Tuna management in the Western and Central Pacific is complicated by the conflicting interests of countries and agents exploiting tuna resources in the region. Historically, regulatory attempts by Pacific Island Countries to control fishing effort within their Exclusive Economic Zones (EEZs) have met with limited success. The introduction of new economic policy instruments by the Parties to the Nauru Agreement (PNA), such as the Vessel Day Scheme (VDS) and Marine Stewardship Council (MSC) certification, has supported and complemented existing conservation and management measures. By bringing in new incentives for the PNA states, greater control over fishing effort and the formulation of perceptibly new sustainable fishing practices have emerged. Using a new institutional economic framework, this paper analyses the shift from regulatory policy to new economic policy instruments through the lens of New Institutional Economics. The results show how the adoption of the VDS and MSC certification program has brought new changes and improvements to tuna negotiations, to agreements, and to outcomes amongst parties. Investing in these new instruments has elucidated ways in which new economic institutions strengthen de jure political control over transboundary fish resources and fishing fleets.