Abstract:

We investigated spatial and temporal variations in the distribution of fish eggs and larvae in the Guareí River, a free-flowing tributary located in the upper section of the Jurumirim Reservoir, Paranapanema River, Brazil. Fish eggs and larvae were sampled ~ fortnightly from November 2017 to March 2018 (Cycle 1), and from November 2018 to March 2019 (Cycle 2), at three sites distributed along the entire course of the Guareí River. We collected 859 fish eggs and 1,340 larvae, comprising 19 taxa. We found reproductive activity of several fish species in the Guareí River, including long-distance migrants. However, there was no spatial differences in the assemblage structure and densities of fish eggs and larvae along the channel of Guareí River. We recorded temporal differences in larvae density only in Cycle 2. The consistent capture of fish eggs and larvae in two reproductive cycles is strong evidence that this river is a spawning site for a variety of fish species. Thus, we recommend maintaining its natural flow regime.

Keywords:
Fish reproduction; Ichthyoplankton; Jurumirim Reservoir; Migratory fish; Prochilodus lineatus

Resumo:

Investigamos as variações espaciais e temporais na distribuição de ovos e larvas de peixes no rio Guareí, um afluente de fluxo livre localizado na parte superior do Reservatório de Jurumirim, bacia do rio Paranapanema, Brasil. Ovos e larvas de peixes foram amostrados ~ quinzenalmente de novembro de 2017 a março de 2018 (Ciclo 1), e de novembro de 2018 a março de 2019 (Ciclo 2), em três locais distribuídos ao longo de todo o curso do rio Guareí. Foram coletados 859 ovos de peixes e 1.340 larvas, compreendendo 19 táxons. Nós encontramos atividade reprodutiva de várias espécies no rio Guareí, incluindo espécies migradoras de longa distância. Contudo, não houve diferenças espaciais na estrutura das assembleias e nas densidades de ovos e larvas e ao longo do canal do rio Guareí. Diferenças temporais foram significativas apenas para densidade de larvas no Ciclo 2. A captura consistente de ovos e larvas de peixes em dois ciclos reprodutivos é uma forte evidência de que este rio é um local para a reprodução de várias espécies de peixes. Assim, recomendamos a manutenção do seu regime de fluxo natural.

Palavras-chave:
Reprodução de peixes; Ictioplâncton; Reservatório de Jurumirim; Peixes migratórios; Prochilodus lineatus

Introduction

The distribution of fish eggs and larvae in freshwater ecosystems reflects the reproductive success of species. Young forms are found in most aquatic environments, such as lakes (e.g., Daga et al. 2009DAGA, V.S., GOGOLA, T.M., SANCHES, P.V., BAUMGARTNER, G., BAUMGARTNER, D., PIANA, P.A., GUBIANI, E.A., & DELARIVA, R.L. 2009. Fish larvae assemblages in two floodplain lakes with different degrees of connection to the Paraná River, Brazil. Neotrop Ichthyol. 7(3):429-438., Tondato et al. 2010TONDATO, K.K., MATEUS, L.A.F., ZIOBER, S.R. 2010. Spatial and temporal distribution of fish larvae in marginal lagoons of Pantanal, Mato Grosso State, Brazil. Neotrop. Ichthyol. 8:123-134. https://doi.org/10.1590/S1679-62252010005000002
https://doi.org/10.1590/S1679-6225201000... ), rivers (e.g., Reynalte-Tataje et al. 2011REYNALTE-TATAJE, D.A., NAKATANI, K., FERNANDES, R., AGOSTINHO, A.A., & BIALETZKI, A. 2011. Temporal distribution of ichthyoplankton in the Ivinhema River (Mato Grosso do Sul State/ Brazil): Influence of environmental variables. Neotrop. Ichthyol.9(2):427-436., Zacardi et al. 2017ZACARDI, D.M., PONTE, S.C.S., FERREIRA, L.C., LIMA, M.A.S., SILVA, A.J.S., CHAVES, C.S. 2017. Diversity and spatio-temporal distribution of the ichthyoplankton in the lower Amazon River, Brazil. Biota Amazônia. 7:12-20.), estuaries (e.g., Sarpedonti et al. 2008SARPEDONTI, V., ANUNCIAÇÃO, E.M.S., & NAHUM, V.J.I. 2008. Ichthyoplankton variations in two mangrove creeks of the Curuçá estuary, Pará, Brazil. Ecotrópicos. 21(1):1-12., Marcolin et al. 2010MARCOLIN, C.R., CONCEIÇÃO, B.L., NOGUEIRA, M.M., JÚNIOR, P.M., JOHNSSON, R. 2010. Mesozooplankton and ichthyoplankton composition in two tropical estuaries of Bahia, Brazil. Check List. 6:210-16.) and others. In rivers, numerous species spawn in the channel, where larvae drift with the flow toward nursery areas located downstream. Depending on the species [e.g., Salminus brasiliensis (Cuvier, 1816)], the larvae may be carried toward downstream reaches for dozens of kilometers (Rosa et al. 2020ROSA, R.R., SILVA, J.C., & BIALETZKI, A. 2020. Long-term monitoring of potamodromous migratory fish larvae in an undammed river. Mar. Fresh. Res. 71(3):384-93). In this sense, spatial and temporal assessments of ichthyoplankton distribution are important to understand reproductive and recruitment dynamics in an ecosystem (e.g., Castro et al. 2002CASTRO, R.J., NAKATANI, K. , BIALETZKI, A., SANCHES, P.V., & BAUMGARTNER, G. 2002. Temporal distribution and composition of the ichthyoplankton from Leopoldo’s Inlet on the Upper Paraná River floodplain (Brazil). J. Zool. 256(4):437-443., Henry & Suiberto 2014HENRY, R., & SUIBERTO, M.R. 2014. Distribuição do ictioplâncton na zona de desembocadura do rio Paranapanema na represa de Jurumirim. In: Henry R, org. Represa de Jurumirim: ecologia, modelagem e aspectos sociais. 1ed. Ribeirão Preto: Holos Editora. p.307-23., Rosa et al. 2020ROSA, R.R., SILVA, J.C., & BIALETZKI, A. 2020. Long-term monitoring of potamodromous migratory fish larvae in an undammed river. Mar. Fresh. Res. 71(3):384-93). This information plays an important role to guide conservation strategies (Nakatani et al. 2001NAKATANI, K., AGOSTINHO, A.A., BAUMGARTNER, G., BIALETZKI, A., SANCHES, P.V., MAKRAKIS, M.C., & PAVANELLI, C. S. 2001. Ovos e larvas de peixes de água doce: Desenvolvimento e manual de identificação. Maringá: EDUEM., Bialetzki et al. 2016BIALETZKI, A. , GARCIA, D.A.Z., & ORSI, M.L. 2016. O estudo de ovos e larvas de peixes. In: ORSI, M.L., ALMEIDA, F.S., SWARÇA, A.C., CLARO-GARCIA, A., VIANNA, N.C., GARCIA, D.A.Z., &BIALETZKI, A., org. Ovos, larvas e juvenis dos peixes da Bacia do Rio Paranapanema, uma avaliação para a conservação. 1ed. Assis: Triunfal Gráfica e Editora. p.17-24.), especially in rivers subject to human impacts (e.g., river regulation, fragmentation, habitat loss, pollution, deforestation).

