The spread of the Nile tilapia into the Limpopo river catchment has been well documented. Originally stocked in Zambian community fish farms by the Peace Corps, the species was translocated to dams in Matabeleland in Zimbabwe in the 1990s, from where it escaped via the Shashi River to the Limpopo River system.
Dr Ben van der Waal, an aquatic scientist, collected samples of Nile tilapia and hybrid fish for scientific identification in Limpopo in 2000 (Van der Waal, B. & Bills, R. 2000). In the same year, Nile tilapia and hybrid species were also identified in the Levuvhu River near Pafuri, and the species has continued its upstream spread ever since.
Threatening the Blue Kurper
The Mozambican government has actively promoted aquaculture using the fast-growing commercial strains of the Nile tilapia, especially in the Chokwe region of the Gaza province (lower Limpopo/Olifants/Letaba river catchment), the Incomati region north of Maputo (Incomati/Sabi catchment), and in southern Mozambique at Bela Vista in the Rio Maputo (Phongola River) catchment.
It is not surprising that the Nile tilapia has penetrated South African rivers; the local aquatic environment is suitable and natural barriers are absent.
The conservation problem with the Nile tilapia is that it hybridises with, and can dominate, our Blue Kurper (Oreochromis mossambicus), leading to genetic contamination and possible local extinction.
The establishment of sanctuaries would conserve our indigenous Blue Kurper in the same way that game reserves conserve terrestrial game.
This would also allow for the utilisation of commercial aquaculture strains elsewhere, as is practised in other agricultural sectors. O. mossambicus is well known as one of the most euryhaline (salt-tolerant) of all tilapia, thus at the climatic extremity of its range it tolerates brackish or even hyper-saline water in estuaries, where
it can survive low temperatures better than in freshwater (James 1991).
The Nile tilapia does not thrive at high salinities (>20ppt) and low temperatures (< 12°C), so these areas exclude invasion and therefore offer a safe refuge for the Blue Kurper. I have seen Blue Kurper in Port Alfred just surviving and covered with fungus at 9,5°C during winter.
The many coastal streams of the Wild Coast therefore offer a safe sanctuary for them, especially as this area is unlikely to attract commercial tilapia aquaculture interest.
Further north, sanctuaries for the Blue Kurper can be established in places where conservation is realistic. The South African record-size Blue Kurper was from Loskop Dam (3,3kg), and this water body and its upstream catchment could serve as a local sanctuary.
Further downstream in the Olifants River, the Flag Boshielo Dam (formerly the Arabie Dam) also has Blue Kurper, and if confirmed that this is a pure population, would also serve as a sanctuary, with the dam walls preventing upstream migration by the Nile tilapia.
Attempting to ‘legislate’ the Nile tilapia from spreading is doomed to failure, and establishing sanctuaries for our indigenous Blue Kurper is the only way to ensure the species’ survival.
Van der Waal, BCW and Bills, R. (2000). ‘Oreochromis niloticus (Teleostei: Cichlidae) now in the Limpopo River system’. South African Journal of Science. 96(1): 47-48. James, NPE. 1991. A life-history approach to the biology of Oreochromis mossambicus (Pisces: Cichlidae) in the Eastern Cape (MSc thesis). Rhodes University.