Port-Cros, a national marine park, and its neighbouring island of Porquerolles are two French Mediterranean islands both managed by the Port-Cros National Park. Numerous visitors are attracted by their preserved environmental heritage. Thus, it is paramount for the Park Board to consider the consequences of the massive influx of sailors and tourists. A precise study of frequentation was started in 2001 with Geomer, an academic geography laboratory. This first stage was the beginning of a fruitful collaboration between scientists and the park managers; it drove the latter to want to continue the study in order to develop a tourist frequentation observatory. This project progressively gathered additional actors, i.e. sector chiefs and field agents of the park and local populations. Bountiles, a helping tool for decision-making, was jointly constructed and permitted appropriation of a part of it by any of the actors to their own profile and expectations. Now, this local context-adapted observatory is operational, and its implementation has been accompanied by the scientists, according to the park request.