Accessing spatial and temporal dynamics of fish eggs and larvae in small and medium-sized rivers has become increasingly necessary in tropical regions (e.g., Reynalte-Tataje et al. 2020REYNALTE-TATAJE, D.A., LOPES, C.A., MASSARO, M.V., HARTMANN, P.B., SULZBACHER, R., SANTOS, J.Á., & BIALETZKI, A. 2020. State of the art of identification of eggs and larvae of freshwater fish in Brazil. Acta Limnol. Bras. 32:e6.), where hydropower expansion has affected the functioning of many lotic systems (Ferreira et al. 2016FERREIRA, J.H.I., CAMACHO, J.R., MALAGOLI, J.A., & JÚNIOR, S.C.G. 2016. Assessment of the potential of small hydropower development in Brazil. Renew Sust. Energ. Rev. 56:380-387., Winemiller et al. 2016WINEMILLER, K.O., MCINTYRE, P.B., CASTELLO, L., FLUET-CHOUINARD, E., GIARRIZZO, T., NAM, S., BAIRD, I.G., DARWALL, W., LUJAN, N.K., HARRISON, I., STIASSNY, M.L.J., SILVANO, R.A.M., FITZGERALD, D.B., PELICICE, F.M., AGOSTINHO, A.A., GOMES, L.C., ALBERT, J.S., BARAN, E., PETRERE, J.R.M., ZARFL, C., MULLIGAN, M., SULLIVAN, J.P., ARANTES, C.C., SOUSA, L.M., KONING, A.A., HOEINGHAUS, D.J., SABAJ, M., LUNDBERG, J.G., ARMBRUSTER, J., THIEME, M.L., PETRY, P., ZUANON, J., TORRENTE VILARA, G., SNOEKS, J., OU, C., RAINBOTH, W., PAVANELLI, C.S., AKAMA, A., VAN SOESBERGEN, A., & SÁENZ, L. 2016. Balancing hydropower and biodiversity in the Amazon, Congo, and Mekong. Science 351:128-129.). Dams cause hydrological changes with direct and indirect negative effects on the aquatic biota (Agostinho et al. 2008AGOSTINHO, A.A., PELICICE, F.M., & GOMES, L.C. 2008. Dams and the fish fauna of the Neotropical region: impacts and management related to diversity and fisheries. Braz. J. Biol. 68: 1119-1132., Couto & Olden, 2018COUTO, T.B.A., OLDEN, J.D. 2018. Global proliferation of small hydropower plants - science and policy. Front Ecol. Environ. 16:91-100.). Moreover, other activities (e.g., agriculture and mining) have caused additional disturbances to small systems, with significant effects on fish biodiversity (Pelicice et al. 2017PELICICE, F.M., AZEVEDO-SANTOS, V.M., VITULE, J.R.S., ORSI, M.L., LIMA-JUNIOR, D.P., MAGALHÃES, A.L.B., POMPEU, P.S., PETRERE, J.R.M., & AGOSTINHO, A.A. 2017. Neotropical freshwater fishes imperilled by unsustainable policies. Fish Fish. 18 (6):1119-1133.; Azevedo-Santos et al. 2019AZEVEDO-SANTOS, V.M., FREDERICO, R.G., FAGUNDES, C.K., POMPEU, P.S., PELICICE, F.M., PADIAL, A.A., NOGUEIRA, M.G., FEARNSIDE, P.M., LIMA, L.B., DAGA, V.S., OLIVEIRA, F.J.M., VITULE, J.R., CALLISTO, M., AGOSTINHO, A.A., ESTEVES, F.A., LIMA-JUNIOR, D.P., MAGALHÃES, A.L.B., SABINO, J., MORMUL, R.P., GRASEL, D., ZUANON, J., VILELLA, F.S. & HENRY, R. 2019. Protected areas: A focus on Brazilian freshwater biodiversity. Divers. Distrib. 25(3): 442-448. https://doi.org/10.1111/ddi.12871
https://doi.org/10.1111/ddi.12871... , 2021AZEVEDO-SANTOS, V.M., ARCIFA, M.S., BRITO, M.F.G., AGOSTINHO, A.A., HUGHES, R.M., VITULE, J.R. S., SIMBERLOFF, D., OLDEN, J.D. & PELICICE, F.M. 2021. Negative impacts of mining on Neotropical freshwater fishes. Neotrop. Ichthyol. 19(3): e210001.). Small to large sized tributaries deserve special attention, since they may act as biodiversity hotspots in riverine networks (Vitorino Júnior et al. 2016VITORINO JÚNIOR, O.B., FERNANDES, R., AGOSTINHO, C.S., PELICICE, F.M. 2016. Riverine networks constrain β-diversity patterns among fish assemblages in a large Neotropical river. Freshw. Biol. 61(10):1733-1745., Silva et al. 2019SILVA, P.S., MIRANDA, L.E., MAKRAKIS, S., DE ASSUMPÇÃO, L., DIAS, J.H.P., & MAKRAKIS, M.C. 2019. Tributaries as biodiversity preserves: An ichthyoplankton perspective from the severely impounded Upper Paraná River. Aquat. Conserv. Mar. Freshwater Ecosyst. 29(2):258-269.), especially in areas affected by dams (Marques et al. 2018MARQUES, H., DIAS, J.H.P., PERBICHE-NEVES, G., KASHIWAQUI, E.A.L., RAMOS, I.P. 2018. Importance of dam-free tributaries for conserving fish biodiversity in Neotropical reservoirs. Biol. Conserv. 224:347-54., Sanches et al. 2020SANCHES, P.V., GOGOLA, T.M., SILVA, R.O., TOPAN, D.A., PICAPEDRA, P.H.S., PIANA, P.A. 2020. Arms as areas for larval development of migratory fish species in a neotropical reservoir and the influence of rainfall over abundances. J. Fish. Biol. 97: 1306-1316. https://doi.org/10.1111/jfb.14474
https://doi.org/10.1111/jfb.14474... ). However, many small tributaries remain uninvestigated, including those that flow into reservoirs, impeding the assessment of their significance in providing critical habitats for fish recruitment.

