Unlike merchant ships, cars, and even cellphones, industrial fishing vessels are not required to have unique, permanent identifying numbers. Fishing vessels do have names, call signs, and other identifiers, but those can be changed by the owner quickly and easily. Further, such identifiers are not systematically included in relevant communications. This makes it easy for owners to hide their vessels' true identities if they want to—for example, if the vessels are being used in illicit activities. Illegal activity on the high seas is rampant. Due to gaps in international fisheries policies, operators can evade accountability in numerous ways, allowing them to ignore catch quotas; fish without licenses; use destructive gear; and otherwise flout rules intended to make fishing fair, sustainable, and environmentally sound. In addition, illegal fishing is linked to other serious crimes in the eastern Pacific Ocean, including drug trafficking and human smuggling, as documented by the U.N. Office on Drugs and Crime. The lack of transparency for identifying fishing vessels helps criminal fishermen conceal their crimes. Thus The Pew Charitable Trusts is calling on the Inter-American Tropical Tuna Commission, or IATTC, to take decisive steps to combat illegal, unreported, and unregulated, or IUU, fishing. This can be done by requiring International Maritime Organization, or IMO, numbers for all vessels at least 24 meters in length that fish in waters managed by the commission. From research presented in this brief, Pew finds numerous errors and inconsistencies in data used to identify vessels authorized to fish in IATTC waters. By mandating IMO numbers, the commission could eliminate such problems.