Ichthyofauna of the hydrographic basin of the Chasqueiro Stream ( Mirim Lagoon system , southern Brazil ) : generating subsidies for conservation and management

Studies that organize lists of species are essential and act as the starting point for future discussions on the ecology of fish in environments that are poorly studied. The present paper describes the fish assemblage of the hydrographic basin of Chasqueiro Stream, an important component of the Mirim Lagoon system. Fish were collected during one year period between August/2012 and July/2013 in six sites, comprising three biotopes: upstream, reservoir and downstream. A total of 22,853 specimens were collected, and were distributed into 83 species, 20 families, and eight orders. The two species with the largest number of individuals captured were Bryconamericus iheringii with 2,904 (12.71%) and Cheirodon ibicuiensis with 2,868 (12.55%). Characiformes and Siluriformes were the most representative orders in terms of richness and abundance. Bryconamericus iheringi and Cyanocharax alburnus were the species with the highest abundance upstream, while Hyphessobrycon luetkenii and Corydoras paleatus contributed more to the abundance downstream. Cheirodon ibicuhiensis and Heterocheirodon jacuhiensis were the most representative species in the reservoir. This study revealed a rich fauna of fish, which should be preserved for future generations and for the maintenance of local and regional biodiversity.


Introduction
Having knowledge of the fish species that colonize a hydrographic basin is the first step in producing information about the structure of the local assemblage, as well as the trophic and reproductive dynamics.The Neotropical region has the highest diversity of freshwater fish species in the world, with estimates of up to approximately 8,000 species (Schaefer 1998, Le ´ve ˆque et al. 2008, Albert & Reis 2011).Lo ´pez-Ferna ´ndez et al. (2012) emphasize that knowledge of the fish fauna is of utmost importance, because it can reveal the relative role of adaptive processes, structural complexity, evolutionary history, and morphological and functional diversification.Indeed, studies that organize lists of species are essential and act as the starting point for future discussions on the ecology of fish in environments that are still poorly studied (Schifino et al. 2004, Vaske et al. 2005).Agostinho et al. (2008) stress that the diversity of fish in the continental ecosystems of Brazil is poorly known, and is greatly associated with the absence of an inventory in these environments.Carrying out studies that include lists of fish species in hydrographic basins is the initial and a fundamental step to the proper management and preservation of fish fauna (Raghavan et al. 2008, Santos et al. 2015).Information about species composition is the basis for recognizing the structure of the assemblage, as well as understanding its dynamics.The detection of these patterns allows the possibility of a more precise conservation plan for the area, defining the priorities in detail.
The fish fauna of freshwater ecosystems have suffered directly from the impacts associated with urban population growth (Pendleton et al. 2014).Other factors such as the introduction of exotic species (Vitule 2012), pollution and deforestation (Carvalho et al. 2012), busbaraquatic ecosystems (Burns et al. 2006, Barletta et al. 2010), and predatory fishing have contributed significantly to the decline in species richness.It is worth mentioning that the changes and loss of natural habitats have been directly influenced by human activities, which have led to loss of biodiversity (Teixeira et al. 2005, Schindler 2007, Barros et al. 2012).For example, Daga et al. (2012), studying the influence of human activity on the fish assemblage in a sub-basin of the San Francisco River, showed that along a longitudinal gradient there is a significant urbanization effect on the fish assemblage, primarily caused by urban organic effluents and urban activity runoff.An aspect that hinders understanding of the impacts of human activities on the fish community in a hydrographic basin is the lack of studies about local or regional species composition.
Among the countries that form the Neotropical region, Brazil has around 2,500 freshwater fish species in its hydrological systems (Reis et al. 2003, Buckup et al. 2007), which is related to the large diversity of aquatic systems present in its different biomes.In southern Brazil, transgression and regression events created the largest lagoon complex in South  and samples sites, 1 and 2 (upstream), 3 and 4 (downstream), 5 and 6 (reservoir) America, formed by the Patos and Mirim lagoons, which together comprise an area of about 14,000 km 2 (Villwock 1987, Kotzian & Marques 2004).It is worth mentioning that the communication between the Patos and Mirim lagoon systems occurs through a natural channel called Sa ˜o Gonc¸alo.In 1977, a dam was constructed to prevent the entry of salt water into the region of Mirim Lagoon, the main water source used for agriculture, particularly for rice cultivation (Burns et al. 2006,

