This study aimed to estimate trophic discrimination factors (TDFs) and metabolic turnover rates of nitrogen and carbon stable isotopes in blood and muscle of the smallnose fanskateSympterygia bonapartiiby feeding six adult individuals, maintained in captivity, with a constant diet for 365 days.TDFswere estimated as the difference between δ13Cor δ15Nvalues of the food and the tissues ofS. bonapartiiafter they had reached equilibrium with their diet. The duration of the experiment was enough to reach the equilibrium condition in blood for both elements (estimated time to reach 95% of turnover: Ct95%blood = 150 days, Nt95%blood = 290 days), whilst turnover rates could not be estimated for muscle because of variation among samples. Estimates of Δ13Cand Δ15Nvalues in blood and muscle using all individuals were Δ13Cblood= 1·7‰, Δ13Cmuscle= 1·3‰, Δ15Nblood= 2·5‰ and Δ15Nmuscle= 1·5‰, but there was evidence of differences ofc.0·4‰ in the Δ13Cvalues between sexes. The present values forTDFsand turnover rates constitute the first evidence for dietary switching in batoids based on long‐term controlled feeding experiments. Overall, the results showed thatS. bonapartiihas relatively low turnover rates and isotopic measurements would not track seasonal movements adequately. The estimated Δ13Cvalues inS. bonapartiiblood and muscle were similar to previous estimations for elasmobranchs and to generally accepted values in bony fishes (Δ13C = 1·5‰). For Δ15N, the results were similar to published reports for blood but smaller than reports for muscle and notably smaller than the typical values used to estimate trophic position (Δ15Nc.3·4‰). Thus, trophic position estimations for elasmobranchs based on typical Δ15Nvalues could lead to underestimates of actual trophic positions. Finally, the evidence of differences inTDFsbetween sexes reveals a need for more targeted research.