Length-weight relationships and condition factor of five ornamental fish species of the middle of the Negro River basin, Amazon, Brazil

Abstract

Length-weight relationships (LWR) were calculated for five freshwater ornamental fish species from the middle Negro River basin in the Brazilian Amazon. Samplings were carried out from October 2011 to February 2012, and the specimens were found close to stream margins and submersed tree trunks and captured with hand nets. This study provides the LWR and condition factor (Kn) parameters for <italic>Carnegiella strigata </italic>butterfly fish, <italic>Dekeyseria brachyura </italic>jaguar fish, <italic>Dicrossus maculatus </italic>chess fish, <italic>Paracheirodon axelrodi</italic> cardinal and <italic>Petitella georgiae </italic>drunk’s nose. Descriptive statistics and estimated LWR parameters, including coefficient of determination (r2) and 95% confidence limits for a and b. Values of a ranged from 0.005 to 0.014, b ranged from 2.555 to 3.418, and r2 ranged from 0.931 to 0.996. Every species of fish displayed allometric growth (b ≠ 3). No difference was observed between the Kn of the five fish species, indicating good body condition. The information gathered here could be helpful for population dynamics research as well as for developing conservation and sustainable management plans for ornamental fish populations in the middle Negro River basin, Amazonas, Brazil.

Key words
aquarium; biometrics; well-being; Amazon

INTRODUCTION

Length-weight relationship (LWR) and condition factor relative (Kn) are regular tools used in fisheries research due to their wide range of applications in population dynamics and fish stock assessments (Froese 2006, Santos et al. 2012, Cunha et al. 2014, Lemos et al. 2015, Benacon et al. 2015). LWR is useful, among other things, for defining growth patterns, condition factors and the nutritional and reproductive state of the fish (Le Cren 1951).

The Negro River basin, located in central Amazonia, has few studies that provide information regarding LWR parameters and body indices of the ichthyofauna (Teixeira de Mello et al. 2009, 2011, Santos et al. 2012, Lemos et al. 2015) when compared to the other basins in central Amazonia. This basin is characterized by black water and a branching floodplain system of fluvial archipelagos, flooded forests, and small streams that host more than 1000 fish species (Beltrão et al. 2019, Chao 2001), many of which are exploited by ornamental fishing (Anjos et al. 2009). According to Yamamoto et al. (2024), the species most utility in ornamental fish are cardinal Paracheiron axeroldi (Schultz, 1956), Marbled hatchetfish Carnegiella strigata (Günther, 1864), Petitella georgiae (Géry & Boutière, 1964), Dirossus maculatus (Steindachner, 1875) and Dekeyseria brachyura (Kner, 1854).

In this sense, studies involving LWR are essential to evaluate population biology, such as ornamental fish species that have commercial value and are frequently removed from the environment. The present study aims to increase scientific information on ornamental fish in the Amazon region to assist the conservation and management of this natural resource that makes up the Amazon biodiversity. This paper reports LWR and Kn of five freshwater ornamental fish species from the middle Negro River basin, Amazonas, Brazil.

MATERIALS AND METHODS

Specimens were captured of the middle Negro River basin in streams of the Mariuá Archipelago, near the Daracuá community (Figure 1), Amazonas, Brazil. Three samplings were carried out during the dry season between October 2011 and February 2012, and the specimens were found close to stream margins and submerged tree trunks. All individuals were captured with hand nets (50 cm × 50 cm metal frame attached to a wooden cable, with mesh size < 1 mm), separated by species and identified by consulting standard fish books and taxonomic keys (Gery 1977, Axelrod et al. 1997, Fisch-Muller 2003, Weitzman & Palmer 2003). Each fish was weighed using an analytical balance with 0.001g precision and measured using a digital pachymeter with 0.01cm precision for the length data (Figure 2).

Figure 1
Collection location for ornamental fish located in the Daracuá Community, Mariuá Archipelago, middle Rio Negro, Amazonas, Brazil.

Figure 2
Summary of the collection and biometric determination of five species of ornamental fish from the middle Rio Negro, Amazonas, Brazil.

The LWR was estimated using the expression W = a.Lb, where W = total weight (g), a = intercept, b = allometric coefficient and L = total length (cm). The parameters a and b were estimated by linear regression after logarithmic transformation of weight and length data (log W = log a + b log L). Additionally, 95% confidence limits (CL) of a and b were estimated. Log-log plots of total length and weight were performed to identify and exclude outliers from the regression (Froese 2006). Student’s t-tests were conducted to determine if the b value was significantly different from isometric growth (b = 3, p > 0.05) (Oliveira et al. 2015). For the relative condition factor (Kn), the formula Kn= W/aLb was used. The Shapiro-Wilk normality test was applied to assess the normality of the data. The homoscedasticity test was also applied. The data’s normality was assessed, and then the Analysis of Variance (ANOVA) test was used, followed by the Tukey test, which assumed 95% reliability. The program used was R®.