Material and methods
The hydrographic basin of Chasqueiro Stream (BHAC) is located in the western region of the hydrographic system of Mirim Lagoon, municipality of Arroio Grande, southern Brazil (31°6'51"S/50°5'17"W) (Figure 1).The region has a subtropical climate (according to Ko ¨ppen), with an average annual rainfall ranging from 1,200 to 1,450 mm, with monthly average temperatures of 25°C in the hottest months and 11°C during the coldest.The BHAC is formed by two principal streams (Chasqueiro and Chasqueirinho) and a reservoir.The area upstream from the reservoir is 248.42 km 2 , and is formed by the Chasqueiro Stream (114.84 km 2 ) and the Chasqueirinho Stream (133.58 km 2 ) basins.Chasqueiro Reservoir has an area of 1,800 ha, and is used primarily for rice monoculture (Sondote ´cnica 1986).Six sampling points were established along the BHAC: upstream -sites 1 (Chasqueiro Stream) and 2 (Chasqueirinho Stream); downstream -sites 3 and 4 (Chasqueiro Stream); Chasqueiro Reservoir -sites 5 and 6 (Figure 1; Figure 2) and a description of the sampling sites is shown in Table 1.Fish were collected monthly for one year between August/2012 and July/ 2013 (SISBIO #n.34389-1), totaling 72 samples.Because of the diversity of habitats that were investigated, it was not possible to maintain a standardized effort for all points and became necessary to employ different collection apparatuses, assuming that the use of the set of fishing gear increased the sampling efficiency.The following apparatuses were used: a) gill net: 75 m long with 10 meshes; 20, 30, 40 and 50 mm (knot to knot) submerged for a period of 24h (checked after 12h); used in reservoir depths between 1.0 and 2.5 m in order to capture larger individuals (4 200 mm); b) beam trawl: 5 m long, 2.25 high, with 5 mm mesh (knot to knot) employed in the coastal zones of the reservoir and downstream, with five trawls at each site per collection; c) two dip nets: 5 mm mesh opening, 35 cm wide and 50 cm long, used upstream and downstream in areas with and without vegetation, and used for 30 minutes at sites with depths  sites 1 and 2 (upstream), 3 and 4 (downstream), 5 and 6 (reservoir) ( Brazil and Uruguay (Kotzian & Marques 2004).However, there is still an absence of studies and knowledge of the fish fauna occurring in the sub-basins located in southern Brazil (Buckup & Reis 1997, Garcia et al. 2006, Ceni & Vieira 2013).In this context, the present study describes the fish assemblage of the hydrographic basin of Chasqueiro Stream, an integral part of the Mirim Lagoon system, with the goal of contributing to the advancement of knowledge on the composition of its ichthyofauna, as well as the production of information that can be used in action protocols aiming the system conservation.
formed by a complex composed of 22 basins, distributed between of 10 cm to one meter and d) one sieve with 5 mm mesh, 80 cm wide and 160 cm long, employed upstream and downstream, and used for 15 minutes per month at each site.The collected fish were stored in plastic bags, labeled, and fixed in 10% formalin, and were then taken to the laboratory where they were transferred to 70% alcohol.Specimens were identified with the help of specialized literature (Reis & Malabarba 1988, Buckup & Reis 1997, Rodriguez & Reis 2008, Caravalho & Reis 2009, Bertaco & Lucena 2010, Carvalho et al. 2012, Lucena et al. 2013, Malabarba et al. 2013), and specialists were consulted when needed for species confirmation.The list of species was formulated according to Wiley & Johnson (2010) and Malabarba et al. (2013).The specimen testimonies were deposited in the ichthyological collection of the Federal University of Rio Grande (FURG).The number of species was estimated for each sampling site, and the Shannon-Wiener diversity index (H') and Pielou's evenness index were also used for calculations with PAST 3.0 statistical software (PAlaeontological STatistics, Hammer et al. 2014).

