This papers aims at understanding the strategies of fishers operating two fisheries which are biologically independent but economically interrelated. The case studied is that of the fishers who dredge scallops, queens and clams in the bay of Brest during winter, and harvest the Laminaria digitata field in the open sea, north-west of Brest in summer. Taking this peculiarity into account, the paper makes a distinction between different strategies, which enable fishers to adapt to a changing environment (abundance variation but also economic and institutional shifts) during recent years. Fishers have to choose among different activities, which bring about various investments. Vice versa, their behaviour has feedback effects on the global institutional environment, and especially on the regulation system of input control. The study is based upon a significant range of individual data, about fishing schedule, productions, and prices.