Large Intraspecific Variation in the Stable Isotope Composition of the Muscle Tissues of Fishes of the Genus Cyprinion (Cyprinidae) from the Middle East

Significant individual variation in the nitrogen and carbon stable isotope composition (δ¹⁵N and δ¹³C values) of white muscle tissues has been found in samples of three cyprinid fish species, Cyprinion macrostomus (periphytonophage), Cyprinion kais (benthophage), and Luciobarbus schejch (omnivore), which were collected from a small tributary of the upper Tigris system (Turkey). In particular, an exceptionally low value (1.7‰) and abnormally wide intraspecific variability (from 1.7 to 15.8‰) of δ¹⁵N have been recorded in the tissues of C. macrostomus. Although all samples were collected synchronously and syntopically, the very broad range of δ¹⁵N values indicates a possible heterogeneity of the samples and the involvement of different processes in the formation of the isotopic composition of fish tissues. The studied watercourse is in the zone of agriculture with intensive chemicalization and pollution of the river with mobile forms of nitrogen (mostly ammonium), which might cause the reduction in the δ¹⁵N value in resident fishes. Fish with high (normal) δ¹⁵N values are supposedly recent migrants that came from a large river for joint spawning with resident fish. The pollution of water bodies, as well as the presence of migrations, should be taken into account in studies of the trophic ecology of fishes based on stable isotope analyses.