, Uele, Volta rivers; in lakes Albert, Chad, and George; and in the Near East in the Jordan River (Thys van den Audenaerde 1968).

In Florida, the species was established in a rockpit near Perrine, Dade County, but was eradicated by personnel of the Florida Game and Fresh Water Fish Commission (Courtenay et al. 1984). Prior to its eradication, it hybridized with introduced spotted tilapia (Taylor et al. 1986). The redbelly tilapia recently was found in several mosquito drainage ditches on Big Pine Key, Monroe County, Florida.

Congo Tilapia (Tilapia rendalli). This species, formerly identified as T. melanopleura, was studied as a possible biological control agent of aquatic plants at Lake Wales, Polk County. The studies revealed that it was highly aggressive, potentially threatening to native vegetation, and capable of causing problematic management of native fishes and waterfowl; therefore, the research was terminated and the population destroyed (Courtenay et al. 1974).