Informal fisheries (including unreported legal, illegal and discarded catch) have significant benefits for communities that official statistics often ignore, yet there is an associated loss in potential economic impacts compared to catch that is processed and marketed formally. This represents a significant global issue, as recent estimates suggest that about half of global fisheries catch is informal. We develop a value of information framework to evaluate monitoring investment policies given uncertainty regarding the size of the informal sector, associated economic losses, and the effectiveness of management, and apply it to an example dataset for Mexico. Results provide advice on optimal investment in fisheries catch formalization to maximize economic impacts. This analysis complements improvements in fisheries policies themselves, also necessary to achieve ecologically and economically sustainable fisheries. Providing estimates of foregone economic benefits due to poor governance can help elicit positive actions, while revealing the costs of inaction.