Tambaqui, Colossoma macropomum (Cuvier 1818), is one of the largest and most commercially important freshwater fishes in South America, attaining a body size of up to 1 m length and a mass of ≥ 30 kg. We hypothesize that the mean length of the captured tambaqui was impacted by the fishery. The present study used GIS information to map the Manacapuru Lake system and record tambaqui caught by fishermen from February 2007 to January 2008. Fork lengths of fish were measured and compared to the legal size limit of 55 cm, corresponding to the reproductive age as set by IBAMA (Brazilian Institute of the Environment and Natural Renewable Resources). Only 1% of 1271 fish measured were at or above this limit. Spatial distribution of the catch within this floodplain lake system varied, depending on the stage of the hydrologic cycle, but the greatest number of fish was caught within the Piranha Sustainable Development Reserve where fishing is prohibited by law. The results of this study indicate an acute need for enforcement of the law to avoid collapse of the fishery.