Fish harvesting operations on 3 Ontario lakes are descr. Catch per unit of effort statistics of these fisheries are used to estimate the populations of fish and also to estimate the efficiency with which the fishing effort was conducted against the various spp. taken. Daily catch per unit of effort is plotted against cumulative catch and the figure produced is a straight line with a characteristic grouping of points. For spp. where this configuration appears, the fishing effort is considered to be efficiently applied. It is revealed that small-mouth black bass (Micropterus dolomieu) and bullheads (Ameiurus nebulosus) are associated, and are vulnerable to an intense trap-net fishery, whereas rock bass (Ambloplites rupestris) and sunfish (Lepomis gibbosus) show different fishing characteristics and are not vulnerable to such a fishery.