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A total of 270 fingerlings (10.7 ± 0.06 g of average weight) were randomly distributed in three treatments with three repetitions, according to a completely randomized design, to evaluate the total substitution of a high fishmeal percentage for silages of fishery by-products in extruded diets for Clarias gariepinus. The treatments were catfish fingerling feed with 25 % of fish meal (control) and two alternative diets, with 10% (dry basis) of silage: one prepared with 98 % of sulfuric acid, and the other with formic acid. Animals were fed for 60 d. No differences were found (P> 0.05) in the final weights (66.63, 66.27 and 70.71 g) and food conversion (1.18, 1.18 and 1.15). However, there were differences for protein efficiency between the diet with fishmeal (2.34) and the silage of fishery products (3.09 and 3.18). Survival was excellent in all treatments (> 96.66%). The economic analysis showed that the profits with silage (US $ 2534.22 and 2430.05/t) were superior to control (US $ 1689.44/t). It is concluded that chemical silages of fishery by-products can replace a high percentage of fishmeal in extruded diets for Clarias gariepinus fingerlings with a positive economic effect, by reducing the importation of fishmeal. [ABSTRACT FROM AUTHOR]