Despite the use of gear requirements and access restrictions to manage lobster fishery interactions with north Atlantic right whales since 1997, the population is likely below 370 animals. The Dynamic Area Management (DAM) program (2002–2009) used “real time” right whale sightings data to provide temporary protection using closures or whale-modified-gear to reduce entanglement. Our ex-post evaluation uses a flexible framework based on a relative risk of entanglement index (RREI) calculated with spatially and temporally explicit data on density of right whales and fishing effort. An illustrative closure optimization model demonstrates the trade-offs between the non monetary benefits of risk reduction and the opportunity cost of closures. Annual aerial sampling to detect DAM areas was low (<3%), and the spatial footprint of dynamic and static measures was small although they may have reduced total risk by 6.5% on average. The optimization results illustrate the lower potential costs to reduce risk under a decision rule that simultaneously considered fishing industry opportunity costs and non-monetary benefits, compared to a decision rule based only on the benefits of risk reduction; the implications of whale-modified-gear and compliance factors are also explored. We recommend that DAMs be considered as part of a suite of policy instruments, and highlight how recent technological advances may support lower cost data collection and faster implementation given limited public sector budgets. This case study highlights the need for empirical evaluation of past policy instruments for right whale protection with a lens beyond biological outcomes, including harvester responses and private and public sector trade-offs.