The biology and impacts of <i>Oreochromis niloticus</i> and <i>Limnothrissa miodon </i>introduced in Lake Kariba

The thesis presents studies on Nile tilapia (Oreochromis niloticus) and a freshwater sardine (Limnothrissa miodon), two exotic fish species introduced into a reservoir, Lake Kariba in Zimbabwe/Zambia. Their growth rates show that conditions in Lake Kariba are comparable to their native lakes. Nile tilapia escaped into Lake Kariba from fish farms and displaced an endemic native tilapia, Oreochromis mortimeri. This provided an opportunity to study the impact of an invading species sharing an ecological niche with a native species, and traits that may give competitive advantage. I conclude that a superior growth rate of Nile tilapia, resulting in a larger size that I showed to be advantageous in aggressive encounters with Oreochromis mortimeri, and a higher reproductive effort have contributed to its success. Additionally, the results support that fish introductions into an occupied niche may not result in higher catches.The sardine was deliberately introduced into a vacant niche, the deep-water habitat in the newly formed reservoir, to increase fish production. An initially highly productive fishery that developed on the sardine collapsed after 16 years of expansion. I conclude that the collapse can be attributed to overfishing and unfavourable environmental conditions associated with rising temperature. Additionally, I conclude that selective fishing pressure reduced growth rate and size at maturity, which slows down the recovery of the sardine population. My results indicate that managers of the sardine fishery need to monitor life-history parameters and environmental variation such as temperature and plankton, and match fishing effort to ecosystem production to safe-keep sustainability