Species Description: Sarotherodon melanotheron, the blackchin tilapia, is a pale (variable light blue, orange, golden yellow) cichlid whose common name refers to the dark pigmentation usually (but not always) concentrated on the underside of the head (the chin) in adult animals. Melanic pigmentation is usually also present on the posterior edge of the gill (the cleithrum) and on the tips of the soft dorsal rays. Irregular bars, spots or splotches on the body are also typical. The mouth is small and filled with up to several hundred very small teeth arranged in 3-6 rows (Trewevas 1983)
Sexual dimorphism is minimal in the blackchin tilapia (Trewevas 1983), although the heads of adult males are usually slightly larger than those of females and some males also have some gold coloration on their opercula. The spine/ray count is: Dorsal = XV-XVI + 10-12; Anal = 8-10 (FishBase).
Potentially Misidentified Species: S. melanotheron is similar in appearance to other tilapine fish species (including the genera Tilapia, Oreochromis, Cichlosoma and Sarotherodon) and to many cichlids in general. Several of these species can be found as co-occurring non-natives in locations where S. melanotheron has become established. The presence and location of the dark coloration on or around the chin aid in species identification.
Regional Occurrence: S. melanotheron is a demersal (bottom-associated) species inhabiting fresh to brackish water where it occurs. It is a tropical west African native occurring from Senegal to Zaire and southern Cameroon (Trewevas 1983, Robbins et al. 1991). The species is common in quiet muddy backwater habitats where aquatic vegetation is abundant (Jennings and Williams 1992). Outside of its native range, blackchin tilapia have been introduced to several countries across Asia, North America, and Europe (Wohlfarth and Hulata 1983).
Florida is the only state in the Gulf region in which S. melanotheron has become established (Courteney et al. 1991, Jennings and Williams 1992). In Florida, blackchin tilapia have been collected on the Gulf coast from freshwater and brackish habitats in Hillsborough County southward to Manatee County including the eastern shore of Tampa Bay (Courtenay et al. 1984). Specimens have also been collected from Alachua, Palm Beach, and Pinellas counties. The species is also established on Florida's east coast within the IRL watershed (see below).
IRL Distribution: Within the IRL watershed, blackchin tilapia have been collected in Brevard, Indian River and Palm Beach counties. It is well established in Brevard County and may be established in the other IRL counties as well. S. melanotheron can now be found from Cocoa Beach in Brevard County south to around Vero Beach in Indian River County. In Brevard County, blackchin tilapia occupy the estuary proper as well as adjacent drainage canal networks and mosquito impoundment marshes (Dial and Wainright 1983, Courtenay et al. 1984, Jennings and Williams 1992).
Northward expansion beyond the IRL is likely limited by winter water temperatures, although southward expansion appears not to be temperature limited (Jennings 1991, Jennings and Williams 1992).
Age, Size, Lifespan: Blackchin tilapia have been reported to reach 28 cm standard length (SL) (Olaosebikan and Raji 1998). In Florida, they commonly attain 22 cm standard length and 24 cm SL is possible (Finucane and Rinckey 1964, Hensley and Courtenay 1980).
Abundance: This exotic species can be extremely abundant on local scales. Courtenay et al. (1974) suggested that the S. melanotheron population in Lithia Springs, Florida, accounted for 90% of the total fish biomass in that system. The authors also speculated that similarly large S. melanotheron populations were established in the Tampa Bay area.
Within a seasonally impounded IRL mangrove ecosystem, Faunce (2000, personal communication) reported that blackchin tilapia were the dominant fish species by weight. Dial and Wainright (1983) report abundance of the species in the IRL in estuarine shallows and within the extensive drainage canal network.
Reproduction: Over its natural range, spawning occurs year-round, but activity declines in periods of heavy rainfall (Trewevas 1983).
In Florida, spawning has been observed form March to November. In the IRL region, Faunce (2000) observed spawning from April to October but gonadosomatic indices revealed that spawning activity peaked in April and May.
Trewevas (1983) reports the formation of stable breeding pairs with females initiating courtship and nest construction and males mouth-brooding the eggs and young. Females will aggressively defend nest sites while males are mouth-brooding (Jennings and Williams 1992).
Trewevas (1983) also indicated that S. melanotheron can achieve sexual maturity at a very small body size, a trait of adaptive advantage for an opportunistic invasive species.
Embryology: Females produce a clutch of approximately 50 orange eggs that vary from 1.5-4.5 mm in diameter depending on the size of the female. Incubation within the buccal cavity ("mouth brooding") by the male ranges from 6-22 days, averaging about 14 days (Finucane and Rinckey 1964, Trewevas 1983).
Temperature: S. melanotheron is a tropical species whose native range in the northern hemisphere extends from the equator to 18ºN (FishBase) and winter temperature extremes are likely the factor most limiting factor northward range expansion (Shafland and Pestrak 1982, Jennings 1991, Jennings and Williams 1992).
