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Diet-tissue discrimination factors (Δ13C and Δ15N) are influenced by variables including the tissues being analysed and the taxon of the consumer and its prey. Whilst differences in Δ13C and Δ15N are apparent between herbivorous and piscivorous fishes, there is less known for omnivorous fishes that consume plant and animal material. Here, the omnivorous cyprinid fishes Barbus barbus and Squalius cephalus were held in tank aquaria and exposed to three diets that varied in their constituents (plant-based to fishmeal-based) and protein content (13 to 45%). After 100 days and isotopic replacement in fish tissues to 98%, samples of the food items, and dorsal muscle, fin tissue and scales were analysed for δ13C and δ15N. For both species and all diets, muscle was always enriched in δ15N and depleted in δ13C compared with fin tissue and scales. Across the different diets, Δ13C ranged between 2.0 and 5.6 ‰ and Δ15N ranged between 2.0 and 6.9 ‰. The diet based on plant material (20% protein), always resulted in the highest discrimination factors for each tissue, whilst the diet based on fishmeal (45% protein) consistently resulted in the lowest. The discrimination factors produced by non-fish diets were comparatively high compared with values in the literature, but were consistent with general patterns for some herbivorous fishes. These outputs suggest that the diet-tissue discrimination factors of omnivorous fishes will vary considerably between animal and plant prey, and these specific differences need consideration in predictions of their diet composition and trophic position.

High variability in stable isotope diet-tissue discrimination factors of two omnivorous freshwater fishes in controlled ex situ conditions