Fish fauna from the Langueyú basin , Argentina : a prairie stream in a heavily modified landscape

On the Pampa plain, one of the most productive modified areas of Argentina, important changes in land uses have drastically altered the landscape during the last decades. This has led to an increased deterioration of surface waters affecting fish that inhabit them. We provide a list of fish species inhabiting an unsurveyed prairie stream of this region. Environmental variables were measured and fish samplings were conducted in 3 sites of the Langueyú stream. A total of 15 species belonging to 10 families and 6 orders were collected. Characiformes and Siluriformes were the richest orders. Characidae was the most representative family. Species richness was highest when compared with other similar regional environments without connection with the Salado river basin. Most of the fish species collected are typical of the region but others are species which typically do not progress beyond the Salado river basin. The role of human intervention in fish species distribution is discussed.


Introduction
South America hosts the richest freshwater fish fauna in the world, with more than 5,000 species (Reis et al. 2003).In Argentina, 548 fish species live in freshwaters, with 17 introduced (Mirande andKoerber 2015, Koerber et al. 2017).Despite this great richness, knowledge on the fish fauna inhabiting rivers and streams is still incipient (Casciotta et al. 1999, Almirón et al. 2000, Paracampo et al. 2015).
The freshwater fishes of Argentina are included in 2 major Neotropical Biogeographic subregions: the Brazilian subregion occupying North and Central-East territories and the Austral subregion extending in Patagonia and part of Central-West territories (Ringuelet 1975, Arratia et al. 1983).The southern boundary of the Brazilian subregion is located in drainages south of Ventania hills and extends to the Río Negro (Almirón et al. 1997).Well before reaching its southern limit, the Neotropical fish fauna already clearly shows a pattern of diversity pauperization southwards (Ringuelet 1975, Gómez 1996).This significant decrease in fish richness is explained by the temperature (by deficit) and dissolved salt concentration (by excess).Particularly, many freshwater fish species that are common in La Plata river drainages are scarcely collected in the lower Salado River and are absent beyond this fluvial network (López et al. 2001).
The Pampa plain, one of the most productive modified landscapes of Argentina (Baldi et al. 2006), is poorly protected from, and one of the most impacted by, anthropic activities.During the last years, important changes in land uses have been produced in the Pampean region as a consequence of the advance of the agricultural frontier (Viglizzo et al. 2001) and the consequent displacement of livestock to marginal flooded areas.This scenario of habitat fragmentation by changes in land use brings an increase in the deterioration of surface waters (Quirós et al. 2006), which has an effect on the biota that inhabits them, including fishes (Granitto et al. 2016).Habitat fragmentation has been implicated as the major threat to the biodiversity of freshwater ecosystems in South America (Reis et al. 2016).Therefore, cataloging the fish species of regions where the landscape has been strongly altered from its original condition is highly relevant, especially if its ichthyofauna has been poorly explored.Information about the geographic distribution of species is also central for conservation purposes, such as by the International Union for Conservation of Nature (IUCN).The IUCN uses diverse knowledge of each species, including geographical distribution (IUCN 2001), and georeferenced distributional data are needed to calculate extent of occurrence, an important piece of information used in extinction risk assessments (Joppa et al. 2016).
The Langueyú stream, belonging to the basin Langueyú (area: 600 km 2 ) is a prairie ecosystem located in the Pampa Plain.This stream drains water from agricultural and livestock lands but also receives effluents from urban and industrial lands (Banda Noriega and Díaz 2010).To be able to evaluate potential impacts of anthropic activities on the fish fauna in this stream, the baseline information of a fish assemblage composition is mandatory.Therefore we provide a list of fishes species inhabiting this unsurveyed prairie stream of the Pampa Plain, Argentina.