The Guareí River is a free-flowing tributary located in the Upper Paranapanema River (Fulan et al. 2012FULAN, J.Á., HENRY, R., & DAVANSO, R. 2012. Os efeitos da ação antrópica sobre a distribuição de macroinvertebrados no Rio Guareí, São Paulo. Estudos de Biologia. 34:51-56.), Upper Paraná River, Brazil. This river flows to the Jurumirim Reservoir (Azevedo-Santos et al. 2020AZEVEDO-SANTOS, V.M., PELICICE, F.M., & HENRY, R. 2020. Knowing biodiversity: Fishes from the Guareí River basin, a tributary of the Jurumirim reservoir, Paranapanema River, Brazil. Biota Neotrop. 20(3):e20201031.), the first impoundment in a “cascade of reservoirs along the Paranapanema River” (Pelicice et al. 2018PELICICE, F.M., AZEVEDO-SANTOS, V.M., ESGUÍCERO, A.L.H., AGOSTINHO, A.A., & ARCIFA, M.S. 2018. Fish diversity in the cascade of reservoirs along the Paranapanema River, southeast Brazil. Neotrop. Ichthyol. 16(2):e170150.: p.1-12). These aspects make the Guareí River an interesting case, with potential to act as spawning site and nursery ground for the regional fish fauna. In this study, we investigated spatial and temporal variations in the distribution of fish eggs and larvae in the Guareí River. In particular, we investigated the hypothesis that the Guareí River is a breeding site for several fish species, including long-distance migratory fish (sensu Agostinho et al. 2003AGOSTINHO, A.A., GOMES, L.C., SUZUKI, H.I., & JÚLIO JR, H.F. 2003. Migratory fish from the upper Parana river basin, Brazil. In: Carolsfeld J, Harvey B, Ross C, Baer A, editors. Migratory fishes of South America: biology, fisheries and conservation status. Victoria: World Fisheries Trust, the World Bank and the International Development Research Centre. p.19-99.), considering that it is a free-flowing river that preserves environmental heterogeneity and habitat diversity. Moreover, adult individuals of several species have been observed in this river (Azevedo-Santos et al. 2020AZEVEDO-SANTOS, V.M., PELICICE, F.M., & HENRY, R. 2020. Knowing biodiversity: Fishes from the Guareí River basin, a tributary of the Jurumirim reservoir, Paranapanema River, Brazil. Biota Neotrop. 20(3):e20201031.), many of which have been reported in Jurumirim Reservoir (see also Kurchevski & Carvalho 2014KURCHEVSKI, G., & CARVALHO, E.D. 2014. Os peixes da represa de Jurumirim: revisão temporal de estudos independentes. In: Henry R, org. Represa de Jurumirim: ecologia, modelagem e aspectos sociais. 1ed. Ribeirão Preto: Holos Editora. p.325-42.). Therefore, we investigated three predictions: (i) the presence of eggs and larvae in the Guareí River, including migratory fishes; (ii) spatial gradients in the distribution of early forms, with more eggs in the upper course of the channel, and the predominance of larvae in lower reaches, as observed in large rivers (e.g., Baumgartner et al. 2004BAUMGARTNER, G., NAKATANI, K., GOMES, L.C., BIALETZKI, A., SANCHES, P.V., & MAKRAKIS, M.C. 2004. Identification of spawning sites and natural nurseries in the upper Paraná River, Brazil. Environ Biol Fish. 71:115-125.); (iii) temporal variations in the density of fish eggs and larvae, considering that reproduction is a seasonal event (e.g., Reynalte-Tataje et al. 2011REYNALTE-TATAJE, D.A., NAKATANI, K., FERNANDES, R., AGOSTINHO, A.A., & BIALETZKI, A. 2011. Temporal distribution of ichthyoplankton in the Ivinhema River (Mato Grosso do Sul State/ Brazil): Influence of environmental variables. Neotrop. Ichthyol.9(2):427-436.).

Material and Methods

1. Study area

The Guareí River (ca. 90 kilometers; Leite et al. 2012LEITE, I., FOGAÇA, C.A., MATOS, D.R.M., FÉLIX, E.S., GOMES, K.F., OLIVEIRA, R.F., HONÓRIO, R.K.A. 2012. A bacia de drenagem, a geomorfologia, os usos do solo, a cobertura vegetal, a precipitação e o canal fluvial do Rio Guareí, p. 31-50. In: Henry R, org. Diagnóstico da qualidade das águas do rio Guareí (Angatuba, SP): uma cooperação ensino superior - educação básica. Botucatu: FUNDIBIO;. 155pp.) is a small tributary flowing into the Jurumirim Reservoir, Upper Paranapanema River (Henry et al. 2016HENRY, R., PAES, J.V.K., KURCHEVSKI, G., SOUSA, J.Q. 2016. A represa de Jurumirim e seus tributários. In: Silva RJ, org. Integridade ambiental da represa de Jurumirim: ictiofauna e relações ecológicas. São Paulo: Editora UNESP;. p.15-30.). Lateral lakes and pools are found along its course, especially in the middle reach (V.M. Azevedo-Santos & R. Henry, personal observation). These lentic environments are connected with the main channel during the wet season. Despite the current hydroelectric expansion in Brazil, which has boosted the construction of small-sized dams (cf., Ferreira et al. 2016FERREIRA, J.H.I., CAMACHO, J.R., MALAGOLI, J.A., & JÚNIOR, S.C.G. 2016. Assessment of the potential of small hydropower development in Brazil. Renew Sust. Energ. Rev. 56:380-387.), the Guareí River still preserves its original lotic condition (i.e., no dams). Currently, the most notable human impacts in this river are the absence of riparian forests in some stretches (Azevedo-Santos et al. 2020AZEVEDO-SANTOS, V.M., PELICICE, F.M., & HENRY, R. 2020. Knowing biodiversity: Fishes from the Guareí River basin, a tributary of the Jurumirim reservoir, Paranapanema River, Brazil. Biota Neotrop. 20(3):e20201031.) and water pollution (e.g., solid wastes).

