Length–weight relationships of eight fish species from the lower section of the Uruguay River (Río Negro, Uruguay)
Author’s address: F. Teixeira de Mello, Grupo de Investigación en Ecotoxicología y, Química Ambiental, Facultad de Ciencias, Universidad de la, República, Iguá 4225, CP 11400, Montevideo, Uruguay.
This study reports length–weight relationships for Apareiodon affinis, Leporinus obtusidens, Cynopotamus argenteus, Loricariichthys melanocheilus, Parapimelodus valenciennis, Iheringichthys labrosus, Rhinodoras dorbignyi and Pachyurus bonariensis. Specimens were collected between 2006 and 2007 from the lower section of the Uruguay River. The study represents the first reference on weight–length relationships for three species and also provides the new maximum size for three species.
Length–weight relationship parameters (a and b) are useful in fisheries science in many ways: to estimate weight of individual fish from its length, to calculate condition indices, to compare life history and morphology of populations belonging to different regions (Petrakis and Stergiou, 1995) and to study ontogenetic allometric changes (Teixeira de Mello et al., 2006).
The aim of the study was to provide information on the length–weight relationships of eight commonly found fish species in three sites from the lower sector of the Uruguay River.
Materials and methods
Fishes used for the calculation of weight–length relationships were captured on four sampling dates between May 2006 and April 2007 in three sites from the lower section of Uruguay River. The sampling sites were settled considering a pulp mill location: one up-stream, Nuevo Berlín; other near the factory, Yaguarete; and the last one down-stream to the pulp mill, Las Cañas.
Fresh fishes were measured (to the nearest 0.1 cm standard length, SL) and weighed (to the nearest 0.01g total weight, WT). The parameters for the equation WT = aSLb (Ricker, 1973) were estimated by linear regression, after a logarithmic transformation of the variables. Voucher specimens were fixed in formalin (10%) and deposited in the Vertebrates Collection of the Facultad de Ciencias (institutional code ZVC-P). In order to check if fish growth was statistically different from isometric growth, a t-test (H0: b = 3), with α = 0.05 was performed; the same test was used to compare the slopes of this work with other data from the literature (H0: b1 = b2, α = 0.05; Ricker, 1973, 1975).
Results and discussion
Totals of 2313 specimens belonging to eight fish species (Table 1) corresponding to seven families (three Characiformes and four Siluriformes) were used for calculation of the SL–WT relationships (Table 1). All regressions were highly significant (P < 0.0001). The lowest r2 values registered were 0.88 for Leporinus obtusidens and 0.95 for Apareiodn affinis, while for the remaining species the value was 0.99.
Parameters of weight–length relationships W = aLb (after logarithmic transformation of variables) of eight species studied from lower Uruguay River
|Parodontidae||Apareiodon affinis||7.1||12.2||70||−1.83 ± 0.16||3.06 ± 0.17||0.95a|
|Anostomidae||Leporinus obtusidens||18.0||27.5||28||−1.44 ± 0.54||2.84 ± 0.40||0.88a|
|Characidae||Cynopotamus argenteus||11.3||24.0||7||−2.17 ± 0.25||3.28 ± 0.20||0.99b|
|Loricariidae||Loricariichthys melanocheilus||7.3||43.0||266||−2.69 ± 0.05||3.38 ± 0.04||0.99b|
|Pimelodidae||Parapimelodus valenciennis||6.9||23.0||183||−2.01 ± 0.05||3.13 ± 0.04||0.99b|
|Pimelodidae||Iheringichthys labrosus||6.4||24.5||1296||−1.93 ± 0.02||3.08 ± 0.02||0.99b|
|Doradidae||Rhinodoras dorbignyi||17.0||22.5||5||−0.89 ± 0.36||2.29 ± 0.28||0.99c|
|Sciaenidae||Pachyurus bonariensis||5.0||18.3||458||−1.82 ± 0.03||3.05 ± 0.03||0.99b|
For five of the considered species, SL–WT estimates were available in Benedito-Cecilio et al. (1997), and for three of them information was also available in FishBase (Froese and Pauly, 2000; Table 2). These previous estimates belong to individuals from Itaipu Reservoir, Paraná, Brazil, a warmer region (Benedito-Cecilio et al., 1997). We found statistical differences for parameters b of three species when compared to previous data (Benedito-Cecilio et al., 1997; Tables 1 and 2). Local environmental conditions, sex or developmental state of the fish considered (Le Cren, 1951), and/or the differences in the range of SL used in the SL–WT relationships (Froese, 2006), could be the cause of finding differences in the slopes of the SL–WT estimates of the same species in different regions. For Leporinus obtusidens and Pachyurus bonariensis no statistical differences were detected between the slopes (b), even when different size ranges were used as in the case of L. obtusidens (Tables 1 and 2).
Parameters of weight (g)–length (cm) relationships W = aLb (after logarithmic transformation of variables) from Benedito-Cecilio et al. (1997), Itaipu Reservoir, Paraná, Brazil
|Parodontidae||Apareiodon affinis||7.4||13.5||0.0720||2.42 ± 0.18*|
|Anostomidae||Leporinus obtusidens||7.5||40.0||0.0171||3.12 ± 0.04|
|Pimelodidae||Iherigichthys labrosus||7.3||26.2||0.0087||3.23 ± 0.0*|
|Doradidae||Rhinodoras dorbignyi||7.1||19.5||0.0203||3.06 ± 0.12*|
|Sciaenidae||Pachyurus bonariensis||12||22.6||0.0304||2.82 ± 0.37|
Three different growth types were exhibited by the studied species. Apareiodon affinis and Leporinus obtusidens showed isometric growth, thus these species grow in length and biomass in equal shares. Only one species, Rhinodoras dorbignyi, showed allometric negative growth, impling allocation of more energy to axial growth rather than biomass. These results need to be corroborated because the number of fish used for parameter estimation was very low as well as the range of size considered. The other five studied species showed allometric positive growth, and thus grew proportionately more in weight than in length.
This study also provides new maximum sizes for three species: Cynopotamus argenteus, Loricariichthys melanocheilus and Parapimelodus valenciennis (Table 1).
For their invaluable collaboration we wish to thank to R. Ballabio, D. Larrea and J. Tana for the data collection and M. Loureiro for support in the taxonomic classification of fishes and language assistance. Comments from Harald Rosenthal and two anonymous referees greatly improved the manuscript.