Abstract
The diet composition and trophic relationships of the blue tilapia, Oreochromis aureus and redbelly tilapia, Coptodon zillii were investigated in the Shatt Al-Arab river from November 2015 to October 2016. Fishes were collected by gill nets, cast net and electro-fishing. The index of relative importance (IRI%) of food items was determined by combining the frequency of occurrence and points methods. There were monthly variations in feeding activity and intensity of both species, which decreased in the colder months. Both species were classified as herbivores. The main food items of O. aureus were macrophytes (44.3%), algae (31.4%), diatoms (9.3%), aquatic insects (6.6%) and detritus (6.3%). The diet of C. zillii is composed mainly of macrophytes (60.8%), algae (23.0%), detritus (8.6%) and diatoms (6.4). The trophic niche breadth values varied between 0.217 for C. zillii and 0.360 for O. aureus. The results demonstrated a high degree of dietary overlap between O. aureus and C. zillii, as both species fed on the same food sources
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