RESUMEN
The relationships between fish assemblage structure and environmental variables along a pollution gradient in the Medina River were analyzed over a year in four sampling sites (S1-S4). The river flows in a mountain-plain transition and is affected by several small town wastewater and sugar cane industries effluents. Environmental variables were divided in two sets, hereafter named -pollution- and -natural-. The first set included water quality variables modified by anthropogenic activities such as D.O. (Dissolved Oxygen), C.O.D. (Chemical Demand Oxygen), and dissolved ion concentrations. Natural variables included altitude, position, and time of the year. The upstream site (S1) had the lowest species richness and C.P.U.E. (catchperunitofeffort). The number of species and density increased downriver(S2-S3). S1 was inhabited by an invertivor especies (Trichomycteruscorduvensis) that has low tolerance to adverse environmental conditions, and has high D.O. requirements. S4 sustained the most tolerant and abundant species (Otocinclus vittatus, Corydoras paleatus), which endure the lowest D.O. and the highest C.O.D. A Canonical Correspondence Analysis for natural variables showed a significant gradient of species composition related to altitude and discharge. Water quality degradation by sugar cane factories and urban development, coupled with natural climatic, topographic and hydrological factors explained a significant amount of spatial and temporal variation in fish community structure (48%). Natural and pollution variables shared about 15% of total variance. However, pollution variables were not significant after partitioning out the effects of natural variables. Natural variability remained significant after removal of pollution effects.
Las relaciones entre la estructura de una comunidad de peces y variables ambientales a lo largo de un gradiente de polución en el Río Medina fue analizado por un año en cuatro sitios muestreados (S1-S4). El río corre en una transición de montaña-llanura y es afectado por descargas de efluentes de pueblos e ingenios azucareros. Las variables ambientales fueron divididas en dos grupos, denominados -polución- y -natural-. El primer conjunto incluyó variables de calidad de agua modificadas por actividades antropogénicas tales como D.O. (Oxígeno Disuelto), C.O.D. (Demanda Química de Oxígeno), y concentración de iones disueltos. Las variables naturales incluidas fueron altitud, ubicación, y tiempo del año. Río arriba (S1) tuvo la más baja riqueza de especies y C.P.U.E. (Captura Por Unidad de Esfuerzo); pero río abajo incrementa el número de especies y densidad (S2-S3). S1 estaba habitado por especies invertívoras (Trichomycterus corduvensis), teniendo baja tolerancia a condiciones ambientales y altos requerimientos de D.O. S4 tuvo las especies más tolerantes y abundantes (Otocinclus vittatus, Corydoras paleatus), soportando los más bajos D.O. y los más elevados C.O.D. Un Análisis de Correspondencia Canónico mostró un gradiente en la composición de especies relacionado a altitud y descarga para las variables naturales. La degradación de la calidad del agua por ingenios azucareros y efluentes urbanos, con factores climáticos naturales, topográfico e hidrológico explicaron la variación espacial y temporal de la estructura de comunidad de peces (=48%). Variables naturales y contaminación compartieron el 15% de la varianza total. Sin embargo, las variables de contaminación no fueron significates después de la partición de los efectos de las variables natural, pero la variabilidad natural permaneció significativa después de remover los efectos de polución.
RESUMEN
The population structure and dynamics of Leporinus acutidens (Valenciennes, 1837), upstream from Yacyretá Dam (Upper Paraná River, Argentina) were analyzed for their relation to age, condition factor, growth, mortality and fishery yield. The pre- (1990-94) and post- (1995-98) filling phases of the reservoir were compared with regard to age structure, condition factor, and the length/weight relationship of fish. Fish sampling was carried out at four sampling sites following a longitudinal gradient along the river. The catches took place monthly using eight monofilament gillnets (mesh 40 to 160 mm). The growth marks on the scales were annual (9-year maximum). The study of the variation in the monthly averages of the scale marginal increments revealed that the period of the growth ring formation shifted from winter to spring in the post-filling phase. The length/weight relationship coefficient (b) was higher after dam closing. The condition factor increased before the reproduction period and during late summer, reaching maximum values in the post-filling phase, and especially at the sampling site placed within the reservoir region. The von Bertalanffy growth coefficient k for both sexes was 0.16, and the asymptotic length L" reached 338 mm. Natural mortality during the whole study period, according to Pauly's equation, was 0.25 years -1 and total mortality was 0.37 years -1. The Beverton-Holt yield per recruit (Y/R) model attained 57.23 g, with a maximum sustainable yield (MSY) of 70.75 g. Low total mortality was associated with the lack of commercial exploitation of the species.
A população de Leporinus acutidens (Valenciennes, 1837), a montante da barragem de Yacyretá (Alto Paraná, Argentina) foi analisada em relação à idade, crescimento, condição nutricional, mortalidade e rendimento da pesca, sendo realizadas comparações na estrutura etária, condição nutricional e relação peso-comprimento nos períodos pré (1990-94) e pós (1995-98) represamento. As amostragens foram conduzidas em quatro localidades ao longo do rio. As capturas foram realizadas mensalmente com oito redes monofilamento (malhagens de 40 a 160 mm). Os anéis de crescimento nas escamas são formados anualmente (máximo 9 anos). A análise das variações mensais na média do incremento marginal das escamas revelou que o período de formação dos anéis de crescimento alterou do inverno para a primavera após o represamento. Já o coeficiente da relação peso/comprimento (b) foi maior, para ambos os sexos, após o represamento. O fator de condição apresentou-se mais elevado antes do período reprodutivo e no final do verão, alcançando valores máximos após o represamento, especialmente em locais dentro do reservatório. O valor do coeficiente k da curva de crescimento de von Bertalanffy foi 0.16 e o comprimento assintótico L" alcançou 338 mm. A mortalidade natural, estimada a partir da equação de Pauly, foi de 0.25 year-1 e a mortalidade total de 0.37 year-1. O rendimento por recruta, estimado pelo modelo de Beverton-Holt (Y/R) atingiu 57.23 g, com um rendimento máximo sustentável (RMS) de 70.75 g. A baixa mortalidade total foi associada com a inexistência de uma exploração comercial relevante do estoque.