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The black mangrove cichlid or ‘spotted tilapia’ is a species of deep-bodied fish belonging to the family Cichlidae. Native to West Africa, its distribution extends from the Ivory Coast through Ghana and Nigeria to Cameroon1. Throughout this range the species is often found in the deeper sections of larger rivers and lagoons but also moves into flowing streams during the rainy season1.
Black mangrove cichlids have become a desirable aquarium species as they are hardy, readily reproduce and have attractive body and fin colouration2. This popularity has led to the species being exported to many countries outside Africa. The ability of black mangrove cichlids to adapt to a wide range of ecological conditions has enabled them to become a highly invasive pest. For example, in the United States the black mangrove cichlid has established self-sustaining populations in several states, including Florida, Arizona, California and Nevada3. Self-sustaining populations of this species reportedly also occur in Russia (unconfirmed) and unfortunately, in Australia.
The Gulf of Carpentaria drainage is at serious threat of being colonised by introduced tilapia
In the United States, studies have found that the species has the potential to affect other introduced and native fishes through aggressive interactions for territories3. They have also become the most abundant species in canal habitats due to their wide ecological tolerances, and compete with small native fish and invertebrates for food3. In Australia, the impacts of black mangrove cichlids are still being assessed, but they are likely to include habitat modification, spread of diseases and reductions in native biodiversity.
The first documented discovery of introduced black mangrove cichlids in Australian waterways was in the 1970s in the Hazelwood pondage, Victoria. During the 1980s, a population of black mangrove cichlids was discovered in coastal rivers in the Wet Tropics World Heritage Area of north Queensland3. These introductions occurred despite the species being on the prohibited import list since the 1960s. The precise origin of the illegal importations is still unknown but it is thought to have been through the ornamental fish trade. More black mangrove cichlids have since been released into the wild, either deliberately or accidently, and have formed self-sustaining populations.
The black mangrove cichlid is present in three eastern catchments draining the Wet Tropics area of north Queensland, and an isolated population persists in the Hazelwood pondage in Victoria. This population is the only known population of black mangrove cichlids outside Queensland.
The distribution of black mangrove cichlids in the Wet Tropics centres on the greater Cairns region. To the north of the city, black mangrove cichlids have been present in small drainages and ponds around Cairns’ northern beaches as well as in the Barron River and associated tributaries since the mid-to-late 1980s5. During the mid 1990s, black mangrove cichlids were introduced to the upland section of the Barron River catchment and are now abundant in all tributaries as well as the large impoundment, Lake Tinaroo6. Feeder creeks and water bodies in the Barron River catchment with self-sustaining populations of this species include (upland section) Peterson Creek, Kenny Creek, Severin Creek, and in the lowlands below Barron River Falls, Freshwater Creek, Cattana Wetlands and Thomatis Creek.
To the south of Cairns, black mangrove cichlids are present in the lower Johnstone River and the Russell-Mulgrave River system. Distributions within both catchments include the main river channels to the base of the coastal escarpment, major tributaries, coastal flood plain drainages and water storages. Black mangrove cichlids were first discovered in the Johnstone River in the early 1990s. They have since colonised the lowland portion of both the North and South Johnstone Rivers including the feeder creeks of Victory Creek, Gracey Creek, Stewart Creek and Utchee Creek. In the Russell-Mulgrave River catchment, black mangrove cichlids are abundant. They occur along the length of both rivers on the coastal plain, as well as Lake Barrine on the Atherton Tablelands, on the headwaters of the Mulgrave River. Feeder streams on the coastal plain include Behana Creek, Fishery Falls Creek and Figtree Creek draining into the Mulgrave River, and Harvey Creek, Babinda Creek and the Alice River draining into the Russell River.
In 2008, a population of black mangrove cichlid was discovered in Eureka Creek, an upper tributary of the Mitchell River, Queensland8. This was the first time this species had been found on the western side of the Great Dividing Range. It was quickly recognised that this population had the potential to spread throughout the Mitchell River catchment and then into the greater Gulf of Carpentaria drainage. At the time of discovery, it was found in relatively low numbers in an isolated section of creek. This suggested that the species was only recently introduced to the area. An immediate eradication operation was carried out by departmental fisheries officers to prevent further spread of the species7. Since then, follow-up surveys have found no evidence of black mangrove cichlids either in Eureka Creek or in other tributaries of the Mitchell River.
A population of black mangrove cichlids persists in the Hazelwood cooling ponds because of heated water being discharged by the power station. The ponds are located on a tributary of the Morwell River in Victoria. Water temperatures in the Morwell River and other temperate areas of Australia are at the extreme lower end of thermal tolerance for the species, and escapees would be unlikely to survive and reproduce here.
There is potential for black mangrove cichlids to spread into other parts of tropical and sub-tropical Australia. Areas most at risk of invasion are usually where colonised and unaffected catchments are close to each other or where there is cross-drainage or cross-catchment distribution of water (eg through irrigation schemes). Some catchments in the Gulf of Carpentaria drainage meet these criteria and are considered to be at high risk of invasion. Apart from natural dispersal, the other major source of spread is through the movement of fish between water bodies by people.
Currently, attempts to stop the spread of black mangrove cichlid are through education and extension programs, spot eradications and installation of screens on irrigation channels and pipelines. There is currently no ‘silver bullet’ option for their management, but ongoing research is looking at alternative control options8. Considering the highly invasive nature of the species, it is likely that the distribution will continue to expand as the species spreads into new waterways in tropical and sub-tropical Australia.