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Use of grape pomace in formulated feed for the rainbow trout fry,
 Oncorhynchus mykiss
 (Walbaum, 1792)

Two different experiments with rainbow trout fry were carried out to evaluate the effect of different grape pomace (GP) levels in feed on growth performance and apparent digestibility. The first experiment used 0, 20, 40, and 60 g kg⁻¹ GP (D0, D2, D4, and D6, respectively); the second experiment used 0, 60, 120, and 180 g kg⁻¹ GP (D0, D6, D12, and D18, respectively). Total polyphenols in the diet ranged from 0 to 8.19 mg g⁻¹ in D0–D18. At the end of the first and second experiments (83 days and 77 days, respectively), no significant differences in fish growth were observed between the different diets. On the other hand, feed containing up to 2.7 mg g⁻¹ total polyphenols (D6) showed a significantly higher digestibility relative to feed with more than 5.5 mg g⁻¹ total polyphenols (D12). Our findings suggest that the inclusion of GP at levels of up to 18% in feed for Oncorhynchus mykiss fry did not affect growth parameters. However, it did influence apparent digestibility; fry that consumed diet D6 showed benefits in terms of nutrient digestibility and feed conversion efficiency.