Methods
Study area.The Langueyú stream has its headwaters in the Tandilia hills (elev.50-250 m) in southeastern Buenos Aires province (Fig. 1).Water flows in a southwest to northeast direction without receiving tributaries of relevance (Ruíz de Galarreta et al. 2013).An artificial channel connects the lowland reaches of this small stream with Samborombón Bay (Río de la Plata estuary).The climate is temperate semi-moist to moist (Thornthwaite and Mather 1957), with annual average rainfall of 838 mm and an average temperature of 13.8 °C.As for many basins in temperate latitudes of the globe (Allan 2004), drastic changes in land uses have been occurred in the Langueyú basin during the last decades (Vásquez and Zulaica 2011).
Data collection.Three sites (1: 37°11ʹ15ʺ S, 059°08ʹ W; 2: 37°05ʹ47ʺ S, 059°06ʹ29ʺ W; 3: 36°55ʹ38ʺ S, 058°56ʹ 08ʺ W) were sampled bimonthly in the Langueyú stream (Fig. 2) during 2 different periods from November 2016 to March 2017 and from October 2017 to February 2018.Field samplings were conducted under scientific fishing permit issued by the Ministerio de Agroindustria of Buenos Aires Province (Permission No. 409).Fish sampling was performed using different nets and fishing devices: trammel nets (12 m), beach seine net (12 m), artisanal tramps and long-lines (9 m).Fish captured were euthanized by an overdose in benzocaine solution, fixed in 10% formalin solution and preserved in 70% ethanol.Fish species were identified following Ringuelet et al. (1967), López et al. (1987), Casciotta et al. (2005), Rosso (2006), Miquelarena et al. (2008), andAlmirón et al. (2015).Valid species names, their authorities, year of publication and synonyms are listed following Eschmeyer et al. (2017) and Mirande and Koeber (2015).Tolerance to diverse environmental variables, ecology, and behavior aspects for all collected species were summarized by Rosso (2006).Voucher specimens were deposited at the fish collection of the Instituto de Investigaciones Marinas y Costeras (UNMDP), in Mar del Plata, Argentina.Environmental variables measured in situ included: dissolved oxygen, water temperature, water specific conductance, and pH.Total suspended solids were quantified after processing water samples in the laboratory.Depth of water column and fine sediment substrate and wet channel width were also reported.

Results
The environmental variables that characterized the study sites are summarized in Table 1.Fish sampling yielded a total of 15 species (Figs 3, 4) belonging to 10 families and 6 orders (Table 2).Characiformes and Siluriformes were the richest orders (Fig. 5) with 5 (33% of species collected) species each.Characidae was the most representative family with 4 species (Fig. 6).The remaining fish families all contained 1 species except for the Heptapteridae and Loricariidae, which were represented by 2 species each.All fish species collected are native, with the exception of Cyprinus carpio.The meristic and morphometric characteristics used to identify each species are provided below.Material examined.Table 2, Figure 3E Large, robust body.Toothless jaws.Nine branched dorsalfin rays.Prepelvic region flattened.Median keel present posterior to insertion of pelvic fin.Pectoral fin does not reach pelvic fin and the latter does not reach the anus.Lateral line with 32-39 scales.Presence of patch of dark pigmentation on the midlateral surface of the caudal peduncle ranging in shape from vertically ovoid to rotund.
In juveniles and many adults a series of spots on the lateral and dorsolateral surfaces of body randomly arranged.Material examined.Table 2, Figure 3H Small, subcilindrical body.A large central scale on the dorsal surface of head.Predorsal series with 13 or 14 scales.Lateral lines series with 30-32 scales.A tubular gonopodium formed by modified anal-fin rays in males.
A swelling between the urogenital papilla and the origin of anal fin in females.Anal fin with 13 or 14 rays.Color pattern: dash-shaped spots arranged in longitudinal lines, with 4 or 5 lines on caudal peduncle.

