African sharptooth catfish, Clarias gariepinus, is a successful aquatic invader as it has a eurytopic physiology, is highly fecund, grows quickly and has an omnivorous diet. Despite possible threats that include predation of and competition with indigenous species, habitat degradation, and the spread of diseases and parasites, there is a lack of knowledge regarding its ecology and population dynamics within invaded systems. This study presents a mark—recapture experiment to investigate its population structure, natural mortality rate, population size and density within a population inhabiting a small, 76 ha invaded water supply reservoir in the temperate Eastern Cape region, South Africa. The adult population was found to be old ( = 120 ± 29 years) and male-dominated (1.76♂:1♀ for fish < 100 cm TL and 3.77♂: 1♀ for fish > 100 cm TL) with its maximum age estimated at one year less than the age of the reservoir. Adult abundance and density (in terms of numbers and biomass) was relatively low at 1655 fish (CV = 18%), 94.32 kg/ha (CV = 18%) and 21.78 fish/ha (CV = 16%), respectively. The estimated mean natural mortality rate was high at 0.80/year (CV = 82%). The skewed length- and age-structure suggests that the adult population is dominated by the fish that were either the initial colonizers during the reservoir filling phase or were the first internally reproduced recruits. The predominance of old, large fish in the population is also probably suppressing recruitment of younger fish. Despite the population being small, the fish are mobile and feed aggressively. Manual removal using baited longlines could be a feasible removal method.