Background. Clupeidae, Nannothrissa stewarti (Poll & Roberts 1976) endemic to Lake Mai-Ndombe is one of most heavily fished fish groups using practices and nets not allowed by the country's legislation. Objective of this study was to determine some aspects of reproductive biology of N. stewarti in Lake Mai-Ndombe. Materials and methods. Fish were sampled monthly from November 2020 to October 2021 breeding parameters were determined : Gonado-somatic index (GSI), Size at first sexual maturity, Absolute fecundity and the relationship between total weight (WT) and total length. Results. Results obtained showed that the sex ratio was in favor of females (1 : 0.8). Estimated absolute fecundity was between 227 and 4080 oocytes for females of total length between 23 and 35 mm with an average of 923 ± 664 g oocytes and a relative fecundity varying between 25115 and 155457 oocytes kg-1. Average oocyte diameter was 0.20 ± 0.14 mm. Distribution of oocyte diameters observed in the population as well as monthly variations of the Somatic Gonado Index (SGI) indicate that the species has two main clutches during the year. LT50 size at first sexual maturity is 27.6 mm for males and 25.5 mm for females. Conclusion. N. stewarti from Lake Mai-Ndombe has multiple reproduction throughout year with however two maximum peaks at the beginning of peak rainfall (February-March and September-October).

Keywords: Breeding strategies, DR Congo, First sexual maturity size, Lake Mai-Ndombe, N. stewarti