As the common carp Cyprinus carpio and African catfish Clarias gariepinus affect biotic assemblages in aquatic ecosystems through their benthivory, their introduction has been considered an environmental concern. We compare the effects of common carp and African catfish on adjacent trophic level assemblages of plankton, periphyton, benthic macroinvertebrates and native tilapia in a pond ecosystem at the National Aquaculture Center, Zomba, Malawi. Four treatments were involved in the study: ponds with common carp (T1), ponds with African catfish (T2), ponds with both common carp and African catfish (T3), and ponds lacking both common carp and African catfish (T4). In all treatments, the endemic Shire tilapia Oreochromis shiranus was also present as part of the local macrofauna. Results showed that T1 had the most abundant phytoplankton but the lowest periphyton, zooplankton and benthic macroinvertebrate abundance and diversity. T4 had the least phytoplankton but the most abundant and diverse zooplankton, periphyton and benthic macroinvertebrates. There was no significant difference between T2 and T3 (both intermediate to T1 and T4) in terms of phytoplankton, periphyton and benthic macroinvertebrate abundance and diversity. Better water quality regime and increased biomass of native tilapia fish were observed in T3 ponds. We conclude that the farming of common carp in aquatic ecosystems containing African catfish may not adversely affect the environment and that the polyculture of African catfish and common carp can be adopted as a management strategy to mitigate the potential adverse effects of common carp.
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