Impact succession of drought and flood on diversity indices, abundance, and size range of fish assemblage in Al-Shafi Marsh, southern Iraq

This study was conducted in the Al-Shafi Marsh, north of Basrah City to investigate the effects of succession drought and flooding on the abundance and size spectrum of fish assemblage. Three stations were selected and samples were collected on a monthly basis using various fishing gears such as electrofishing, gill nets, and cast nets. The temperature of the water was 12.50-34.53⁰C, and the salinity fluctuated from 1.64 PSU in May to 4.03 PSU in October. The pH ranged from 7.38 in September to 8.17 in March. The water depth in the low flat areas ranged from 33 cm in October to 71 cm in May, while the average depth in the seasonal flat areas ranged from zero in June, July, August, September, and October to 34cm in April. A total of 19 fish species were collected, representing 17 genera and 11 families. Eight fish species were native, eight exotics, and two marines. Cyprinidae was the most abundant family, with four species. The most abundant species were the Abu mullet, Planiliza abu, Prussian carp, Carassius gibelio, and blue tilapia, Orechromis aureus (28.42, 20.97, and 14.90%, respectively). The D3 dominance index was 64.29%. The length groups of the most important sixth commercial fish species ranged from 5 cm in P. abu to 31 cm in Leuciscus vorax. The diversity index ranged between 1.84 and 2.25, the evenness index 0.74 and 0.88, and the richness index between 1.63 and 2.37. The resident species accounted for 94.82% of the total catch. Seasonal species account 2.76% of all samples. The occasional fish species account for 2.42% of total collected fishes. Based on the results, there was insufficient flood pulsation to stimulate native species to reproduce, combined with overfishing, which resulted in a depletion of the species' biomass, and an increase in the abundance of small medium-sized fish species and small invasive species.