The three sampling sites cover the entire course of the Guareí River (Fig. 1). Site 1 (S1: ~5 m wide and ~ 629 m asl) is located in the upper course (23°21’56.83”S, 48°10’54.47”W), within the urban perimeter of the municipality of Guareí, São Paulo, Brazil. Site 2 (S2: ~13 m wide and ~ 593 m asl) is located in the middle course (23°27’54.27”S, 48°25’18.95”W), close to the Mineiros bridge, in the municipality of Angatuba, São Paulo. Site 3 S3: ~20 m wide and ~ 579 m asl) is located in the lower course (23°27’51.44”S, 48°34’40.34”W), near Aterradinho village, also in the municipality of Angatuba, São Paulo. Site 3 is located about 5 km (straight line) upstream from the confluence of the Guareí River with the Jurumirim Reservoir.

Figure 1
Location of the three sampling sites (in yellow) along the Guareí River, Paranapanema River basin (upper Paraná River system), São Paulo (modified from Azevedo-Santos et al., 2020AZEVEDO-SANTOS, V.M., PELICICE, F.M., & HENRY, R. 2020. Knowing biodiversity: Fishes from the Guareí River basin, a tributary of the Jurumirim reservoir, Paranapanema River, Brazil. Biota Neotrop. 20(3):e20201031.).

2. Sampling and laboratory analysis

Fish eggs and larvae were sampled about fortnightly (Table S1) from November 2017 to March 2018 (Cycle 1), and from November 2018 to March 2019 (Cycle 2). The sampling period was chosen based on Henry & Suiberto (2014)HENRY, R., & SUIBERTO, M.R. 2014. Distribuição do ictioplâncton na zona de desembocadura do rio Paranapanema na represa de Jurumirim. In: Henry R, org. Represa de Jurumirim: ecologia, modelagem e aspectos sociais. 1ed. Ribeirão Preto: Holos Editora. p.307-23., who indicated a high fish reproductive activity during these months. Collections occurred under license provided by Brazilian Institute of the Environment and Renewable Natural Resources (SISBIO 57047).

Sampling took place at night, as previous studies have shown higher densities of fish eggs and larvae during this period (e.g., Garcia et al. 2018GARCIA, D.A.Z., COSTA, A.D.A., ALMEIDA, F.S., BIALETZKI, A., & ORSI, M.L. 2018. Spatial distribution and habitat use by early fish stages in a dammed river basin, Southern Brazil. Rev. Biol. Trop. 66(2):605-621. http://dx.doi.org/10.15517/rbt.v66i2.33384
http://dx.doi.org/10.15517/rbt.v66i2.333... ). We used an ichthyoplankton net (50 cm diameter and 300 µm mesh) equipped with a flowmeter (“General Oceanics” model 2030BR). The net was positioned near the water surface (~10 cm depth), at the center of the Guareí River channel for 10 minutes (passive collection). We repeated this procedure three times at each sampling event, totaling thirty minutes of water filtered at each site and collection date. The material collected was euthanized with eugenol (procedure frequently used for adult fishes; see Vicentin et al. 2019VICENTIN, W., FERREIRA, F.S., & SÚAREZ, Y.R. 2019. Ichthyofauna of lotic environments in the Ivinhema river basin, upper Paraná river, Mato Grosso do Sul state, Brazil. Biota Neotrop. 19(4):e20190735.) and fixed in formalin with CaCO3_, as recommended in Nakatani et al. (2001)NAKATANI, K., AGOSTINHO, A.A., BAUMGARTNER, G., BIALETZKI, A., SANCHES, P.V., MAKRAKIS, M.C., & PAVANELLI, C. S. 2001. Ovos e larvas de peixes de água doce: Desenvolvimento e manual de identificação. Maringá: EDUEM..

In the laboratory, fish eggs and larvae were washed and separated from other materials (e.g., leaves and debris) using a 50 µm sieve. The resulting sample was screened under a stereomicroscope to separate and count fish eggs and larvae. Fish larvae were determined to the lowest taxonomic level (including species) based on Nakatani et al. (2001)NAKATANI, K., AGOSTINHO, A.A., BAUMGARTNER, G., BIALETZKI, A., SANCHES, P.V., MAKRAKIS, M.C., & PAVANELLI, C. S. 2001. Ovos e larvas de peixes de água doce: Desenvolvimento e manual de identificação. Maringá: EDUEM.. When necessary, a specialist confirmed the identification (see Acknowledgments section). In addition, the fish larvae were classified according to each development stage and degree of notochord flexion, following Ahlstrom & Ball (1954)AHLSTROM, E.H., & BALL, O.P. 1954. Description of eggs and larvae of jack mackerel (Trachurus symmetricus) and distribution and abundance of larvae in 1950 and 1951. Fish Bull. 56:209-245. and adapted by Nakatani et al. (2001; p. 33)NAKATANI, K., AGOSTINHO, A.A., BAUMGARTNER, G., BIALETZKI, A., SANCHES, P.V., MAKRAKIS, M.C., & PAVANELLI, C. S. 2001. Ovos e larvas de peixes de água doce: Desenvolvimento e manual de identificação. Maringá: EDUEM.: “yolk-sac larvae”, “pre-flexion”, “flexion” or “post-flexion”.

