The hydrological system plays a vital role in ecosystem functions that reflects on organism species populations. The study of fish species populations and ecological factors was carried in three sites in the northern part of Shatt Al-Arab River, during the period October 2018 to September 2019. A total of 35 species of fish were captured from the area of the sites which included 30 genera and belong to 17 families. In total of 2731 individuals were caught from site one and varied from 45 individuals in September to 420 in June and the number of species differed from ten species in October, January, and September to 15 species in December. The total number of 5631 individuals was recorded from site two it recorded 272 individuals in March to 928 in December, while the number of species ranged from eight species in September to 17 species in March. Site three included 5381 individual and varied from 297 in January to 617 individual in April and most of the fish species recorded in this site and ranged between 11 species in May and November to 19 species in March. The number of fish members in the study sites was 13743 individuals, the minimum number (908) was caught in August and the maximum (1800) in December, the number of species differed from 17 in November to 22 in December and February, the relative abundance varied from 24.62% to 0.01%. The values ranged of ecological indices in the study area; Shannon and Weaver diversity index (H) 0.43 in September to 1.99 in March, the values varied of Evenness (E) 0.15 in August and September to 0.65 in March and the minimum of Margalef richness (D) 2.30 in November to the highest 3.02 in February. Three fish species ( and which included 70.21% of the total number according to dominance index C. gibelio, P. abu, O. aureus)(D). Fish species were classified into three groups according to their presence in the monthly samples, the common fish species were formed 3 16 species and in proportion 98.741270% of the total number. The seasonal fish contain five species and represented 0. 93% of the catch, the occasional fishes comprised 14 species and formed 0.320163% from the catch.