RESULTS

A total of 1709 fish specimens were captured, representing five species belonging to four families. The descriptive statistics and estimated LWR parameters such as a and b and their range in 95% confidence limits and coefficient of determination (r2) are presented in Table I. The r2 ranged from 0.931 to 0.996, values of a varied from 0.005 to 0.014 and b ranged from 2.555 to 3.418. All fish species presented allometric growth (b ≠ 3). Kn demonstrates that the five ornamental fish species presented ideal biometric conditions, and there were no statistical differences between the species (Figure 3).

Table I
Descriptive statistics and length-weight relationship parameters of five ornamental fish species from the middle Negro River basin, Amazonas, Brazil.

Family	Species	.n	TL range (cm)	WT range (g)	a (CL 95% a)	b (CL 95% b)	Growth type	r2
Characidae	Paracheirodon axelrodi (Schultz 1956) cardinal	778	1.5–2.6	0.04–0.15	0.014 (0.013–0.015)	2.555 (2.535–2.575)**	Negative allometry	0.988
Characidae	Petitella georgiae (Géry & Boutière 1964) drunk’s nose	370	2.7–4.3	0.14–0.73	0.005 (0.005–0.006)	3.405 (3.370–3.440)**	Positive allometry	0.990
Gasteropelecidae	Carnegiella strigata (Günther 1864) butterfly fish	336	2.7–4.1	0.20–0.79	0.007 (0.006–0.008)	3.418 (3.293–3.544)**	Positive allometry	0.931
Loricariidae	Dekeyseria brachyura (Kner 1854) jaguar fish	70	3.4–14.6	0.78–27.40	0.010 (0.008–0.012)	2.951 (2.907–2.995)*	Negative allometry	0.996
Cichlidae	Dicrossus maculatus (Steindachner 1875) chess fish	202	1.5–3.6	0.04–0.48	0.012 (0.010–0.014)	2.910 (2.876–2.944)**	Negative allometry	0.993

• n, sample size; TL, total length; WT, weight; range, maximum and minimum values; a and b, parameters of the equation; CL, confidence limit 95%; r 2, coefficient of determination. Significant differences (t-test): *p< 0.05; **p< 0.001.

Figure 3
Relative condition factor (Kn) of five species of ornamental fish from the middle Rio Negro, Amazonas, Brazil.

DISCUSSION

Of the five species of ornamental fish evaluated, all are commercially important and generate income for fishermen and riverside dwellers of the middle Rio Negro (Ladislau et al. 2020, 2021, Ribeiro et al. 2021). Despite their commercial importance, none of the species is at risk or threatened with extinction. Another factor to consider is the geographic distribution pattern, in which the five species are not classified as endemic.

LWR is typically used to calculate the biomass of fish populations (Froese 2006, Fricke et al. 2025), estimate fish weight from length observations (Teixeira de Mello et al. 2011, Matos et al. 2019, Fricke et al. 2025) or study the ontogenetic allometric changes in fish growth (Teixeira de Mello et al. 2006). In this study, b values estimated for the five fish species are within the expected range of 2.5 to 3.5 (Froese 2006). The species D. brachyura, D. maculatus and P. axelrodi presented negative allometric growth (b< 3), indicating a relatively higher growth rate in length than in weight, expected for fish species with elongated body shape (Oliveira et al. 2018). This is contrary to the result found in C. strigata and P. georgiae with positive allometric growth (b> 3), meaning the species becomes relatively stouter or deeper-bodied as it increases in length (Riedel et al. 2007).

Regarding the Bayesian LWR predictions available in FishBase (Froese & Pauly 2019), only P. georgiae had a and b values outside the 95% confidence limits shown in the database, as well as P. axelrodi that showed lower b values and C. strigata with higher b value. This difference may occur because the Bayesian LWR estimates were based on data of similar body shape taxon (Froese et al. 2014). Furthermore, many factors can influence these values, including sex, life stage, gonadal development, food availability and habitat characteristics (Koushlesh et al. 2018).

For Kn, the five species of ornamental fish studied presented a value of 1.0, with no difference between them. Santos et al. (2012) state that when Kn is equal to 1.0, the fish populations indicate that they are in perfect biometric conditions. In this sense, the fish species in the present study present ideal health conditions, considering that they are species from a natural environment.

CONCLUSIONS

The results obtained here may be helpful in studies of population dynamics and in formulating strategies for sustainable management and conservation of ornamental fish populations from the middle Negro River basin. Therefore, due to commercial exploitation and climate change, overexploitation, mortality and population reduction may occur. In this sense, the biometric information proposed in this work is a reference for future comparative studies with these species, assisting in management and conservation.

Acknowledgements

The authors thank Fundação de Amparo à Pesquisa do Estado do Amazonas (FAPEAM) for financial support. ATO (process 315713/2020-8) is Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) research fellow.

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Publication Dates

• Publication in this collection
  15 Sept 2025
• Date of issue
  2025

History

• Received
  07 Jan 2025
• Accepted
  04 May 2025