Fish movement patterns in a Neotropical free-flowing tributary located downstream from a large dam Ruanny Casarim A,D , Ivo Gavia ˜o Prado A,B,C , Raquel Coelho Loures A,B and Paulo Santos Pompeu A A Laborato ´ rio de Ecologia de Peixes. Setor de Ecologia, Departamento de Biologia – Universidade Federal de Lavras. Campus Universita ´rio, Lavras, Minas Gerais, 37200-000, Brazil. B Compainha energe ´tica de Minas Gerais (Cemig) Gerac ¸a ˜o e Transmissa ˜o, Programa Peixe Vivo, Avenida Barbacena 1200, Belo Horizonte, Minas Gerais, 30161-970, Brazil. C Pisces – Consultoria e Servic ¸os Ambientais, Rua Comandante Vilas Boas 35, Lavras, Minas Gerais, 37200-000, Brazil. D Corresponding author. Email: rucasarim@gmail.com Abstract. In dammed rivers, the conservation of free-flowing tributaries is considered an important strategy to mitigate the negative effects of the dam on fish communities. In this study, we evaluated the importance of a free-flowing tributary of a dammed river as a reproductive migration route. From August 2012 to July 2013, hydroacoustics data were collected alongside active fish sampling using cast nets and ichthyoplankton nets in the Sa ˜o Francisco and Abaete ´ rivers. Cast net sampling captured 738 individuals and hydroacoustics detected 42 196 fishes. In almost all samples and sampling sites, preferential movements to the Abaete ´ River and the main Sa ˜o Francisco River were observed. The increase in the flow rate of the Abaete ´ coincided with a greater incidence of fish movements to upstream areas of the tributary. A higher density and proportion of larger fishes, as well as a higher density of ichthyoplankton, were observed in the Abaete ´ River compared with the main river. Because this tributary is an alternative route for migratory fishes, its protection is essential for fish conservation and therefore maintenance of local fisheries Additional keywords: horizontal acoustic scanning system, hydroacoustics, migratory fishes. Received 7 October 2016, accepted 24 March 2018, published online 17 July 2018 Introduction In the Neotropics, most economically valuable fish species are migratory (Ferraz de Lima 1986; Bittencourt and Cox-Fernandes 1990; Carolsfeld et al. 2003; Resende 2003). In this region, fish migrate upstream to spawn, returning to downstream feeding sites after spawning. This phenomenon normally occurs during the rainy season, when the river water level rises (Agostinho et al. 1992, 2003; Lowe-McConnell 1999; Carolsfeld et al. 2003; Godinho et al. 2017). However, one of the main threats to migratory fishes is the construction of dams for hydropower purposes (Godinho and Godinho 2003; Agostinho et al. 2007; Pelicice et al. 2015). These barriers block free access to feed and spawning areas, and obstruct the downstream dispersion of eggs and larvae (Agostinho et al. 1992, 2007; Godinho 1993; Pompeu et al. 2011; Pelicice et al. 2015). The development and implementation of long-term manage- ment strategies for migratory fish populations in South America is lacking (Pompeu et al. 2012). In this context, the conservation of free-flowing tributaries is an important strategy to attenuate negative effects suffered by ichthyofauna when the main river is dammed (Antonio et al. 2007; Moyle and Mount 2007). Where the natural hydrological regime is maintained, tributaries of larger rivers may be important for the conservation of migratory species (Agostinho et al. 2004; Pracheil et al. 2009; Suzuki et al. 2009). This is because tributaries may permit the conservation of fish populations that complete their life cycles in the tributary, or they may provide an alternative route for species that previously used the main river (Antonio et al. 2007). In Brazil, some tributaries have been considered fundamental areas for spawning in systems fragmented by dams (Agostinho et al. 2004; Baumgartner et al. 2004; Nakatani et al. 2004; Suzuki et al. 2004, 2009). In the Sa ˜o Francisco River basin, the Abaete ´ River, located downstream from the Tre ˆs Marias Dam, is known to be an important spawning area for long-distance migratory species, including the dourado Salminus franciscanus (Lima & Britski 2007), curimba Prochilodus argenteus Spix & Agassiz 1829 and surubim Pseudoplastysoma corrucans (Spix & Agassiz 1829) (Sato et al. 1987, 2005; Godinho and Godinho 2003; Sampaio and Lo ´pez 2003; Godinho and Kynard 2006; Godinho et al. 2007, 2017; Agostinho et al. 2007). However, fish movement patterns and the use of the Abaete ´ River by migratory species in relation to the Sa ˜o Francisco River have never been quantified. CSIRO PUBLISHING Marine and Freshwater Research https://doi.org/10.1071/MF17216 Journal compilation Ó CSIRO 2018 www.publish.csiro.au/journals/mfr