The Iroise Sea, at the northwest end of Brittany in France, hosts one of the largest kelp forests in Europe. Kelp is large brown marine seaweed that has been used for centuries by small-scale coastal communities for various agricultural and industrial purposes. For the past few decades, the kelp fisheries have been managed through a co-governance system by kelp harvesters in collaboration with processing industries, fisheries administration and scientists, to provide a sustainable supply to local alginate factories. The recent creation of a marine protected area in the Iroise Sea has resulted in the emergence of new actors and new concerns have arisen, modifying thus the system-to-be-governed and its governance. This case study is an example of how socio-ecological system and their governance can rapidly evolve according to changing circumstances. From the governability perspective, we examine how such change affects the viability of small-scale kelp harvesters.