This study examines the processing of fins from blue sharks Prionace glauca caught by the Spanish longline fleet and landed in Vigo, Spain, and implications of these practices for enforcing the E.U. ban on shark finning, which relies on a maximum fin‐to‐carcass‐mass
ratio. Two major sources of variability in fin‐to‐carcass ratios are quantified and discussed: (1) the fin set (type and number of fins retained from each shark) and (2) the cutting method used to separate fins from carcasses. The significant differences in fin‐to‐carcass‐mass
ratios between fin sets or cutting procedure demonstrates that the ratio limit is problematic and, conclusively, in order to facilitate proper enforcement, fishermen should be required to land all sharks with the fins still naturally attached to the bodies.