Purse seine fishing of skipjack tuna in the Western and Central Pacific Ocean (WCPO) raises concern over increasing impacts on yellowfin and bigeye tuna by-catch. To address sustainability concerns, the Parties to the Nauru Agreement (PNA) adopted the vessel day scheme (VDS) for the management of purse seine fishing in the WCPO. The VDS has the potential to improve economic benefits for PNA members and to contribute to sustainability of the tuna stocks, and since 2012, has become an important mechanism to regulate fishing access rights in PNA waters. Despite this, monitoring and enforcement remains weak leading to violations of the agreement. Using a game theoretic framework, this paper examines the effectiveness of the VDS and its implications for fisheries regionalism. We examine the payoffs of member countries of the Nauru Agreement under full compliance and payoffs under the currently observed partial compliance. Our findings indicate that member states partial compliance with VDS rules plays a role in stabilising the agreement. Requiring full compliance, on the other hand, may encompass strong incentives for PNA members to deviate from the VDS since third parties offer attractive benefits in return for privileged access to fishing grounds. However, pragmatic tolerance of deviations from full VDS compliance seems to play a facilitative role in promoting cooperation and fisheries regionalism.