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Trophic interrelationships between the exotic Nile tilapia and indigenous tilapiine cichlids in the Limpopo river system

Mozambique tilapia

Mozambique tilapia, O. mossambicus from the Limpopo River, South Africa.

Native to the Nile River basin, Lake Chad, south-western Middle East and the Niger, Benue, Volta and Senegal Rivers, Oreochromis niloticus (Nile tilapia) has been widely introduced in southern Africa for aquaculture. In South Africa, the species was at first introduced in the Cape Flats area and in KwaZulu-Natal Province in the 1950s. Its range has since expanded to include the Limpopo and other eastern rivers in South Africa and Mozambique. The introduction of O. niloticus in the Limpopo river system is a cause for concern for the conservation of indigenous species, especially for the Mozambique tilapia, Oreochromis mossambicus, which is likely to become locally extinct from the river system through hybridization and competition arising from its habitat and trophic overlaps with that of O. niloticus.

The ecology of seasonal rivers within the Limpopo river system is poorly understood and the effect that O. niloticus may have on these indigenous species is largely unknown. It is therefore important to establish whether these species are sharing resources because adverse impacts of O. niloticus invasions on indigenous species due to habitat and trophic overlaps are well-documented elsewhere.

In a recent paper in the journal Environmental Biology of Fishes, C·I·B PhD student, Tsungai Zengeya and co-authors Professor Tony Booth (Rhodes University), C·I·B Research Associate Professor Amanda Bastos (University of Pretoria) and C·I·B core team member Professor Chris Chimimba (University of Pretoria) investigated the feeding ecology of tilapiine cichlids in the Limpopo River system, with a specific focus on dietary overlap between the invasive O. niloticus and indigenous tilapiine species.

Typical tilapiine habitat

Typical tilapiine habitat Mogalakwena River, Limpopo River Basin, South Africa.

The study combined both stomach content and stable isotope analysis to determine whether tilapiine cichlids divide resources seasonally, by comparing diets in wet and dry months, as well as throughout ontogeny, by separating diets based on size class. They also speculate on whether the introduced O. niloticus will adversely affect co-occurring indigenous fish populations through predation and/or competition.

Stomach contents of O. niloticus and O. mossambicus indicated high dietary overlap across size class, habitat and season, with both species primarily feeding on vegetative detritus. However, stable isotope analysis revealed that the two Oreochromis species had different stable isotope ratios derived from different food sources. The relatively δ13C-depleted O. niloticus indicates a phytoplankton-based diet, while the δ13C-rich O. mossambicus indicates a macrophagous diet dominated by vegetative detritus and periphyton. The high similarity in stomach contents and the interspecific differences in isotopic composition reveal fine-scale patterns of food resource division that could be achieved through selective feeding. Tilapia rendalli (Redbreast tilapia) fed mainly on aquatic macrophytes and had a low dietary overlap with both O. niloticus and O. mossambicus.

In the Limpopo River, detritus and algae are probably the most abundant food resources and the causal factors responsible for the observed patterns of resource partitioning among the tilapiines are usually difficult to ascertain. Fish may be able to perceive food resources in terms of the dynamics that determine their availability. Detailed studies of variation in food resource availability and fish habitat use within the system are needed to evaluate this hypothesis.

Read paper:

Zengeya TA, Booth AJ, Bastos ADS, Chimimba, CT (2011) Trophic interrelationships between the exotic Nile tilapia, Oreochromis niloticus and indigenous tilapiine cichlids in a tropical African river system, Limpopo River, South Africa. Environmental Biology of Fishes

For further details, email Tsungai Zengeya at tzengeya@zoology.up.ac.za or ctchimimba@zoology.up.ac.za