Aquaculture carrying capacity of Nile tilapia Oreochromis niloticus and Nile crocodile Crocodylus niloticus in Lake Kariba, Zambia and Zimbabwe

Increased aquaculture production can improve food and nutrition security and economic growth in developing countries, but comes with environmental risks. In recent years, aquaculture has increased tremendously in Lake Kariba, which is located on the border between Zambia and Zimbabwe. We calculated the carrying capacity of aquaculture production in Lake Kariba using the Beveridge P balance model with data for lake-wide total-P concentrations, river flows and data from aquaculture farms and feed producers in the Lake Kariba area. The maximum permissible P load for aquaculture farms in Lake Kariba is 1.2 × 106 kg P yr-1. Average loss of P to the environment in Lake Kariba through farming is 13.92 kg P t-1 for Nile tilapia Oreochromis niloticus and 92.5 kg P t-1 for Nile crocodile Crocodylus niloticus. Consequently, sustainable aquaculture carrying capacity in Lake Kariba is 86900 t yr-1 for O. niloticus and 13000 t yr-1 for C. niloticus. We expect aquaculture production in Lake Kariba to account for 71% of the total allowable aquaculture P load by 2028. The total-P concentration is expected to increase to 28.5 mg m-3 by 2028 due to growth in aquaculture, and its development should be carefully monitored. In future, we recommend that our predictions should be compared to observed changes in order to validate the model. We also illustrate that companies can greatly improve the carrying capacity of a system by improving the feed conversion ratio, the efficiency of nutrient uptake and the processing of mortalities.