The volume of water filtered by the planktonic net was calculated based on Nakatani et al. (2001)NAKATANI, K., AGOSTINHO, A.A., BAUMGARTNER, G., BIALETZKI, A., SANCHES, P.V., MAKRAKIS, M.C., & PAVANELLI, C. S. 2001. Ovos e larvas de peixes de água doce: Desenvolvimento e manual de identificação. Maringá: EDUEM.. Fish eggs and larval densities (number/10m³) were calculated based on Tanaka (1973)TANAKA S. 1973. Stock assessment by means of ichthyoplankton surveys. FAO Fish. 122:33-51. apud Nakatani et al. (2001)NAKATANI, K., AGOSTINHO, A.A., BAUMGARTNER, G., BIALETZKI, A., SANCHES, P.V., MAKRAKIS, M.C., & PAVANELLI, C. S. 2001. Ovos e larvas de peixes de água doce: Desenvolvimento e manual de identificação. Maringá: EDUEM., considering adaptations made by these last authors.

3. Data analysis

We ran a Non-Metric Multidimensional Scaling analysis (NMDS) (Clarke & Warwick, 2001CLARKE, K, & WARWICK, R.M. 2001. Change in Marine Communities: An Approach to Statistical Analysis and Interpretation. Plymouth: National Environment Research Council.) to investigate assemblage structure patterns among sites (S1, S2, and S3). For this analysis, fish larvae were grouped at the family level, as many individuals were not identified at the species or genus levels. Density data were log (x + 1) transformed, and the analysis employed Bray-Curtis similarity (Clarke & Warwick, 2001CLARKE, K, & WARWICK, R.M. 2001. Change in Marine Communities: An Approach to Statistical Analysis and Interpretation. Plymouth: National Environment Research Council.)with the insertion of Dummy (0.01). Subsequently, a PERMANOVA was applied to test for differences in assemblage structure (p < 0.05) among sites. The NMDS and PERMANOVA were run in PRIMER 6.0.

Significant differences (p < 0.05) in egg and larval densities were investigated with non-parametric Anova (Kruskal-Wallis, H), considering sites (S1, S2 and S3) and sampling events (e.g., Jan A, Jan B, so on) as factors. This same analysis was used to test differences between larval development stages in each sampling site. Data in each cycle were investigated separately. The Dunn test (Dunn, 1964DUNN, O.J. 1964. Multiple comparisons using rank sums. Technometrics. 6:241-52.) indicated differences among groups. Both tests, H and Dunn, were run in the FSA package (Ogle et al. 2019OGLE, D.H., WHEELER, P., & DINNO, A. 2019. FSA: fisheries stock analysis. R package version 0.8, 24.. Available from: https://cran.r-project.org/web/packages/FSA/FSA.pdf
https://cran.r-project.org/web/packages/... ), R environment (R Development Core Team, 2018R DEVELOPMENT CORE TEAM. 2018. R: A Language and Environment for Statistical Computing. R Foundation for Statistical Computing, Vienna, Austria: ISBN 3-900051-07-0. Available from: www.r-project.org/
www.r-project.org/... ).

Results

We collected 859 fish eggs and 1,340 larvae in the Guareí River: 567 eggs and 307 larvae in Cycle 1, and 292 eggs and 1,033 larvae in Cycle 2. Captured larvae were identified up to the maximum level of class (total of one), orders (two), families (seven), genera (two), or species (seven). A total of 19 taxa was recorded (Table 1; see also Table S2): 15 in Cycle 1, and 15 in Cycle 2. Seven taxa were identified at the specific epithet level. Among them is Prochilodus lineatus (Valenciennes, 1837) [Prochilodontidae], a long-distance migratory fish, but these larvae were recorded only in two sites during Cycle 2 (Table 1).

Assemblage structure of larvae was similar among sampling sites in both cycles (Fig. 2a-b). Within each cycle, no significant differences in assemblage structure between sampling sites were observed (Permanova, p = 0.24 and p= 0.11).

Figure 2
NMDS depicting spatial variation in fish larvae structure (Family level) in sampled sites along the Guareí River, Paranapanema River basin (upper Paraná River system), São Paulo, Brazil, in the: (a) Cycle 1; (b) Cycle 2.

In Cycle 1, egg density ranged between 3.3 and 12.5 eggs/10m³, and larval density ranged between 8.7 and 14.9 larvae/10m³ among sampling sites (Fig. 3a-b). In Cycle 2, egg density ranged between 1.8 and 7.4 eggs/10m³, and larval density ranged between 18.0 and 46.7 larvae/10m³ among sampling sites (Fig. 3c-d). There were no significant spatial differences in the density of fish eggs (Kruskall-Wallis, Cycle 1: p = 0.72; Cycle 2: p = 0.72) and larvae (Kruskall-Wallis, Cycle 1: p = 0.97; Cycle 2: p = 0.78) among sites in both cycles.

Figure 3
Average density and standard deviation of the ichthyoplankton collected at each site along the Guareí River: (a) eggs and (b) larvae, Cycle 1; and (c) eggs and (d) larvae, Cycle 2.

Temporally, egg density ranged between 0 and 36.4 eggs/10m³, and larvae between 0.5 and 42.1 larvae/10m³ in Cycle 1 (Fig. 4a-b). On the other hand, egg density ranged between 0 and 17.9 eggs/10m³, and larvae between 1.1 and 158.0 larvae/10m³ in Cycle 2 (Fig. 4c-d). Statistical differences were found only for fish larvae in Cycle 2 (Table S3).

Figure 4
Average density of the ichthyoplankton collected at each sampling occasion in all sites along the Guareí River: (a) eggs and (b) larvae, Cycle 1; and (c) eggs and (d) larvae, Cycle 2.

We recorded four different developmental stages for fish larvae (Fig. 5). In Cycle 1, the yolk-sac larvae stage was predominated (Fig. 5b; Table S4).

Figure 5
Average density of each larvae stages collected in (a) Cycle 1 and (b) Cycle 2 in each site of the Guareí River.

