Abundance and length structure of Brycon nattereri (Osteichthyes, Bryconidae), an endangered fish species in central Brazil

Brycon nattereri is a threatened fish species restricted to small headwater rivers of the Paraná, São Francisco and Tocantins rivers. This species has been extirpated from many sites, and little is known about its ecology. In this sense, the present study investigated temporal variations in the occurrence, abundance and length structure of B. nattereri in the Dois Irmãos river, a small headwater river from the Tocantins river basin. Samples were taken monthly between January and November of 2009 and 2010. The studied site comprised a 4.5 km river stretch, sampled with an active fishing method (fly fishing). Individuals caught were counted, measured and returned immediately to the river. We caught 302 individuals along two years. Brycon nattereri was present in the studied stretch in all sampling months, with higher abundance between November and May (wet season). We registered a wide range of lengths, with predominance of adult fish; young were captured in July and August, indicating recruitment in the region. The present study, therefore, showed that populations of B. nattereri, an endangered fish species, still persist in small headwater rivers of the upper Tocantins river.


Introduction
The genus Brycon includes 42 valid species (ESCHMEYER;FONG, 2014), distributed across different river systems of South America.Most species are rheophilic and migratory, but some have specific habitat affinities, such as the pirapitinga Brycon nattereri Gunter, 1864.This is a medium-sized fish (maximum standard length = 27.9 cm) found in headwaters of the Paraná, São Francisco and Tocantins river basins, restricted to lotic environments with the presence of riparian vegetation (LIMA et al., 2008).Because of these specific demands, populations of B. nattereri have been seriously threatened by dams, land use (deforestation) and other disturbances (LIMA et al., 2008).Common in the past, it has been extirpated from many headwater systems (LIMA et al., 2008;ROSA;LIMA, 2008); as a consequence, B. nattereri is currently listed as 'threatened with extinction' in the Brazilian official list of threatened species (ROSA;LIMA, 2008).
Personal observation and sport fishermen indicate that remnant populations of B. nattereri still persist in small rivers of the upper Tocantins river basin, Central Brazil.However, this watershed has been targeted for hydropower development in the last decade: large dams were constructed in the main channel (AGOSTINHO et al., 2011;ARAÚJO et al., 2013) and several small dams have been installed in the tributaries (TOLLEFSON, 2011).Moreover, the Cerrado biome in Central Brazil has been extensively converted to agriculture and pastures (KLINK; MACHADO, 2005), especially in recent years.This scenario of extensive landscape change is causing a progressive loss of habitats that are important to B. nattereri.Therefore, the confirmation of remnant populations in the upper Tocantins river basin would indicate that these headwaters have high conservation value, particularly because this species has been extirpated or is now rare in other basins and localities (LIMA et al., 2008).Also, we highlight that ecological aspects of B. nattereri remain largely unknown.
In this sense, the present study investigated the existence of relict populations of B. nattereri in the upper Tocantins river basin.To this end, we analyzed temporal variations in the occurrence, abundance and size structure of B. nattereri in the Dois Irmãos river, a small headwater river.The existence of persistent populations in this area emphasizes the need for land use and conservation planning that explicitly take into account the presence of B. nattereri, particularly because this fish is now threatened with extinction.We hope these results inspire more comprehensive surveys in the headwaters of the Paraná, São Francisco and Tocantins rivers.

Study area
The study was conducted in the Dois Irmãos river, a second order tributary of the Almas river and headwaters of the Tocantins river basin (Goiás State, Brazil).This lotic system is characterized by variable altimetric profiles, strong water flow (Figure 1), the presence of rapids, waterfalls, and high water transparency.The rainy season, with increased river flow, occurs between November and May (Figure 2), with a clear peak between February and April (AGÊNCIA NACIONAL DE ÁGUAS, 2005).The studied site comprised a 4.5 km river stretch (between 15° 14' 43'' S/49° 02' 43'' W and 15° 43' 50''/49° 01' 47'' W), located in the lower reach (736-748 a.s.l.), near the confluence with the Almas river.The study area is extensively covered by riparian vegetation (Figure 1), with closed canopies found in some areas.

Sampling
The strong water flow and the canyon-like structure of the Dois Irmãos river (Figure 1) impeded the use of conventional fishing gears, such as gill nets or cast nets.Therefore, we used fly fishing to sample individuals (hook 1/8), on a catch-and-release basis.This sampling method allows the easy handling of captured specimens and the immediate return to the environment, with minimal injuries.The bait used was the winged female of the ant Atta spp., which is widely used by local fishermen and seems to present high effectiveness irrespective of season and fish size (VITORINO JR., personal observation).Sampling was standardized by time effort (about 6 hours to cover 4.5 km) and number of fishermen (one angler, with experience in the use of fly fishing).Overall, fishing effort was about 750 m hour -1 , employed in the downstream-upstream direction, where the bait was continuously cast into rapids and runs.Samples were taken monthly between January and November of 2009 and 2010, totaling 22 samples.In each month, we recorded the number of individuals caught in the stretch.Between May and November 2010 we measured total length of each fish (TL, cm).All individuals were returned alive to the river, except one specimen, deposited in the fish collection of the Laboratory of Ichthyology Systematics of the Federal University of Tocantins, Porto Nacional, Brazil (UNT 11550).

