Effects of the interannual variations in the flood pulse mediated by hypoxia tolerance: the case of the fish assemblages in the upper Parana River floodplain

The effects of the duration of the floods on abiotic variables and attributes of species tolerant and intolerant to hypoxia (STH and SIH respectively) were evaluated in rivers and lakes of the upper Paraná River. Fish were sampled once a year, in six sampling stations, during the high water period from 2000 to 2003. There were overall reductions in dissolved oxygen levels and increases in transparency of water in lakes in years of moderate floods. The duration of floods influenced species differentially based on their tolerance to hypoxia: in moderate floods, richness of STH increased and numerical abundance and biomass of SIH reduced significantly. Opposite relationships were detected between dissolved oxygen and the attributes of STH and SIH. Dissolved oxygen was the best predictor of variability of STH and SIH in years of moderate floods, whereas water transparency predicted significant amounts of STH in years of short floods. Being positively affected by dissolved oxygen reductions, STH seem to take advantages in persisting in seasonally harsh lentic habitats. The incorporation of abiotic data as well the differential tolerance of species to hypoxia would improve further investigations of the effects of interannual variations in the flood pulse on tropical fish assemblages.

Os efeitos da duração da cheia sobre algumas variáveis abióticas e atributos das assembleias de peixes (espécies tolerantes e intolerantes à hipóxia, STH e SIH, respectivamente) foram avaliados em rios e lagoas do alto rio Paraná. Os peixes foram amostrados uma vez ao ano, em seis localidades, durante o período de águas altas entre 2000 e 2003. Houve reduções generalizadas nos níveis de oxigênio dissolvido e aumentos na transparência da água em lagos em anos de cheias moderadas. A duração das cheias influenciou as espécies diferencialmente em função de sua tolerância à hipóxia: em anos de cheias moderadas, a riqueza das STH aumentou e a abundância numérica e de biomassa das SIH reduziram significativamente. Relações opostas foram detectadas entre o oxigênio dissolvido e os atributos das STH e SIH. O oxigênio dissolvido foi o melhor preditor da variabilidade de STH e SIH em anos de cheias moderadas, enquanto a transparência da água explicou uma significativa proporção das STH em anos de cheias curtas. Por serem positivamente afetadas pelas reduções no oxigênio dissolvido, as STH parecem ter vantagens em persistir em ambientes lênticos sazonalmente inóspitos. A incorporação de dados abióticos bem como da tolerância diferencial das espécies à hipóxia deve aprimorar as investigações dos efeitos das variações interanuais no pulso de inundação nas assembleias de peixes tropicais.

Estudio preliminar de la ictiofauna y los hábitats acuáticos del Río Bajo Paraguá, Santa Cruz, Bolivia / Preliminary study of fish fauna and aquatic habitats in the Lower Paraguá River, Santa Cruz, Bolivia

A fin de conocer la riqueza íctica del Río Bajo Paraguá (cuenca del Iténez o Guaporé) se realizaron colectas de peces y análisis físico-químicos del agua en tres localidades del norte de Santa Cruz, sobre el límite oeste del Parque Noel Kempff Mercado. Los muestreos duraron 5 días en cada sitio (Piso Firme, Florida y El Refugio) y en dos épocas del año (aguas altas: Abril-Mayo; aguas bajas: Septiembre). Utilizando diferentes artes de pesca se capturaron 4098 peces pertenecientes a 124 especies, de las cuales 35 fueron nuevos registros para el Parque Noel Kempff Mercado. De las 124 especies, el 56 por ciento correspondió al orden Characiformes, 22 por ciento a Siluriformes (con 34 spp), 15 por ciento a Perciformes (18 cíclidos), 5 por ciento a Gymnotiformes y 2 por ciento a Synbranchiformes, Rajiformes y Beloniformes. La riqueza de especies fue menor en el curso superior del río (Piso Firme: 90 especies, Florida: 71 especies y El Refugio: 23 especies), al igual que la concentración de oxígeno que en El Refugio alcanzó niveles críticos en la época de escaso caudal por la mayor cobertura de plantas flotantes. La proporción de nuevos registros geográficos y los interrogantes sobre distribución y abundancia de los peces del Rio Paraguá indican la necesidad de implementar monitoreos sistemáticos asociados a los planes de manejo de la zona.

With the aim of improving our knowledge about the fish fauna of the Lower Paragua River (Iténez or Guaporé basin), fish collections and physic-chemical analyses of water were conducted in three localities in northern Santa Cruz, on the western border of the Noel Kempff Mercado National Park. Sampling periods lasted 5 days on each of three sites (Piso Firme, Florida y El Refugio) and two seasons (high waters: April-May, low waters: September). By using various fishing methods, 4098 fish belonging to 124 species were collected, 35 of them were new geographic records for the National Park. From all the species, 56 percent belonged to the Characiformes, 22 percent to Siluriformes (34 spp.), 15 percent Perciformes (18 spp. of cichlids), 5 percent Gymnotiformes and 2 percent Synbranchiformes, Rajiformes and Beloniformes. Species richness was low in the upper course of the river (Piso Firme: 90 species, Florida: 71 species, El Refugio: 23 species) as well as oxygen levels which reached a critical minimum at El Refugio during low waters and increased cover of floating vegetation. The proportion of new geographic records and the uncertainties about fish distribution and abundance indicate the need to conduct monitoring actions associated to the management plans in the area.