Discussion

Our results support the hypothesis that the Guareí River is a reproductive area for rheophilic and migratory fishes. We expect the same pattern in other small rivers flowing to hydroelectric reservoirs in Brazil. In general, the role of small tributaries in areas affected by impoundments - like the Guareí River - as spawning areas for fishes are often overlooked. In fact, studies usually focus on medium or large-sized rivers (e.g., Silva et al. 2019SILVA, P.S., MIRANDA, L.E., MAKRAKIS, S., DE ASSUMPÇÃO, L., DIAS, J.H.P., & MAKRAKIS, M.C. 2019. Tributaries as biodiversity preserves: An ichthyoplankton perspective from the severely impounded Upper Paraná River. Aquat. Conserv. Mar. Freshwater Ecosyst. 29(2):258-269.). Therefore, we fill an important knowledge gap concerning the distribution of fish eggs and larvae in these small watercourses.

We recorded larvae of the migratory fish Prochilodus lineatus, although in low density. This species is able to migrate hundreds of kilometers to spawn (Agostinho et al. 1993AGOSTINHO, A.A., VAZZOLER, A.E.A.M., GOMES, L.C., & OKADA, E.K. 1993. Estratificación espacial y comportamiento de Prochilodus scrofa en distintas fases del ciclo de vida, en la planicie de inundación del alto río Paraná y embalse de Itaipu, Paraná, Brazil. Rev. Hydrobiol. Trop. 26:79-90., Agostinho et al. 2003AGOSTINHO, A.A., GOMES, L.C., SUZUKI, H.I., & JÚLIO JR, H.F. 2003. Migratory fish from the upper Parana river basin, Brazil. In: Carolsfeld J, Harvey B, Ross C, Baer A, editors. Migratory fishes of South America: biology, fisheries and conservation status. Victoria: World Fisheries Trust, the World Bank and the International Development Research Centre. p.19-99.), but dams block migration routes and affect spawning dynamics - and populations usually decline in the impoundment and adjacent areas. The presence of preserved tributaries (i.e., free-from-dams) can sustain populations (e.g., Agostinho et al. 2003AGOSTINHO, A.A., GOMES, L.C., SUZUKI, H.I., & JÚLIO JR, H.F. 2003. Migratory fish from the upper Parana river basin, Brazil. In: Carolsfeld J, Harvey B, Ross C, Baer A, editors. Migratory fishes of South America: biology, fisheries and conservation status. Victoria: World Fisheries Trust, the World Bank and the International Development Research Centre. p.19-99., Lopes et al. 2019LOPES, J.D.M., POMPEU, P.S., ALVES, C.B.M., PERESSIN, A., PRADO, I.G., SUZUKI, F.M., FACCHIN, S., & KALAPOTHAKIS, E. 2019. The critical importance of an undammed river segment to the reproductive cycle of a migratory Neotropical fish. Ecol. Freshw. Fish. 28(2):302-316.), and the Guareí River seems to offer conditions for spawning. Our sampling also detected the presence of Schizodon nasutus Kner, 1858, a medium-sized rheophilic fish (maximum ~ 30 cm) (Nobile et al. 2015NOBILE, A.B., BRAMBILLA, E.M., LIMA, F.P., FREITAS-SOUZA, D., BAYONA-PEREZ, I.L., & CARVALHO, E.D. 2015. Length-weight relationship of 37 fish species from the Taquari River (Paranapanema Basin, Brazil). J. Appl. Ichthyol. 31(3):580-582.), that migrates small distances to spawn (Carvalho et al. 1998CARVALHO, E.D., FUJIHARA, C.Y., & HENRY, R. 1998. A study of the ichthyofauna of the Jurumirim reservoir (Paranapanema River, São Paulo State, Brazil): fish production and dominant species in three sites. Verh Int Verein Limnol. 26:2199-2202.). Other species found were Apareiodon affinis (Steindachner, 1879), Bryconamericus aff. iheringii (Boulenger, 1887), Hoplias malabaricus (Bloch, 1794), Hoplosternum littorale (Hancock, 1828), and Serrasalmus maculatus Kner, 1858, non-migratory fishes (sensu Vazzoler 1996VAZZOLER, A.E.A.M. 1996. Biologia da reprodução de peixes teleósteos: teoria e prática / Anna Emília Amato de Moraes Vazzoler; Apresentação de Paulo de Tarso da Cunha Chaves -- Maringá: EDUEM ; São Paulo : SBI. 169 p.: il. 1996) with populations in the Jurumirim Reservoir (Kurchevski & Carvalho 2014KURCHEVSKI, G., & CARVALHO, E.D. 2014. Os peixes da represa de Jurumirim: revisão temporal de estudos independentes. In: Henry R, org. Represa de Jurumirim: ecologia, modelagem e aspectos sociais. 1ed. Ribeirão Preto: Holos Editora. p.325-42.). The reproduction of these species - which include several with commercial importance (see Novaes & Carvalho 2009NOVAES, J.L.C. & CARVALHO, E.D. 2009. Recursos pesqueiros oriundos da pesca artesanal no reservatório de Jurumirim, Rio Paranapanema, Alto Paraná, Brasil. Bol. Inst. Pesca 35: 553-565.) - is indication that the Guareí River serves as a spawning ground for fishes with different reproductive strategies.

The fish larval structure was similar among sampling sites. However, it should be noted that the analyses were carried out mostly at the family level, since the identification of species is difficult (see Bialetzki et al. 2016BIALETZKI, A. , GARCIA, D.A.Z., & ORSI, M.L. 2016. O estudo de ovos e larvas de peixes. In: ORSI, M.L., ALMEIDA, F.S., SWARÇA, A.C., CLARO-GARCIA, A., VIANNA, N.C., GARCIA, D.A.Z., &BIALETZKI, A., org. Ovos, larvas e juvenis dos peixes da Bacia do Rio Paranapanema, uma avaliação para a conservação. 1ed. Assis: Triunfal Gráfica e Editora. p.17-24., for a broader discussion on this issue). This aspect may have precluded the observation of differences in structure among sites. For example, representatives of Characidae may be widely distributed along the river, but with different species occupying the gradient headwater-mouth (Azevedo-Santos et al. 2020AZEVEDO-SANTOS, V.M., PELICICE, F.M., & HENRY, R. 2020. Knowing biodiversity: Fishes from the Guareí River basin, a tributary of the Jurumirim reservoir, Paranapanema River, Brazil. Biota Neotrop. 20(3):e20201031.).

