Length-weight relationship of trahira Hoplias aff . malabaricus ( Bloch , 1794 ) in a regulated stream of southern Brazil

This paper presents an allometric study of the length-weight relationship of Hoplias aff. malabaricus collected in the Chasqueiro Reservoir and Stream in southern Brazil. During the study, 112 specimens were collected with total lengths ranging from 124 to 386 mm. The estimated parameters of the length-weight model were statistically signiicant and showed positive allometric growth for this species.

Studies of ish populations related to trophic ecology, reproduction, and estimates of growth parameters are very important because they generate information needed to understand ecological relationships and this contributes to the management of ish stocks.Estimates of the length-weight relationship of a species provide relevant information for fishery management and conservation, as well as estimate the growth pattern of a species (LE CREN, 1951;FROESE, 2006;CHU et al., 2012;ANTONETTI et al., 2014).According to Froese (2006), there are few published studies about lengthweight relationships and these are only for a limited number of species, especially marine species, which makes modeling aquatic ecosystems more complicated.
Hoplias aff.malabaricus (Bloch, 1874), popularly known as trahira, has a wide distribution in freshwater aquatic ecosystems in South and Central America, and is found in rivers, streams, wetlands, lakes, lagoons, and reservoirs (REIS et al., 2003).It is a piscivorous species that has an important role in the structure of food webs; the young have a diet based on insects and the adults consume ish (CORRÊA et al., 2012).The species reproduces mainly during periods of higher temperatures; females lay their eggs in vegetation and parental care is performed by males (BEMVENUTI; MORESCO, 2005).Studies that estimate the growth model of H. aff.malabaricus have been conducted in other regions of Brazil (BARBIERI, 1989;CHAVES et al., 2009;NOVAES;CARVALHO, 2011); however, in the state of Rio Grande do Sul there are few studies about this species in regards to the hydrographic of Mirim Lagoon system.
The trahira plays an important economic role for ishermen from the Mirim Lagoon system (PIEDRAS et al., 2012), so it is essential to conserve the species in this region.Mitigation measures for the conservation of the species include, for example, subsidizing information that should be recorded for the conservation of this important species in the system.This study provides the length-weight relationship of H. aff.malabaricus in a hydrographic basin of the Mirim Lagoon system in southern Brazil.The hydrographic basin of the Chasqueiro Stream (BHAC) is located in the city of Arroio Grande (31°6'51"S/50°5'17"W) (Figure 1) (CORRÊA et al., 2015b).The region has a humid  The Chasqueiro Reservoir has a surface area of 1,800 ha and is used primarily to irrigate rice ields (SONDOTÉCNICA, 1976;CORRÊA et al., 2014;2015a).The specimens were collected from the following six points: 1 and 2 (upstream), 3 and 4 (downstream), and 5 and 6 (reservoir) (Figure 1).Fish were collected monthly for a one-year period between the months of August 2012 to July 2013 (SISBIO n. 34389-1).To collect the specimens, the following was used: a) gill net, 75 m long with 10, 20, 30, 40 and 50 mm mesh (knot to knot), submerged for a period of 24 h and checked after 12 h, which were used at reservoir depths between 1.0 and 2.5 m to capture larger individuals (> 200 mm); b) trawl net, 5 m long, 2.25 high, with 5 mm mesh (knot to knot), used in the coastal zones of the reservoir and downstream, with ive trawls at each site per collection; c) landing net, 5 mm mesh opening, 35 cm wide, and 50 cm long, used upstream and downstream, in areas with and without vegetation, for 30 minutes at each site at depths ranging from 10 cm to one meter.The collected ish were stored in labeled, plastic bags, and ixed in 10% formalin, then taken to the laboratory where they were transferred to 70% alcohol.
After each sample was taken, the specimens were transferred to the Laboratory of Ichthyology, at the Federal University of Pelotas (UFPeL), where they were measured for total weight (W) in grams (g) and total length (L) in millimeters (mm), and the standard error (SE) was calculated.The parameters (a) and (b) were estimated by linear regression expressed by the equation log (W) = log (a) + b log (L); extreme values were excluded from the analysis (FROESE, 2006), and for the adjusted model the statistical software PAST 3.0 (PAlaeontological STatistics, HAMMER et al., 2013) was used.Where W is the total weight of ish (g), a is the coeficient related to body shape, L is the total length (mm), and b is an exponent that is related to the growth of ish (isometric or allometric) (FROESE, 2006).The data was analyzed together because the number of captured specimens was not similar between different biotopes, and possible differences among the average size of the ish captured downstream and from the reservoir were evaluated by the non-parametric Mann-Whitney test (with Monte Carlo resampling 10,000 "replication", n = 90 and 22 for downstream and the reservoir, respectively) with the statistical package PAST 3.0 (HAMMER et al., 2013).Voucher specimens were deposited in the Ichthyological Collection of the Federal University of Rio Grande (#ICFURG-0029).
During the study, 112 specimens were collected with a total average length of 170.3 ± 6.3 mm and a total average weight of 97.6 ± 14.1 g.The 22 individuals captured in the reservoir had an average total length of 255.7 ± 18.1 mm and an average total weight of 284.4 ± 54.4 g.Downstream, 90 individuals were collected with an average length of 150.4 ± 4.2 mm and an average weight of 51.9 ± 4.5 g.When comparing the average length of individuals between biotopes, we observed a signiicant difference (p = 0.001).During the study, the abundance of Hoplias aff.malabaricus was lower in the reservoir (n = 22) (limnetic zone); however, these individuals had a significantly higher mean length (p = 0.001) than individuals that were captured downstream (shallow zone) (Figure 2).The parameter of length-weight relationship estimated was statistically signiicant (p = 0.001, R 2 = 0.98) and the linear regression showed a value of b = 3.11 (W = 0.0000148 * L 3.11 ) (Figure 3).More trahira were captured downstream compared with the reservoir.This may be associated with the environmental structure in the downstream area, for example, in this region there are aquatic macrophytes, which favors ish reproduction and feeding (THOMAZ; CUNHA, 2010).In addition, the downstream environment has a smaller area compared to the reservoir.According to Hermes-Silva and Zaniboni (2012), the presence of macrophytes provides different microhabitats that act as a refuge from predators as well as feeding sites.It is worth noting that gillnets were not used downstream because it is a wading area habitat.Indeed, the larger  individuals that were captured in the reservoir may be associated with the type of sampling devices used for this biotype because gillnets tend to be more size selective, capturing larger ish in deeper areas.This pattern is described by other authors, such as Artioli et al. (2009) that studied the structure of the ish assemblage in Mangueira Lagoon in southern Brazil.These authors noted that in shallow areas the dominant species are small ish, such as minnows, while deeper areas are dominated by larger species, such as H. aff.malabaricus.Similar patterns were also observed by Garcia et al. (2006) and Ceni and Vieira (2013).Barbieri et al. (1982) and Barbieri (1989) studied Hoplias aff.malabaricus in two reservoirs in Brazil, the Lobo and Manjolinho (b = 3.0 and 3.2, respectively).
According to Araújo and Vicentini (2001), values lower than 3 indicate that individuals become more longilineal during growth and values higher than 3 indicate that individuals become rounder.Chu et al. (2012) indicate that the value of b for the length-weight ratio can be used as a biological attribute, which may be related to predation as well as growth.In this context, ontogenetic changes in ish lead to changes in feeding ecology where the ish adapt to consume the diversity of food items available in the ecosystem (MAZZONI;COSTA, 2007;CORRÊA et al., 2012).In conclusion, H. aff.malabaricus demonstrated positive allometric growth in the study site.Furthermore, future studies related to feeding ecology, reproductive, age and growth, as well as continuous monitoring of the ish stocks in the region, would generate more detailed information that could aid in the preservation of these ish in the hydrographic system of the Mirim Lagoon and adjacent areas.Mitigation measures should be taken for the conservation of this species because it is a top predator and its decline may create disturbances in the ish community.
subtropical climate, described by the Köppen-Geiger climate classiication as Cfa, with an average annual rainfall ranging from 1,200 to 1,450 mm and monthly

