The National Marine Fisheries Service (NMFS) developed the Atlantic Large Whale Take Reduction (ALWTR) plan to protect endangered large whales from entanglements in commercial fishing gear, which can cause injury or death. Because whales continued to die after the ALWTR plan went into effect, NMFS proposed revisions in 2005. GAO was asked to review these proposed revisions, including (1) their scientific basis and uncertainties regarding their effectiveness, (2) NMFS's plans to address concerns about the feasibility of implementing them, (3) the extent to which NMFS fully assessed the costs to the fishing industry and impacts on fishing communities, and (4) the extent to which NMFS developed strategies for fully evaluating their effectiveness. GAO reviewed the proposed changes to the ALWTR plan and obtained the views of NMFS officials, industry representatives, scientists, and conservationists. GAO recommends that NMFS revise its economic analysis to present a range of possible costs, expand its proposed gear-marking requirements, and develop a strategy to assess industry compliance. The agency reviewed a draft of this report and did not agree to revise its economic analysis or expand gear markings but did agree to develop a strategy to assess industry compliance.