Technical contribution Length–weight relationships of 26 fish species from the middle section of the Negro River (Tacuarembo´-Durazno, Uruguay) By F. Teixeira-de Mello 1 , I. Gonzalez-Bergonzoni 1,2 , F. Viana 2 and C. Saizar 2 1 Grupo de Ecologı´a y Rehabilitacio ´n de Sistemas Acua ´ticos, Departamento de Ecologı´a y Evolucio ´n, CURE-Facultad de Ciencias, Universidad de la Repu ´blica, Maldonado, Uruguay; 2 Departamento de Medio Ambiente, Laboratorio Tecnolo ´gico del Uruguay (LATU), Montevideo, Uruguay Summary This study provides data on the length–weight relationship (LWR) for 26 species of neotropical fishes. Specimens were collected between 2008 and 2009 from a dam-enclosed section of the middle Negro River (Uruguay). This study represents the first reference on length–weight relationships for 17 species, and also provides new maximum sizes for eight species. Introduction Length–weight relationships (LWR) can be a useful tool in the management of fisheries and environmental monitoring pro- grams in many ways, such as making weight estimations of individual fish based on length and vice versa and calculating a condition index to allow for morphology comparisons of populations from different regions (Petrakis and Stergiou, 1995). It also allows for the study of the ontogenetic allometric changes in fish growth (Teixeira-de Mello et al., 2006) and possible effects from parasites (Teixeira-de Mello and Eguren, 2008). Uruguay has a high diversity of freshwater fish, with some 230 species being reported (Nio´n et al., 2002). Previous LWR studies involved eight species from the Uruguay River (Teixeira-de Mello et al., 2009a) and 21 from Santa Lucia River (Teixeira-de Mello et al., 2009b). The aim of this work was to estimate LWR of 26 common fish species from the main course of the Negro River. Materials and methods The Negro River is the most important tributary to the lower Uruguay River (La Plata River Basin), and crosses Uruguay from NW to W. The two studied sampling sites are within a dam-enclosed area, between Rinco´n del Bonete and Baygorria dams, in the middle section of the river. Fishes used for length– weight relationship estimation were captured during four sampling campaigns between November 2008 and July 2009. Fishes were collected in the framework of a baseline monitor- ing program commissioned to the Technological Laboratory of Uruguay (LATU) by the Stora Enso company, under Collection Permit No. 424 ⁄ 2008, issued by the National Directorate for Aquatic Resources (DINARA). Sacrificed with an overdose of anesthesia (2-phenoxy-ethanol, 1 ml L )1 ) the fishes were identified using various taxonomic keys available for the various fish families present in the region then measured (to the nearest 0.1 cm standard length, S L ) and weighed (to the nearest 0.01 g total weight, W T ). Parameters for the equation W T = aS L b (Ricker, 1973) were estimated by linear regression, following the logarithmic transformation of the variables (Log 10 W T = Log 10 a+bLog 10 S L ). Spelling of all scientific names were checked against the FishBase (Froese and Pauly, 2008). Obvious outliers observed in the log–log curves were excluded from the regression. Voucher specimens were fixed in 10% formalin and 70% alcohol, and deposited in the Coleccio´n de Zoologı´a de Vertebrados of the Facultad de Ciencias (institutional code ZVC-P). Results and discussion A total of 1836 specimens belonging to 26 fish species corresponding to 15 families from eight orders (Siluriformes, Characiformes, Perciformes, Atheriniformes, Cyprinodonti- formes, Pleuronectiformes, Clupeiformes and Gymnotiformes) were used for calculation of the S L –W T relationships (Table 1). All regressions were highly significant (P < 0.0001). The data for Oligosarcus jenynsii, Diapoma terofali, Pime- lodella gracilis, Parapimelodus valenciennis, Corydoras palea- tus, Rhinelepis strigosa, and Hypostomus commersoni must be applied considering the size ranges analyzed in this study. The ranges used were narrow for these species, as major size ranges can present different LWR (Froese, 2006). Along the length range analyzed, 17 species presented isometric growth, whereas the other species showed allometric growth (three positive and six negative) (Table 1). Worth highlighting is the case of P. valenciennis, a species that showed a high Wt variability for the same SL, revealing many scraggy individuals. This results in a low coefficient of determination (r 2 ) (Table 1). When compared with the data of the Uruguay River (considering the same size range), b is significantly smaller (Table 2). Given seasonal abundance variation in the Uruguay River it is possible that P. valenciennis moves large distances for reproductive and ⁄ or feeding purposes, as the dam is an enclosed area, the fish may not be able to completely satisfy their food demands. Several LWR estimated for same species were available from the Uruguay (Teixeira-de Mello et al., 2009a) and Parana´ rivers (Benedito-Cecilio et al., 1997). The same size range was used in comparing the data from the Uruguay River. The two species compared in the three different systems (Negro, Uruguay and Parana´) failed to show any significant differences in the slope value (b) of the Uruguay and Negro rivers, but both were different from the species J. Appl. Ichthyol. 27 (2011), 1413–1415 Ó 2011 Blackwell Verlag, Berlin ISSN 0175–8659 Received: September 22, 2010 Accepted: November 11, 2010 doi: 10.1111/j.1439-0426.2011.01810.x U.S. Copyright Clearance Centre Code Statement: 0175–8659/2011/2706–1413$15.00/0 Applied Ichthyology Journal of