In Yucatan, Mexico, the sea cucumber fishery started informally in 2000, with a low-intermittent fishing effort. The first permits ware given in 2006 and in 2010 prices, buyers’ interest and the fishing effort substantially augmented, being followed by illegal fishing-fishers and traders, creating informal fishing-trade channels and severe social and biological concerns. This paper aims to give account of the emergence and dynamics of the informal institutions which seem to currently dominate this fishery. To what extent rules and regulations are not being respected and are affecting fish resources and coastal communities. We use qualitative and quantitative methodologies considering formal and informal interviews applied to key actors, a questioner applied to fishers and an estimation of the degree of compliance from three of the main management meaures. Socio-biological negative impacts were identified in Yucatan’s coastal communities and its fisheries. Foreign buyers and local middlemen exert high pressure on fishers to exceed the quota and catch the highest possible volumes enhancing the fisheries decline. This and the growing economic interest gave place to develop strong informal institutions supporting illegal fishing and informal trade. Low levels of compliance of the main management measures allowed illegal fishers from other states to enter in other fisheries (grouper, octopus and lobster). Social problems emerged or intensify: divers decompressions or death, alcoholism, drug consumption and prostitution. Women have been particularly affected, either because they have become widows or the arrival of young sexual workers is causing families breakdowns.