Results and Discussion
A total of 22,853 individuals were collected.They were represented by 83 species, 20 families, and eight orders (Table 2).The two species with the greatest number of individuals captured were Bryconamericus iheringii (2,904; 12.71%) and Cheirodon ibicuiensis (2,868; 12.55%).When the data of the orders were compared among different biotopes (upstream, downstream, and  The rarest species in the system were Trachelyopterus lucenai and Gymnotus pantherinus (0.004%), which corroborate data reported by Artioli et al. (2009) in Mangueira Lagoon, Rio Grande do Sul state.Trachelyopterus lucenai is not considered a native species of the region, and might have come to the hydrographic basin through irrigation channels (rice fields) (Bertoletti et al. 1992, Becker et al. 2013).
The number of species collected represents a total of 51.87% of the 160 total species recorded for the Patos-Mirim Lagoon system (Becker et al. 2013).Studies to expand lists of fish species in the Mirim Lagoon system were conducted in the Taim Ecological Reserve (Buckup & Malabarba 1983, Grosser et al. 1994, Garcia et al. 2006), where 51 to 62 fish species were recorded.Based on analysis of the different sampling sites, the stretch that showed a larger number of individuals consisted of sites 3 (downstream) (5,665; 24.79%) and 5 (reservoir) (5,600; 25.5%).Note that downstream site 3 presents a large predominance of aquatic macrophytes that provide refuge from predators, which are important for feeding and reproduction (Lowe-McConnell 1999).However, at site 5, the high abundance may be associated with the high reproduction rate of the forage species (e.g.Cheirodon ibicuiensis and Astyanax aff.fasciatus).Agostinho et al. (1999) describe how the high abundance of forage species is associated with a high reproduction rate, as well as how feeding flexibility favors the colonization of these reservoir species.
Regarding ecological indices, the upstream Shannon diversity index values were 2.57 and 2.22 at sites 1 and 2, respectively.However, the downstream Shannon diversity index values at sites 3 and 4 were greater (3.00 and 2.83, respectively), while the reservoir was the least diverse biotope (2.04 and 2.09) (Table 2).The same pattern can be observed in relation to the evenness index of Pielou, as the downstream sites demonstrated the highest values (0.75 and 0.72), while the reservoir presented the lowest equitability values (0.30 to 0.63) (Table 2).At the six BHAC sampling sites, two species expanded their spatial distribution: Acestrorhynchus pantaneiro, an invasive species in the Patos-Mirim Lagoon system (Einhardt et al. 2014) and Cyphocharax spilotus, which was recorded in the northern part of this system and in the BHAC (Corre ˆa et al. 2014).The richness found in the BHAC may be associated with broad environmental heterogeneity, since the basin is formed by a large variety of ecosystems (rivers, streams, wetlands and reservoir).Landscapes with different lotic, lentic and semilentic aquatic systems tend to present larger functional habitats and greater microhabitat diversity for fishes, increasing the availability of feeding resources, breeding sites and refuges against predation (Matthews 1998).BHAC also showed high species diversity compared to similar studies in aquatic systems.For example, in the coastal streams of austral Brazil, Bastos et al. (2013) recorded a total of 41 species, while Artioli et al. (2009) documented the occurrence of 52 species in Mangueira Lagoon, located in the same coastal plain, with the largest values comparable to those found in other aquatic environments in southern Brazil (Lucena et al. 1994, Tagliani 1994, Malabarba et al. 2009, Carvalho et al. 2012).Lower numbers of species in the studies described above can be associated with the type of sampler, as well as lower environmental heterogeneity; however, the outstanding abundance of Characiformes and Siluriformes are documented for every location (Artioli et al. 2009, Carvalho et al. 2012, Bastos et al. 2013, Silva et al. 2014).The downstream biotope presented the greatest richness, which may be related to increased diversity of habitats and micro habitats observed in this biotope, which corroborates the river continuum theory (Vannote et al. 1980), where changes along the longitudinal gradient (upstream to downstream) tend to increase the biological diversity.This is related to higher solar incidence and temperature variations, which increase structural complexity, e.g. the presence of macrophytes with this higher primary productivity.Another aspect that may influence the increase in fish diversity in a hydrographic basin is the presence of environmental barriers.Winemiller et al. (2008) explain that environmental barriers limit dispersion, which tends to influence diversity along a longitudinal gradient.Therefore, this study provides important data about ecosystems that compose the hydrographic basin of Chasqueiro Stream.Indeed, the richer biotopes were upstream and downstream, where there was better ecological and functional structure.Furthermore, there was a strong predominance of Characiformes and Siluriformes, reflecting the environmental heterogeneity of these environments.Downstream environments were However, the reservoir has the lowest diversity, reflecting a more homogeneous environment and the presence of invasive species (e.g.Odonthestes spp.) (Figure 4).The information mentioned above can be used for management plans and regional conservation, adopting mitigation measures for the preservation of this important ecosystem in southern Brazil.Further studies about the spatial and temporal distribution of species can help answer important questions about the dynamics of the assemblage organization, as well as provide additional assistance for formulating conservation programs for the ichthyofauna basin and greater control over environmental impacts, such as those resulting from irrigation and rice cultivation.
in the fieldwork and laboratory, and two anonymous referees by their valuable suggestions.We are grateful to SISBIO license for the field collection, Mirim Lagoon Agency (ALG) and Coordenaca ˜o de Aperfeic¸oamento de Pessoal de Nı ´vel Superior (CAPES) in terms of field study and financial support.

