The aim of the current study was to address length‐weight relationships (LWRs) in seven ornamental fish species from the Unini River basin—the main tributary of lower Negro River (Brazilian Amazon). Specimens were seasonally sampled by using hand nets and beach seines (mesh size < 1 mm) in five different occasions (August 2009—December 2010). Preserved specimens (initially kept in 10% formalin and transferred to 70% ethanol 10 days later) were measured and weighted in laboratory. The LWR was calculated based on a log‐transformed linear regression. The maximum standard length of each species was different from the ones previously reported at Fishbase. In addition, b values were different from the ones reported for the four species whose data were available in the literature.

We explored patterns of phenotypic variation in Hemigrammus coeruleus from the Unini River basin, a blackwater river in the Brazilian Amazon. Geometric morphometrics was used to evaluate variation in body shape among populations from four tributaries (UN2-UN5). We found no evidence for sexual dimorphism in body size and shape. However, morphological differences among populations were detected as the analyses recovered significant groups corresponding to each sub-basin, with some overlap among them. The populations from UN2, UN3 and UN5 had more elongate bodies than fish from UN4. The most morphologically divergent population belonged to UN4, the tributary with the most divergent environmental conditions and the only one with seasonally-muddy waters. The morphological variation found among these populations is likely due to phenotypic plasticity or local adaptation, arising as a product of divergent ecological selection pressures among sub-basins. This work constitutes one of the first to employ a population-level geometric morphometric approach to assess phenotypic variation in Amazonian fishes. This method was able to distinguish subtle differences in body morphology, and its use with additional species can bring novel perspectives on the evaluation of general patterns of phenotypic differentiation in the Amazon.Keywords: Geometric morphometrics, Morphology, Ornamental fish, Phenotypic plasticity, Populations.Neste estudo foram explorados os padrões de variação fenotípica em Hemigrammus coeruleus da bacia do rio Unini, um rio de água preta na Amazônia brasileira. Métodos de morfometria geométrica foram aplicados para avaliar as variações na forma do corpo entre populações provenientes de quatro tributários (UN2-UN5). Os resultados mostraram ausência de dimorfismo sexual relacionado ao tamanho e formato do corpo. Entretanto, diferenças morfológicas entre populações foram detectadas, uma vez que as análises apontaram agrupamentos correspondendo a cada sub-bacia, com certo grau de sobreposição entre populações. As populações dos rios Preto, Arara e Pauini (UN2, UN3 e UN5) apresentaram formato de corpo mais alongado do que a amostra do igarapé Solimõezinho (UN4). A população mais divergente morfologicamente pertenceu ao igarapé Solimõezinho (UN4), o tributário que apresentou a condição ambiental mais divergente e o único com águas sazonalmente barrentas. A variação morfológica encontrada nessas populações de H. coeruleus é provavelmente devido a plasticidade fenotípica ou adaptação local por seleção induzida por diferentes pressões seletivas entre as sub-bacias. Este estudo constitui a primeira contribuição científica usando métodos de morfometria geométrica para avaliar a variação fenotípica entre populações em peixes amazônicos. Esse método foi capaz de distinguir diferenças sutis na morfologia e sua replicação em outras espécies amazônicas pode trazer novas perspectivas na avaliação de padrões gerais de diferenciação fenotípica na região.

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Length at first maturity (L 50) is an important tool for the management and conservation of fish populations. Traditional approaches based on macroscopic and microscopic maturity staging exhibit high accuracy and precision, while alternative approaches (e.g., I g-based staging, stanza changing point) are less resource-demanding. Herein, we compare four approaches to estimate L 50 in a population of the heptapterid Rhamdioglanis transfasciatus from Atlantic Forest streams. Fish were sampled monthly during a year by using electrofishing. We measured the length (cm), mass (g), and gonad mass (g) of each specimen, then classified their maturity status macroscopically and microscopically. Alternative approaches were strongly discordant from traditional ones. Logistic curves considering mature individuals as those displaying at least 1% of the maximum I g in the sample greatly underestimated L 50 for females and overestimated L 50 for males. The stanza changing point derived from the polyphasic growth model underestimated L 50 in both cases. Despite the increasing development of less onerous approaches, it seems that they are not suitable for all fish populations and the requirements to use such approaches demand further investigation.

Movement is a key factor in spatial dynamics and the size of fish populations. It is one of the main mechanisms for maintaining viable populations and also accounts for the dispersion, colonization and performance of individuals life stages. The currently refuted restricted movement paradigm predicted that freshwater fish would not move. However, studies show that in addition to moving on a short and large scale, the movement patterns of these fish have, in general, reproductive, food, search for shelter, colonization or exploitation motivation. Studies on the movement of Neotropical fish are concentrated on the large migrators from large basins, such as Amazonas and Paraná. For streams, where small species predominate, little is known about the movement and its motivations. In this review, we show that studies on the movement of fish in neotropical streams are still very scarce. We propose a protocol for obtaining data and analytical tools for determining fish movement in streams. We highlight three tools for that: (i) analysis of the spatio-temporal structure of the size of the fish, (ii) mark-recapture experiments and (iii) molecular analyzes. We conclude that, given the limitation of each analysis, the use of the three tools must be carried out jointly.

We provide length‐weight relationships (LWR) estimates for 45 species from the upper Tocantins River basin. Specimens were sampled by a standardized set of gill nets varying from 15 mm to 150 mm mesh size for 15 years (from 1995 to 2010) of field works. Field expeditions were carried out in three temporal bins with bimonthly samplings, (i) December 1995 to February 2000, (ii) February 2001 to December 2002, and (iii) February 2009 to December 2010, totalizing 46 field works. For all species, standard length (SL; 0.1 cm precision) and total weight (Wt; 0.01 g precision) were determined using a scale and a digital balance, respectively. From these 45 species, 28 have no previous LRW information in Fishbase, and we also provide new maximum standard length for 16 species. When data for at least 10 male or female individuals were available, we also provided the LWR information separately by gender.