Fisheries have long been agents of ecological change, often altering ocean environments to their own disadvantage. The social mechanisms of formal or informal fisheries management can, in principle, control fishing so that it does not overshoot sustainable limits and collapse. In practice, such control proves difficult to achieve. Fisheries are complex and dynamic systems of interaction between humans and their biophysical environment. We propose a simple conceptual model for a fishery system. This model is not definitive or complete, but it helps to integrate discussion of selected fisheries issues, such as the unintended consequences of management change, or other systems implications of findings from more narrowly focused research. Specific propositions from the model, and directions for future research, are illustrated using examples from northwest Atlantic fishing communities.