This paper addresses a systemic approach for the study of fishers' ecological knowledge in order to describe fishers' ways of knowing and dealing with complexity in ecosystems, and discusses how knowledge is generated through, e.g. apprenticeship, experiential knowledge, and testing of hypotheses. The description and analysis of fishers' ecological knowledge has been done using the "Structure-Dynamics-Functions" conceptual framework. Fishers identify 5-50 feeding interactions ("Structure"), recognize populations' dynamics over time, and, the impact of external factors (climate change, water quality and overfishing) ("Dynamics") and finally, acknowledge different values or services ("Functions") of the ecosystem (drinking water and fishing). Knowing about these three main aspects seems to be core knowledge embedded in fishers' ecological knowledge, which comprises systems thinking. Systems thinking is arguably part of fishers' professional skills and significant for sustainable natural resource management yet understanding ecosystem complexity is also a cognitive challenge.