A lethal low temperature of 10.3ºC has been established by Shafland and Pestrak (1982) who suggested that Daytona Beach represented the probable northern limit of the expanding range of this species in Florida. Other reports indicate Florida blackchin tilapia populations can survive seasonal low temperatures of around 9ºC (Springer and Finucane 1963, Crittenden in Finucane and Rickley 1964).
In the late 1980s in the IRL near Merritt Island, blackchin tilapia suffered a widespread thermal fishkill when winter air temperatures fell to 0ºC (Snodgrass 1989).
Finucane and Rickley (1964), suggested spawning would cease below 25ºC for spawning to occur while Schreitmuller in Trewevas (1983) reported failure of breeding pairs to successfully rear young at temperatures below 23ºC.
Salinity: Blackchin tilapia are broadly euryhaline, primarily inhabiting estuarine habitats such as mangrove marshes, and travel freely between fresh and saltwater environments (Trewevas 1983, Shafland 1996). They frequent the saline lower reaches of streams and are also tolerant of hypersaline conditions that may arise in enclosed lagoons and impounded marshes (Page and Burr 1991).
Although the IRL watershed canals and ditches that are home to many blackchin tilapia typically do not exceed 10 ppt, individuals have survived experimental exposure to salinities that are ten times that level (Jennings and Williams 1992). This fish is capable of spawning both in full seawater and in pure freshwater (Shaw and Aronson in Trewevas 1983, Jennings and Williams 1992).
Within the Brevard County portions of the IRL watershed, S. melanotheron persists in freshwater as well as at salinities of up to 30 ppt (Dial and Wainright 1983, Snodgrass 1989).
Trophic Mode: Blackchin tilapia exhibit an ontogenic dietary shift, switching from a more carnivorous habit as juveniles to an adult diet that focuses mainly on detritus, algae, periphyton and the organisms and material inhabiting or fouling submerged hard surfaces (Hensley and Courtenay 1980, Diouf 1996). The many small teeth, with their spoon-shaped crowns, are well-suited to such a diet (Trewevas 1983).
Associated Species: Associations with other species within the non-native range of the blackchin tilapia are likely to be competitive in nature, with food and space being the disputed resources.
Invasion History: The blackchin tilapia was the first of at least six species of tilapia that have been released into Florida environments (Shafland 1996). The species was originally imported into the United States to be raised and sold for the aquarium trade (Axelrod and Schultz 1955). Initial release to the natural environment was most likely an accidental escape from a west coast fish farm in the mid-1950s.
Introduction of blackchin tilapia to Florida waters initially occurred near Tampa Bay in the 1950s, through release associated with the aquarium trade (Springer and Finucane 1963, Courtenay and Robins 1973, Hensley and Courtenay 1980, Jennings and Williams 1992). The first records confirming collection of this species from the wild on the west coast of Florida date to 1959 (Springer and Finucane 1963).
The east coast IRL watershed blackchin tilapia population became established substantially later; the first reported collection from the wild dates to 1980 from Satellite Beach in Brevard County (Dial and Wainright 1983).
The east coast population may also derive from aquarium releases or aquaculture escapes, although there is some speculation that it resulted from an intentional introduction by fishermen (Dial and Wainright 1983, Jennings and Williams 1992). One additional IRL introduction pathway has been proposed that points to escape of this species from an ornamental pond located at the Satellite Beach Civic Center where it was used to control the growth of algae (Dial and Wainright 1983). Regardless of the actual mechanism, authors have suggested Satellite Beach as the likely epicenter for the east coast S. melanotheron introduction (Dial and Wainright 1983, Jennings and Williams 1992).
Thermal tolerance limits may restrict northward expansion of the Florida range of blackchin tilapia, but there appears to be no similar mechanism limiting southward range expansion (Snodgrass 1989, Jennings and Williams 1992).
Potential to Compete with Natives: Large populations of S. melanotheron likely compete with native fish populations for resources. Courtenay et al. (1974) provide circumstantial evidence, noting a malnourished and diseased appearance in largemouth bass and bluegills co-occurring with non-native blackchin tilapia in a Florida freshwater spring. In addition to dietary items, blackchin tilapia may compete with other species for breeding and nesting space, as is typical for cichlids.
Direct predation of S. melanotheron on co-occurring native species may be less important than competitive interactions. Given their adaptability and euryhaline habit, however, the species nevertheless has the ability to dominate systems it invades potentially resulting in biodiversity reduction (Dial and Wainright 1983). FishBase rates the resilience of S. melanotheron as "medium" based on an estimated minimum population doubling time of 1.4 - 4.4 years.
Established populations of blackchin tilapia have been associated with a reduction of aquatic vegetation due to overgrazing (Courtenay et al. 1974).
Possible Economic Consequences of Invasion: Blackchin tilapia have been exploited as a food resource in their native range, and have been utilized as an aquaculture and non-native fishery species elsewhere as well. Early records of commercial utilization in Florida date from 1959, when the species was marketed under the name "African sunfish" (Springer and Finucane 1963). S. melanotheron taken from Florida waters are included as commercial fishery landings, but no estimate on the value of this fishery component has been assessed.
No studies have been reported that fully evaluate the economic impacts this exotic fish has had on freshwater, marine, and estuarine systems in Florida.
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