Alien fish species have negative effects on the abundance, diversity and richness of native fish species in southern Iraq. The numbers of alien fish species are constantly increasing due to the entry of invasive species. This has resulted in shifts in the historical composition of fish community structure and scarcity of some native species which represent the keystone in the building of fish populations.     Fish samples were monthly collected from three sites in southern Iraq (Al-Chibyaish marsh, the lower parts of Euphrates River, and the northern part of Shatt Al-Arab River) from April 2017 to June 2018. A total of 14,853 individuals of fish were sampled, which represented 26 species, 24 genera and 13 families of bony fishes. Ten alien fish species were recorded, affiliated to nine genera and five families, namely Cichlidae, Cyprinidae, Heteropneustidae, Poeciliidae and Xenocyprididae. Three alien fish species dominated the abundance in the three sites; the Prussian carp Carassuis gibelio comprised 12.58%, 26.19%, and 13.84%, the Blue tilapia Oreochromis aureus formed 16.78%, 13.66%, and 18.79%, , and the Redbelly tilapia Coptodon zillii comprised 7.37%, 7.71%, and 14.66% of the total number of species in three study sites respectively. (  …. )      These alien fish species created serious shifting in fish composition, diversity, richness, and abundance of native fish populations in comparison to the results of the historical survey in southern Iraq.

A total of 114 (76 female and 38 male) specimens of Shelig Leuciscus vorax were monthly collected from Al-Huwaiza marsh southern Iraq from November 2016 to October 2017. The samples were caught utilizing gill net and electrofishing. The highest values of gonadosomatic index (GSI) were found in February (18.35 and 5.33 for female and male respectively). Only mature individuals of L. vorax female were used to estimate absolute-relative fecundity and measure eggs diameters of very ripe gonads. Absolute fecundity ranged from 77612 eggs at 56 cm in total length (1672 g) body weight to 378391 eggs 79.5 cm in total length and (5001 g) boy weight. Highly significant differences relationships were observed among to total length, body weight and reproductive parameters. The relative condition factor differs from 0.85±0.11 in October to 1.37±0.09 in June. The results showed one spawning peak on reproductive strategy of represented species, which is preceding other species of fish to enables the small fish of L. vorax feeding on larvae and other small fish species in assemblage.

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Diversity, abundance and community structure of fish species in the lower part of the Euphrates River was investigated during May 2016 to April 2017. Three stations were selected to conduct the work. Monthly variance of water temperature ranged from 11ºC in January and February to 31ºC in August, turbidity differs from 2.31 in June to 24.10 NTU in November. The overall values of hydrogen-ion differ from 7.52 in April to 8.27 in November and December. The overall values of salinity in the work area were 0.91 ‰ in October to 1.52 ‰ in May. Several fishing methods were used to collect the samples of fishes. A total of 4380 specimens' of fish were collected included 23 fish species belonging to 23 genera and 11 families, ten of them represented native fish species, eight alien fish species and five marine species. Oreochroims aureus was the most abundance species formed 34.27 % of the total number then Liza abu (24.66 %) of the total number of species and Carassius gibelo (14.36 %). Values of three dominate species (D3) was 73.29 %. The mean values of diversity index (H) in the study area ranged from 1.11 to 1.92, evenness index (J) from 0.53 to 0.90 and richness index (D) from 1.15 to 2.33. The study showed the dominance of small species of exotic and native fish in the study area and the scarcity of large economic species due to the overfishing.