Knowledge of the length at first maturity (L50) is an important tool for fish stock management and conservation. This study estimated the L50 values of 10 fish species in two reservoirs of the Apodi/Mossoró River in the tropical semiarid region of Brazil. The following species were studied: Curimatella lepidura (Eigenmann et Eigenmann, 1889); Prochilodus brevis Steindachner, 1875; Leporinus piau Fowler, 1941; Leporinus taeniatus Lütken 1875; Triportheus signatus  (Garman, 1890); Hoplias malabaricus (Bloch, 1794); Hypostomus pusarum (Starks, 1913); Loricariichthys derbyi Fowler, 1915; Trachelyopterus galeatus (Linnaeus, 1766); Crenicichla menezesi Ploeg, 1991. The captured individuals were identified, separated by species, measured for total length (TL), and categorized as reproductive or immature after dissection. The L50 values for both sexes were estimated via a logistic regression for each species. Significant differences between values were tested for the species that occurred in both reservoirs (ANCOVA). The estimated L50 value varied between 7.37 cm (C. lepidura, female) and 19.40 cm (H. malabaricus, female). Only the L50 value of the males of the species C. lepidura and P. brevis differed between the two reservoirs. This study is the first report of the L50 for seven species and the first report for tropical semiarid river basins for two species. These results are important for fish resource management and species conservation.

artisanal fisheries, reproductive strategy, freshwater fish, fisheries management