The Siazakh Reservoir, located in the upper reaches of the Qezel-Ozan River approximately 7 km from Divandarreh, Kurdistan Province, Iran, supports a diverse fish community whose composition and biological characteristics are important for effective reservoir management. This study investigated the relative abundance, length and weight composition, and length–weight relationships of fish species in the reservoir from summer 2024 to spring 2025. Fish were collected using gillnets with mesh sizes ranging from 8 to 60 mm. A total of 11 species belonging to two families, Cyprinidae (7 species) and Leuciscidae (4 species), were recorded. Goldfish (Carassius auratus), gibel carp (Carassius gibelio), and Cyprinus rubrofuscus were identified as non-native species in Iran. Species composition and dominance varied considerably among gillnet mesh sizes. Kura bleak was dominant in the 8-mm (93%) and 15-mm (35%) mesh nets, whereas Transcaucasian chub was the dominant species in the 23-mm mesh net (53%). Razi scraper dominated catches in the 30-mm mesh net (52%), while common carp and Bulatmai barbel were the most abundant species in the larger mesh nets. Overall, Transcaucasian chub, Razi scraper, and Kura bleak accounted for approximately 85% of total fish abundance. The estimated length–weight relationship coefficient (b) ranged from 2.87 to 3.15. Based on Pauly’s classification, Blackbrow bleak, Transcaucasian chub, Razi scraper, and common carp exhibited isometric growth, whereas Kura bleak showed positive allometric growth. Bulatmai barbel and gibel carp exhibited negative allometric growth. The findings indicate that the reservoir fish community is dominated by a limited number of species and includes several non-native taxa. Given the limited representation of economically valuable phytoplanktivorous and zooplanktivorous species, the stocking of small numbers of silver carp and bighead carp (100–200 g) has been proposed as a management measure to assist in controlling algal blooms and enhancing fish production.