Data analysis
To investigate the variation in fish abundance over months and years, we calculated the catch per unit effort (CPUE: fish/4.5 km/ 6 hours).To investigate the seasonal variation in fish abundance, we plotted total CPUE against months, for each year.Differences in mean CPUE between years were tested with a t-test for independent samples, considering months as replicates.The length structure in 2010 was assessed by calculating the number of individuals within different size classes (2 cm intervals).The percentage of young and adults was also calculated.Lima et al. (2008) provide data about standard length (SL, cm) at first maturity in the upper Tocantins river basin: 11.4 cm SL for females and 9.8 cm for males.Once we did not investigate gonadal development and sex, we considered immature those fish < 12 cm SL.To obtain SL values, we converted TL values into SL with the equation SL = 0.9291 + 0.8173 * TL (linear regression: R 2 = 0.99; n = 4).We then considered immature those fish < 13 cm TL.

Results
A total of 302 individuals were caught during 2009 and 2010.Brycon nattereri was present in the river stretch during all sampling months (Figure 2), except in October 2010, when the stretch was disturbed by fire and sampling was not performed.Total CPUE ranged between 6 and 26 individuals/ 4.5 km/ 6 hours, with higher values between November and May in both years (Figure 2), and lower values between June and September.Mean CPUE was 14.27 ± 1.78 SE in 2009 and 14.5 ± 2.18 SE in 2010, and with no significant difference in mean CPUE between years (t = -0.08;p = 0.936).
The total length ranged from 10.4 to 36.1 cm (fish caught between May and November 2010), with predominance of adult fish (86% with TL > 13 cm), particularly within classes between 18 and 22 cm (Figure 3).We observed a variation in length structure over the months (Figure 4); immature fish (10.4 -12.9 cm) were caught only in July and August.

Discussion
This study registered the presence of B. nattereri in the Dois Irmãos river, a headwater tributary of the Tocantins river.The population apparently lives in the area, considering that individuals were consistently caught for two consecutive years.In addition, the presence of immature fish suggests recruitment, indicating that populations of B. nattereri persist in these headwaters.The existence of this remnant populations confirms the high conservation value of the upper Tocantins river headwaters, especially because B. nattereri is now considered threatened with extinction (ROSA; LIMA, 2008).
We caught specimens of B. nattereri throughout the study period, with a clear seasonal variation in fish abundance within the 4.5 km stretch.This pattern suggests that the population lives in the region, performing short migrations between river stretches.This seasonal pattern indicates the use of different habitats (or river stretches) to complete its life cycle, as known for other Brycon species (GONÇALVES et al., 2006;LIMA;RUFFINO, 2003).In the present study, higher abundances coincided with the wet season (February to May), when the species probably use the studied site for feeding.During the dry season, fish abundance declined in the area, probably because the population moved downstream, where more suitable habitats (i.e.refuges) remain available during the low water period.Brycon nattereri is omnivorous (AGOSTINHO et al., 2006;LIMA et al., 2008), consuming mainly allochthonous resources from the surrounding riparian vegetation (LIMA et al., 2008) -in our study we used an allochthonous resource (Atta spp.) to capture the species.It is worth noting that the Dois Irmãos river is surrounded by riparian vegetation, a basic condition to maintain viable populations of B. nattereri (LIMA et al., 2008;ROSA;LIMA, 2008).A wide range of body lengths were recorded in the stretch, with prevalence of intermediate body size.An important finding was the presence of young fish, which evidences that the population is probably recruiting in the region.In addition, breeding individuals that released eggs and sperm (when handled) were consistently observed during June and November, which represent the beginning and the end of the dry season, respectively.In both years, more than half of the individuals caught in these months released gametes (VITORINO JR., unpublished data).Lima et al. (2008) also reported reproductive activity during the dry season, probably to avoid high variability in flow and strong flows during rainy periods (ABILHOA et al., 2011;WINEMILLER et al., 2008).Some studies (LIMA et al., 2008;VIEIRA et al., 2005) have suggested that this species does not perform long-distance migration, but our results suggest short-distance seasonal movements within the basin, since abundance declined consistently in the studied stretch during drier months.Future studies should investigate spatial requirements of B. nattereri, such as the location of spawning, feeding and nursery habitats, and how it is related to habitat availability (e.g.low water level) and migration needs.
This type of information is necessary to set up effective plans to conserve this threatened species.At present, small hydroelectric plants (PCH) represent the most important threat, since they have been installed in many small tributaries of the basin (TOLLEFSON, 2011), affecting fluvial connectivity, local hydrology and the structure of riparian forests.

Conclusion
The present study confirmed the existence of a persistent population of B. nattereri in the Dois Irmãos river, a headwater tributary of the Tocantins river basin.Further research should be done in these headwaters, to map the occurrence of B. nattereri and to describe population structure in detail (e.g.age structure, feeding, reproduction and recruitment).Future development plans (e.g.hydropower, agriculture), therefore, must evaluate carefully the existence of remnant populations of B. nattereri in the target area; otherwise, this species may also go extinct in these headwaters in the near future.

Figure 2 .
Figure 2. Monthly variation in the abundance of Brycon nattereri (CPUE: individuals/ 4.5 km/ 6 hour), in 2009 and 2010.The solid line is the mean river flow (m 3 s -1 ), measured downstream from the sampling site, at the Almas river station, between 2000 and 2012 (AGÊNCIA NACIONAL DE ÁGUAS, 2005).

Figure 3 .
Figure 3. Length structure of Brycon nattereri in the Dois Irmãos river (May to November 2010).The figure shows the number of fish within size classes (2 cm intervals).

Figure 4 .
Figure 4. Variation in length structure of Brycon nattereri over the months (May to November 2010).The figure shows the number of fish within size classes (2 cm intervals).