J Appl Ichthyol. 2018;1–9. wileyonlinelibrary.com/journal/jai | 1 © 2018 Blackwell Verlag GmbH 1 | INTRODUCTION The hydrologic regime is an important structural force in river/ floodplain systems (Lowe‐McConnel, 1999). The seasonality be‐ tween flood and dry periods may promote or restrict access to new habitats, according to the Flood Pulse Concept (Junk, Bayley, & Sparks, 1989). During dry periods, fish are mainly restricted to the river channel, but during floods, when river water achieves high levels extending to adjacent areas of the river, fish are able to reach new habitats (Lowe‐McConnel, 1999) that might be more suitable for new food supplies, spawning, sheltering for larvae and developing juveniles (Pompeu & Godinho, 2006; Vazzoler, Suzuki, Received: 21 March 2018 | Revised: 20 June 2018 | Accepted: 25 June 2018 DOI: 10.1111/jai.13770 ORIGINAL ARTICLE Population parameters and reproduction of the piranha Serrasalmus marginatus in the Negro river, Pantanal, Brazil Wagner Vicentin 1 | Karina Keyla Tondato 2 | Fabiane Silva Ferreira 1 | Fábio Edir dos Santos Costa 1 | Yzel Rondon Súarez 3 1 Universidade Estadual de Mato Grosso do Sul, Rod. Dourados‐Itahum, Dourados, Mato Grosso do Sul, Brasil 2 Laboratório de Biodiversidade, Núcleo de Pesquisa em Produção e Conservação do Cerrado, Universidade Federal de Mato Grosso, Rondonópolis, Mato Grosso, Brasil 3 Laboratório de Ecologia, Centro de Estudos em Recursos Naturais, Universidade Estadual de Mato Grosso do Sul, Dourados, Mato Grosso do Sul, Brasil Correspondence Wagner Vicentin, Universidade Estadual de Mato Grosso do Sul, Rod. Dourados‐Itahum, 79804‐970, Dourados, Mato Grosso do Sul, Brasil. Email: wagnervicentin.bio@gmail.com Funding information Fundação de Apoio ao Desenvolvimento do Ensino, Ciência e Tecnologia do Estado de Mato Grosso do Sul (FUNDECT); Centro de Pesquisas do Pantanal/ Ministério da Ciência, Tecnologia, Inovações e Tecnologia (CPP/MCTI); Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES); Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Abstract This paper aimed to define the reproductive period and population parameters of Serrasalmus marginatus relative to local environmental features, such as day length, rainfall and mean river level. The study site was a floodplain in the Negro river, Pantanal, Brazil, and samples were collected bimonthly using gill nets and cast nets with meshes from 1.5 to 8 cm between adjacent knots. The reproduction period, as determined by gonadosomatic index (GSI) and percentage of gonad stages, varied significantly along the year (F 4,116 = 77.5; p < 0.01), but it was well defined from October to December. Reproduction period was positively correlated with rainfall (rs = 0.97; p < 0.01) and photoperiod (rs = 0.92; p = 0.02), but not with the rise of river level (rs = 0.10; p = 0.86). Rainfall and photoperiod may act as predictive factors, pro‐ viding cues to fish to begin reproduction in order to find the best conditions for off‐ spring in the following months, during the flooding season, when the river level reaches its peak and extends to the plain. We have herein reported the first popula‐ tion parameters described for this species in the Brazilian Pantanal, including growth rate (k = 0.53/year), asymptotic length (L∞ = 32.74), lifespan ( A 0.95 = 5.65) and mortal‐ ity (M = 1.11), showing that this population has better growth performance when compared to data reported for this same species in the Paraná river since the popula‐ tion in the Negro river is characterized by lower growth and mortality rates, and S. marginatus achieved longer lengths and lifespan. This latter parameter was compati‐ ble with six cohorts estimated by Electronic Length Frequency Analysis (ELEFAN). The size at first maturity (L50 = 18.26 cm A50 = 1.56 years) was also larger and later than the population in the Paraná river, likely a consequence of the the lower mortal‐ ity and the maintenance of larger specimens in the Negro river. KEYWORDS ELEFAN, floodplain, f, gonadosomatic index, length–frequency data, size at first maturity