Summary The length‐weight relationships (LWRs) are described for 29 fish species in the region of the Tucuruí Reservoir, located in the Tocantins/Araguaia Basin, northern Brazil. Fishes in fourteen sites were sampled seasonally for 10 years, from 2001 to 2010. Significant differences between sexes were found for two species, while 27 species had similar LWRs for males and females. The relationship of standard length and total length is presented for all species as well as the minimum and average size of reproductive females and seasonality of occurrence. No information on LWRs of these 29 species was heretofore available in FishBase.

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The continuous interference of human activities in continental aquatic systems results in direct and indirect impacts, with consequences for water quality, aquatic biota and the dynamics of water resources. In this context, the Tucuruí Hydroelectric Power Plant (HPP) acquired relatively importance, as it was the first large dam carried out in the Amazon, in the 1980s, presenting important technical/economic results to justify the environmental costs associated with the project. The objective of this study was to apply a new methodology to represent through maps georeferencing biological information and evaluate the impact of the dam on the following fish species, which have different physiologies and taxonomies, in addition to not having much biological information about the species: Botinho (Hassar wilderi, Doradidae), Mandubé (Ageneiosus ucayalensis, Auchenipteridae), Sardinha Papuda (Triportheus trifurcatus, Characidae) and Uéua Cachorrinho (Acestrorhynchus falcirostris, Acestrorhynchidae). The condition factor related to the relationship between the individual's weight and body length, which derives from an expansion of the cubic law of living beings, being very important in ecological studies, even though they depend on simple to the most complex scientific methods, to estimate this bodily condition. Furthermore, this biological parameter influenced by environmental conditions and characteristics, due to food availability and ecological relationships in the environment. This information can support an adequate fisheries management, as there is an integration of fisheries biology and the characteristics of the environment in which they live. Thus, the methodology applied in this study was successful in its application proposal.