GOMES, Luiz Carlos; MIRANDA, Leandro Esteban; AGOSTINHO, Angelo Antonio. Fishery yield relative to chlorophyll a in reservoirs of the Upper Paraná River, Brazil. Fisheries Research, New York, v.55, no.1-3, p.335-340, 2002.Fishery yields in reservoirs of the Upper Paraná River basin are small when compared to those in other parts of the world. Several hypotheses have been formulated to explain the small yields, but low primary production has not been seriously considered as a possibility. In a sample of seven reservoirs, yield was directly related to fishing effort and chlorophyll a concentration; thus, enhancement of these factors may boost yield. However, primary production is difficult to augment in large reservoirs, plus current levels of effort seem to be nearing maximum sustainable yields in some reservoirs, and therefore increased fishing effort is not likely to boost yields to levels recorded in other geographical regions. Low primary production in reservoirs of the Upper Paraná River basin appears to be a result of interactions between physical characteristics of the reservoirs and climatic conditions of the region.