Figure 3 )
.Artioli et al. (2009), studying dominance patterns of an aquatic ecosystem in southern Brazil, also recorded high abundance of Characiformes species (e.g.Bryconamericus iheringii).Similarly,Becker et al. (2013)  studied the fishes of the Taquari-Antas River basin and Garcia et al. (2006) compared the dominance patterns of the fish fauna of Taim wetlands.The predominance of species belonging to Characiformes and Siluriformes corroborates studies conducted in Neotropical aquatic ecosystems (Lowe-McConnell 1999, Becker et al. 2013, Viana et al. 2013).The high diversity, as well as the abundance of Characiformes and Siluriformes in the Neotropical region, is related to several factors, e.g. the ecological and behavioral patterns of the species (Carvalho et al. 2012), diversity of ecosystems (Oliveira et al. 2012), as well as evolutionary and geological processes (Le ´ve ˆque et al. 2008).Teixeira et al. (2005) emphasize that the differences in physiography over a longitudinal gradient corresponded to the changes in the diversity and abundance of the fish assemblage, where species of Characiformes and Siluriformes have wide spatial distribution.

Figure 3 .
Figure 3. Number of fish species by taxonomic order, for the three biotopes (upstream, downstream and reservoir) of hydrographic basin Chasqueiro Stream, Mirim Lagoon system, southern Brazil.

Figure 4 .
Figure 4. Picture of the 83 species of fishes collected in the hydrographic basin Chasqueiro Stream, Mirim Lagoon system, southern Brazil.

Table 1 .
Characterization of sampling sites, upstream, downstream and reservoir, presenting the biotopes, general characteristics, substrate and geographic coordinates, the sites located in the hydrographic basin of Chasqueiro Stream, Mirim Lagoon system, southern Brazil.Eichhornia crassipes and rooted Polygonum punctatum.Groups of shrubs and tree species on both sides of individuals such as Calliandra brevipes.Average width of 6.2 m and average depth of 31.9 cm.Presents with a flow site and with a predominance of local shallows.
reservoir), Characiformes and Siluriformes were the most representative in terms of abundance and richness (Table2;