FIGURE 2 :
FIGURE 2: Number of individuals (A) and average total length (mm ± SE) (B) of individuals captured by biotope, hydrographic basin of Chasqueiro stream, Arroio Grande city, Rio Grande do Sul state, Brazil.

FIGURE 3 :
FIGURE 3: Model linear regression for length-weight relationship of Hoplias aff.malabaricus, in the Chasqueiro stream, Arroio Grande city, Rio Grande do Sul state, Brazil.
The data obtained from the regression values (b) for H. aff.malabaricus in BHAC indicate a positive allometric growth, showing an ontogenetic pattern where species increase their body mass along with their length.A comparison of allometric growth for H. aff.malabaricus is presented below.Chaves et al. (2009), when studying the species in northeastern Brazil, recorded a lower value of b for January and February (n = 27, b = 2.25) that indicated negative allometric growth, and a higher value of b for May and June (n = 22, b = 3.20) that indicated positive allometric growth; these values can be associated with the sample size, type of environment and seasonal variation.Lower values were also found by Mello et al. (2009) in the Santa Luiza River, in Uruguay (n = 22, b = 2.97).Novaes and Carvalho (2011) studied the population structure of trahira in the Jurumirim Reservoir, Brazil, and found negative allometric growth for b (n = 426, b = 2.75) that indicated the species grows more slowly in terms of weight than length, which differs from the type of growth of trahiras in the BHAC.Antonetti et al. (2014) described a similar value (n = 57, b = 3.09) compared to BHAC individuals, indicating positive allometric growth.