Contrary to our prediction, we observed no spatial gradient in the distribution of fish eggs and larvae along the Guareí River. The predominance of non-migratory fish in this river (Azevedo-Santos et al. 2020AZEVEDO-SANTOS, V.M., PELICICE, F.M., & HENRY, R. 2020. Knowing biodiversity: Fishes from the Guareí River basin, a tributary of the Jurumirim reservoir, Paranapanema River, Brazil. Biota Neotrop. 20(3):e20201031.) may explain this pattern. Non-migratory fish release eggs in different localities along the river (Baumgartner et al. 2004BAUMGARTNER, G., NAKATANI, K., GOMES, L.C., BIALETZKI, A., SANCHES, P.V., & MAKRAKIS, M.C. 2004. Identification of spawning sites and natural nurseries in the upper Paraná River, Brazil. Environ Biol Fish. 71:115-125.), probably in the confluence with streams; they do not need to reach upper stretches to spawn, which is a common behavior among migratory fishes (Agostinho et al. 2003AGOSTINHO, A.A., GOMES, L.C., SUZUKI, H.I., & JÚLIO JR, H.F. 2003. Migratory fish from the upper Parana river basin, Brazil. In: Carolsfeld J, Harvey B, Ross C, Baer A, editors. Migratory fishes of South America: biology, fisheries and conservation status. Victoria: World Fisheries Trust, the World Bank and the International Development Research Centre. p.19-99., Baumgartner et al. 2004BAUMGARTNER, G., NAKATANI, K., GOMES, L.C., BIALETZKI, A., SANCHES, P.V., & MAKRAKIS, M.C. 2004. Identification of spawning sites and natural nurseries in the upper Paraná River, Brazil. Environ Biol Fish. 71:115-125.). Our results show that in small rivers like the Guareí River (with few migratory species), different reaches may provide adequate conditions for spawning.

Contrary to our prediction, no significant temporal differences were found during Cycle 1, as reproduction occurred in all sampling months. Significant differences between peaks were found only for fish larvae in November A and December A during Cycle 2, reflecting the increased reproductive activity in these occasions. Our results suggest that small rivers like the Guareí do not present a clear temporal pattern of reproduction peaks. Other possible explanation is that sampling occurred only in six months, when most species in the region reproduce (Henry & Suiberto 2014HENRY, R., & SUIBERTO, M.R. 2014. Distribuição do ictioplâncton na zona de desembocadura do rio Paranapanema na represa de Jurumirim. In: Henry R, org. Represa de Jurumirim: ecologia, modelagem e aspectos sociais. 1ed. Ribeirão Preto: Holos Editora. p.307-23.). Therefore, the detection of fine scale patterns and spawning peaks would require a different sampling design (i.e., considering samplings in all months of the year).

At each site, the larval development stage was variable. In general, after hatching, the fish larvae develop during the drifting process in the river (Baumgartner et al. 2004BAUMGARTNER, G., NAKATANI, K., GOMES, L.C., BIALETZKI, A., SANCHES, P.V., & MAKRAKIS, M.C. 2004. Identification of spawning sites and natural nurseries in the upper Paraná River, Brazil. Environ Biol Fish. 71:115-125.). Then, results indicate that the river channel is an area of dispersion and growth. Considering the two cycles, fish larvae in more advanced development stages (i.e., flexion and post-flexion) had lower densities when compared to the other stages (e.g., yolk-sac larvae). As we used passive collection, we believe that larvae in flexion and post-flexion stages show some type of mobility (se also Sanches et al. 2020SANCHES, P.V., GOGOLA, T.M., SILVA, R.O., TOPAN, D.A., PICAPEDRA, P.H.S., PIANA, P.A. 2020. Arms as areas for larval development of migratory fish species in a neotropical reservoir and the influence of rainfall over abundances. J. Fish. Biol. 97: 1306-1316. https://doi.org/10.1111/jfb.14474
https://doi.org/10.1111/jfb.14474... ), impeding them to drift with the river flux up to the ichthyoplankton net. Another possibility is that fish larvae in these more advanced stages are able to remain in areas less affected by the current (i.e., backwaters, including those close to vegetation), or in lateral lakes and ponds, which offer favorable conditions for feeding and survival.