Discussion
The species richness of the Langueyú stream almost parallels the mean species richness that can be found in any locality in temperate streams worldwide (Matthews 1998).Moreover, the 10 fish species observed in site 3 of the Langueyú stream coincides with the mode of the average species per site reported by Matthews (1998) for temperate streams.When compared with its counterparts of the southern Pampa Plain, the observed fish species richness in the Langueyú stream is still high.Particularly, this stream showed the richest fish communities among other similar streams and rivers of the Pampa Plain without connection with the Salado river basin.For instance, 11 species were recorded in 35 lotic systems in the Southern Pampean region with Atlantic slope (Casciotta et al. 1999).In a study encompassing 23 streams in the Ventania hills, Menni et al. (1988) recorded only 7 species.Later, 9 species were collected in the Sauce Grande River (López Cazorla et al. 2003).Ringuelet et al. (1967) mentioned Australoheros facetus, Cheirodon interruptus and Synbranchus marmoratus near the city of Tandil without precise an exact locality.All the species mentioned in these previous studies were collected by us.More than 65% of the species collected in the Langueyú stream were grouped in only 2 orders: Characiformes and Siluriformes.A fish community dominated by species of Characiformes and Siluriformes is a common pattern in Pampean region (Rosso 2006) as it is in most of the Neotropical fish assemblages (Lowe-Mc Connell 1987).Even being a marginal Neotropical fluvial ecosystem, the Langueyú stream shares this pattern of fish assemblage organization with other Neotropical streams of lower latitudes.However, the marked proliferation of species richness within 1 family explaining the local species diversity in fish assemblages is not a universal pattern.In the Pampa Plain, a marked proliferation of species within Characidae with the remaining families accounting for no more than 2 species is commonly observed (Rosso and Quirós 2010).Conversely, in tropical fish assemblages (Angulo et al. 2013, Casatti et al. 2013), as in many other streams worldwide (Matthews 1998, Muchlisin et al. 2015), an increased local richness is explained by an increased number of families instead a proliferation of species within families.
As it is observed in many freshwater ecosystems of Argentina and in the Pampa Plain (Maiztegui et al. 2016), the exotic and invasive common carp Cyprinus carpio was present in the Langueyú stream.This species has a wide trophic spectrum, high fecundity, lacks predators, and displays high tolerance to environmental change (Colautti 1997).These characteristics could explain its success in Argentina and fast colonization since its introduction at the end of the 19th century, extending its distribution as far as Río Negro (Liotta 2017).The presence of the common carp in this environment could generate resuspension of sediments increasing turbidity which would finally affect the visibility of sight-oriented species such as O. jenynsii and other small species of Characidae.In addition, given the food spectrum of this invasive species, it could compete for food with native species, mainly with H. commersoni and L. anus (detritivorous-alguivorous species) and C. voga (a detritivorous species).
Most collected fish species are typical of the marginal fish fauna of the Pampa Plain (Ringuelet 1975) but others are species which typically do not progress beyond the Salado river basin.For instance, the occurrence of Cyphocharax voga, Hypostomus commersoni, and Loricariichthys anus in the Langueyú stream challenges the historical biogeography of these latitudes (Ringuelet 1961).Indeed, they are representatives of the typical fauna of the Salado river basin (Rosso 2006).According to Gómez (2008), the distributions of some parano-platense species in the Pampa Plain are enhanced by channelization and by increases in temperature and rainfall.Langueyú stream is artificially connected with Samborombón Bay by means of an artificial channel, the Canal 1.Before the construction of Canal1, Langueyú stream, as well as most of the northern streams of the Tandilia System flowed to a very low elevation area of the Buenos Aires province without a natural drainage to the Samborombón Bay (Fucks et al. 2012).It could be hypothesized, therefore, that migration of H. commersoni, C. voga, and L. anus from Samborombón Bay to Langueyú stream could have been facilitated by the anthropogenic modification of the natural fluvial network.
The freshwater fish distribution is a result of historical, ecological, and zoogeographical factors (Ringuelet 1961), but man-made interventions has drastically modified these historical patterns and homogenized the freshwater faunas (Rahel 2007).A better knowledge of the geographic distribution of species in various environments is needed for defining priorities areas in conservation planning, evaluating the anthropic impacts on communities and proposing successful management measures.

Material examined :
Table 2, Figure3BBlunt snout.Mouth opening at the ventral margin of the orbit.Upper jaw slightly larger than lower jaw.Two rows of teeth on the premaxilla with four teeth in the inner series and maxilla with 2 to 4 teeth.A large third infraorbital contacting the preopercle along its posterior and ventral margins; a complete lateral line with 35 to 40 perforated scales.Two to two and a half scales in contact with the supraoccipital bone.Anal-fin rays 18-23.Cheirodon interruptus (Jenyns, 1842)Material examined.Table2, Figure3DA large, oblique head.Orbital diameter 25-34% of the head length.Upper jaw slightly larger than lower jaw when the mouth is closed.Both jaws with canine teeth.Maxilla with 19-28 teeth.Large (may reach 220 mm SL) compressed body with 54-65 scales in the lateral line system.Ten to 13 scales from lateral line origin to dorsal fin.Anal-fin rays 26-30.Scale around lateral peduncle 21-25.

Figure 6 .
Figure 6.Number of species of each family expressed as percentages of the total of species collected in the Langueyú stream.

Figure 5 .
Figure 5. Number of species for each order expressed as percentages of the total of species collected in the Langueyú stream.

Table 1 .
Environmental characteristics in different reaches of the Langueyú stream.

Table 2 .
List of species collected in the Langueyú stream, Buenos Aires province, Argentina.