Texas Invasives

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Oreochromis aureus

Blue Tilapia

Synonym(s):
Class: Actinopterygii
Order: Perciformes
Family: Cichlidae

Photographer: Michael Rupert Hayes
Source: nas.er.usgs.gov

Description

Adults typically range from about 5-8 inches in length and can weigh 5-6 pounds; however, the largest recorded specimen was up to 21 inches and weighed more than 10 pounds. The anterior portion of the dorsal and anal fins have hard spines while the posterior portion has softer rays. Oreochromis aureus has a blue-grey body with a white belly. The head of the male fish will change into a bright metallic blue shade during the breeding season and he will also display a red or pink band on the edge of the dorsal and tail fins. A breeding female fish will develop a pale orange color on the edges of her dorsal and tail fins.

Ecological Threat: Blue Tilapia have been found to negatively impact plant, fish, shrimp, and native mussel populations and degrade habitat through voracious herbivory and prolific nesting activity. Some streams where Oreochromis aureus is plentiful have lost most vegetation and nearly all native fishes.

It has been shown in several states that the blue tilapia's local abundance and high densities in certain areas have resulted in marked changes in fish community structure. These invasive fishes compete with centrarchids (e.g., black bass) for food and spawning areas. Blue tilapia are also known to impact catfish and a number of minnows and topminnows such as shiners, chub, pupfish, and mosquitofish -- including some imperiled species -- through habitat degradation, competition for incidental consumption of eggs through herbivory, predation on small and/or young fishes, and disturbance of nesting males in the case of pupfish. They can impact catfishes through competition for food and predation on young.

Biology: Blue Tilapia are primarily a freshwater species but can also survive and reproduce in brackish waters. This species inhabits warm ponds well as lakes and streams, in open water as well as among stones and vegetation. Blue Tilapia exhibit extremely broad, generalist diets, consuming detritus, algae, vegetation, invertebrates and their larvae, and smaller fishes and fish larvae and are more piscivorous as adults. These invasive fish have high tolerance for heat whereas cold temperatures may be the only factor limiting their distribution in Texas and die-offs have been observed when water temperatures dropped to 50°F. Tilapias have broad tolerances for pH and oxygen and are capable of surface respiration in anoxic conditions. Tilapias—and cichlids in general—are also known to be highly adaptable to salinity and can tolerate fairly brackish conditions.

Male tilapias dig nests to which they attract mates and, following spawning, females take eggs into their mouths (mouthbrooders) and move into deeper waters for a few days until they hatch and the fry school around the mother for several more days. Broods range from 160 to 1,600 eggs per female. Sexual maturity can be reached at an age of 5-6 months in ponds.

History: Blue Tilapia have been introduced throughout their non-native range (Texas, U.S., and internationally) through a combination of means including stocking and experimental work by states and releases by individuals seeking to have another species of sport fish. This species has also been introduced as food for predatory fish, as a human food source, and as a means of aquatic plant control. However, the impact of this invasive species was not taken into consideration when introductions occurred. The exact reasons for and sources of some introductions in Texas are uncertain. However, many introductions occurred in power plant lakes when power companies became interested in using "tropical fishes" for food or sport in lakes used to cool both fossil fuel fired and nuclear generating plants where temperatures often became too high to support populations of native fishes. Blue Tilapia were first introduced in power plant cooling reservoirs in Texas in the 1960s and have since spread to other reservoirs and rivers in basins across East, Central, and South Texas. It is believed that they may have been spread via bait bucket transfers.

U.S. Habitat: Primarily freshwater but can also survive in brackish water. Especially prevalent in some power plant cooling reservoirs due to warmer winter temperatures but are also likely able to survive winters in rivers, particularly spring-fed rivers and near wastewater outfalls. Tilapias are known to congregate around thermal refuges such as these during winter months.

Distribution

Native Origin: Tropical and subtropical Africa and the Middle East

U.S. Present: AL, AR, AZ, CA, CO, FL, GA, ID, KS, LA, NC, NV, OH, OK, PA, PR, SC, TX

Distribution in Texas: Blue Tilapia are established in numerous river drainages across the state of Texas. Although tilapia introduced to Texas water bodies other than power plant cooling reservoirs may experience die-offs during winter months, springs and even wastewater outfalls can provide thermal refuges, allowing them to survive in rivers. For example, Blue Tilapia are established in the spring-fed San Marcos and Comal Rivers, which are critical habitat for the federally endangered Fountain Darter.

SIMILAR SPECIES

Blue Tilapia might most easily be confused with its congeners, the Nile Tilapia (Oreochromis niloticus) and the Mozambique Tilapia (Oreochromis mossambicus) and there are no reliable characteristics that can be used to distinguish among these invasive species. Mozambique Tilapia have been found in the past in some Texas waters and there are isolated records of Nile Tilapia near the coast.

Rio Grande Cichlid (Herichthys cyanoguttatus; a.k.a. Rio Grande Perch). This species is native to the lower Rio Grande but has been introduced outside its range into several rivers of the Edwards Plateau including the San Marcos, Guadalupe, San Antonio, and Colorado Rivers. Rio Grande Cichlids can be distinguished from tilapias by the presence of five to six spines in the anal fin, whereas tilapias typically have only three.

Management

There are limited options for management of tilapia populations. Harvest/removals could reduce their numbers but not eliminate them altogether and is labor intensive, although targeted removals of tilapia may hold some potential in sensitive areas where imperiled fishes are present and likely to be negatively affected. Chemical treatments would harm native fishes and is not feasible in most locations. Encouragement of fishing for tilapia, without the intent of "catch and release", could help to reduce their numbers and potentially reduce their impacts to some degree. It is important to note that all tilapia species found in Texas are prohibited species and must be beheaded or gutted upon possession when caught. In power plant cooling lakes, plant shutdowns during winter can result in high tilapia mortality but these are inadvertent and planned shutdowns are not feasible.

Text References

USGS Nonindigenous Aquatic Species database profile
Global Invasive Species Database profile
U.S. Fish & Wildlife Service Ecological Risk Screening Summary: