Council Regulation (EC) No 1005/2008 provides that a third country may be identified and subject to measures if it fails “to discharge the duties incumbent upon it under international law as flag, port, coastal or market state, to take action to prevent deter and eliminate IUU fishing”. In assessing the promise or limits of this unilateral listing mechanism, the question arises as to what influence, if any, it has had on third-country legislation. This article uses a case study on port state duties, demonstrating the use and impact of the unilaterally defined port state duties and their implementation in EU practice. The latter part addresses trends in the prescriptive responses of port states, including whether the EU listing process was a contributing factor to legislative reform. Prescriptive trends highlighted are the implementation of generally accepted treaty-based port state measures, followed by three examples of expansion into port state offences.