The Atlantic goliath grouper ( Epinephelus itajara ) is the largest species of grouper in the Atlantic Ocean and is classified as critically endangered in Brazil. This study compared the isotopic ratios of carbon (δ 13 C) and nitrogen (δ 15 N) between the muscle tissue and dorsal‐fin rays of 37 juveniles to assess fin rays as a non‐lethal alternative to muscle tissue sampling. The δ 13 C values in rays were significantly highe...