A number of studies (e.g., Nunes et al. 2015NUNES, D.M.F., MAGALHÃES, A.L.B., WEBER, A.A., GOMES, R.Z., NORMANDO, F.T., SANTIAGO, K.B., RIZZO, E., & BAZZOLI, N. 2015. Influence of a large dam and importance of an undammed tributary on the reproductive ecology of the threatened fish matrinxã Brycon orthotaenia Günther, 1864 (Characiformes: Bryconidae) in southeastern Brazil. Neotrop. Ichthyol. 13(2):317-324., Silva et al. 2015SILVA, P.S., MAKRAKIS, M.C., MIRANDA, L.E., MAKRAKIS, S., ASSUMPÇÃO, L., PAULA, S., DIAS, J.H.P., & MARQUES, H. 2015. Importance of reservoir tributaries to spawning of migratory fish in the upper Paraná River. River Res. Appl. 31:313-322., Marques et al. 2018MARQUES, H., DIAS, J.H.P., PERBICHE-NEVES, G., KASHIWAQUI, E.A.L., RAMOS, I.P. 2018. Importance of dam-free tributaries for conserving fish biodiversity in Neotropical reservoirs. Biol. Conserv. 224:347-54., Silva et al. 2019SILVA, P.S., MIRANDA, L.E., MAKRAKIS, S., DE ASSUMPÇÃO, L., DIAS, J.H.P., & MAKRAKIS, M.C. 2019. Tributaries as biodiversity preserves: An ichthyoplankton perspective from the severely impounded Upper Paraná River. Aquat. Conserv. Mar. Freshwater Ecosyst. 29(2):258-269., Sanches et al. 2020SANCHES, P.V., GOGOLA, T.M., SILVA, R.O., TOPAN, D.A., PICAPEDRA, P.H.S., PIANA, P.A. 2020. Arms as areas for larval development of migratory fish species in a neotropical reservoir and the influence of rainfall over abundances. J. Fish. Biol. 97: 1306-1316. https://doi.org/10.1111/jfb.14474
https://doi.org/10.1111/jfb.14474... ) have shown or suggested the importance of tributaries for the maintenance of fish diversity in impounded areas. Therefore, the data presented here indicate that the Guareí River is important to the maintenance of fish assemblages in the Jurumirim Reservoir, given the connectivity between the two areas (i.e., free-flowing river and dam). First, it is possible that migratory species (short or long distance) found in the Guareí River, and that also occur in the Jurumirim Reservoir (e.g., S. nasutus, P. lineatus; Kurchevski & Carvalho 2014KURCHEVSKI, G., & CARVALHO, E.D. 2014. Os peixes da represa de Jurumirim: revisão temporal de estudos independentes. In: Henry R, org. Represa de Jurumirim: ecologia, modelagem e aspectos sociais. 1ed. Ribeirão Preto: Holos Editora. p.325-42.), are using this tributary as a route to complete their reproduction; since this affluent has important characteristics that many species require for spawning (e.g., rapids, free-flow). Secondly, is possible that larvae of migratory or sedentary species that reproduce in the Guareí River may reach the Jurumirim Reservoir through drift; and in viable development stages for feeding and displacement. For example, Rosa et al. (2020)ROSA, R.R., SILVA, J.C., & BIALETZKI, A. 2020. Long-term monitoring of potamodromous migratory fish larvae in an undammed river. Mar. Fresh. Res. 71(3):384-93 estimated that fish larvae can drift long distances until they reach growth areas. In the case of the Guareí River, larvae of P. lineatus (especially in stages more susceptible to drift, e.g., pre-flexion) may reach the Jurumirim Reservoir and, once in it, individuals will be able to find growth areas with food such as zooplankton, a resource present in dammed environments and freshwater areas adjacent to them (e.g., Arcifa 1984ARCIFA, M.S. 1984. Zooplankton composition of ten reservoirs in southern Brazil. Hydrobiologia 113:137-145., Nogueira 2001NOGUEIRA, M.G. 2001. Zooplankton composition, dominance and abundance as indicators of environmental compartmentalization in Jurumirim Reservoir (Paranapanema River), São Paulo, Brazil. Hydrobiologia 455:1-18., Casanova & Henry 2004CASANOVA, S.M.C., & HENRY, R. 2004. Longitudinal distribution of Copepoda populations in the transition zone of Paranapanema river and Jurumirim Reservoir (São Paulo, Brazil) and interchange with two lateral lakes. Braz. J. Biol. 64:11-26., Coelho & Henry 2021COELHO, P.N., & HENRY, R. 2021. Functional groups of microcrustaceans along a horizontal gradient in a Neotropical lake colonized by macrophytes. Aquat. Sci. 83:3. https://doi.org/10.1007/s00027-020-00759-3
https://doi.org/10.1007/s00027-020-00759... ). This second explanation is reinforced by the data of Henry & Suiberto (2014)HENRY, R., & SUIBERTO, M.R. 2014. Distribuição do ictioplâncton na zona de desembocadura do rio Paranapanema na represa de Jurumirim. In: Henry R, org. Represa de Jurumirim: ecologia, modelagem e aspectos sociais. 1ed. Ribeirão Preto: Holos Editora. p.307-23., who showed the presence of P. maculatus larvae and other species at the confluence of the Guareí River with the Jurumirim Reservoir. Third, species may reproduce in the Guareí River, grow in this tributary, and then move to the Jurumirim Reservoir in short (seasonal) or long term (years) for different reasons, especially in search of food, space, or refuge (e.g., macrophytes) (Fig. 6). In other words, we believe that the Guareí River may serve as a “biodiversity source” to the Jurumirim Reservoir. Thus, due to its importance for the reproduction of several fish species - and because it is a lotic tributary of a reservoir -, we emphasize the need to maintain its natural flow regime.

Figure 6
Conceptual model showing how the Guareí River may serve as a “biodiversity source” for the Jurumirim Reservoir, São Paulo, Brazil.

Conclusion

Our results confirmed the hypothesis that the Guareí River is a spawning area for several species, including long-distance migratory fish. However, we detected no spatial differences in assemblage structure or in the densities of fish eggs and larvae along the Guareí River. In addition, we recorded temporal differences in densities of fish larvae only in Cycle 2. As an important watercourse for fish reproduction, we recommend maintaining natural flow regime.

Acknowledgments

We are grateful to Andréa Bialetzki (Nupélia/UEM), for helping with the identification of Prochilodus lineatus larvae and other taxa, and Eduardo M. Brambilla (UNESP), for helping with the first identification of larvae during Cycle 1 and for running NMDS and PERMANOVA for larvae structure. We also thank Paula N. Coelho (UNESP), for helping with statistical analysis (including NMDS and Anova) and data interpretation, and Marcelo F. G. Brito (UFS), for providing relevant suggestions for the title. Hamilton A. Rodrigues (UNESP) and Paula N. Coelho provided assistance during fieldwork. Andréa Bialetzki (Nupélia/UEM), Angelo A. Agostinho (Nupélia/UEM), Reinaldo J. Castro (UFSCar), and André B. Nobile (UNESP), for provided comments and important suggestions in a version in Portuguese. Robert M. Hughes (OSU), for providing relevant suggestions in a version in English. This study was funded by CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - 001; VMAS) and CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico; FMP).

Supplementary Material

The following online material is available for this article

Table S1 - Detailed information about the months, their acronyms, and dates of collections at the three sample sites along the Guareí River, São Paulo, Brazil.

Table S2 - Classification (sensu Fricke et al., 2018a, Fricke et al., 2018b) of each taxon captured in the Guareí River, São Paulo, Brazil.

Table S3 - Significant results of Dunn’s test for larvae collected in the Cycle 2 in the Guareí River, São Paulo, Brazil.

Table S4 - Results of the statistical analysis of the larval stages.

References

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