Influence of a large dam and importance of an undammed tributary on the reproductive ecology of the threatened fish matrinxã Brycon orthotaenia Günther, 1864 (Characiformes: Bryconidae) in southeastern Brazil

A total of 312 adults of matrinxã were captured bimonthly, between 2011-2012, in two sites of the rio São Francisco: immediately downstream of the Três Marias Dam (site 1) and in the confluence with the rio Abaeté (site 2). Specimens of Brycon orthotaenia from site 2 presented higher values of total length (32.24 ± 5.70 cm for females and 26.64 ± 3.79 cm for males) and body weight (506.66 ± 332.17 g for females and 267.36 ± 145.84 g for males), when compared with the site 1. In site 2 was registered the highest GSI means (9.97 ± 2.96 for females and 0.93 ± 0.52 for males), compared with the site 1 means. Reproduction period occurred from October to February. Unlike site 2, no spawned females or spent males were captured in site 1. Significant differences were found between females and males in site 2 (χ2 = 26.84, df = 1, P < 0.05). Physical-chemical parameters of the water presented higher values in site 2. The canonical correlation test indicated that the reproduction of males is less susceptible to environmental factors than that of females. These results show that B. orthotaenia finds favourable conditions to reproduction just in site 2