How well are international resource management regimes equipped to handle climate change? Are such regimes able to adapt to and include new players, or do the stakeholders prefer status quo? This article will explore this phenomenon by using the case of the Mackerel War in the Northeast-Atlantic during 2008–2014. The mackerel stock in the Northeast-Atlantic changed its migratory pattern, which triggered the Faroe Islands to renegotiate its deal with Norway and the EU. Simultaneously, Iceland became a new major player in the mackerel fisheries. The mackerel regime consisting of Norway, EU and the Faroe Islands experienced a temporary breakdown during the conflict. This led to substantial overfishing. Due to differing interpretations of international conventions as well as the addition of a newcomer, the regime was not able to adapt well to the changing situation. Research on climate change was doubted by the states and was used strategically to legitimize resource claims. The fisheries regime needs stricter and more consistent rules regarding the inclusion of newcomers in fisheries, as well as how to handle dramatic changes in migratory patterns.