Abstract
The diversity, abundance and distributional pattern of freshwater fish communities in the Térraba River, south Costa Rica, were investigated from the early dry season of 2004 to early rain season of 2005. There have been no preview studies on the freshwater fish distribution in Térraba. Therefore, the aims of this study were to determine fish species richness, abundance and distribution there. Fish sampling was done using a combination of gears such as gill net, fine mesh net and visual observation. Thirty three species, 26 genera and 14 families were collected in four sampling sites along the river. The number and biomass captured during the entire study was 984 individuals and 147 410.9 g respectively. Most of them are carnivorous species (48%), 33.3% are omnivorous and 12% detritivorous, and only two species are herbivorous. The most important species in relative abundance (56.5%) and biomass (53.7%) in the study area was the machaca (Brycon behreae). It should be clear that although the list of fish species that occur in Térraba River is reasonably complete, knowledge of their ichthyogeographic history patterns is superficial. The main community component was secondary freshwater species; with 17 invading brackish water species and one introduced species (tilapia O. niloticus). Nine species are reported for the first time in this river. The diversity index H' varied from 2.32 (El Brujo) to 1.67 (Coto), a similar pattern was also showed for the other indexes. Most of our results were similar to those of previous studies on freshwater fish distribution elsewhere, however no significant correlation between species distribution and environmental variables was found, and we hypothesized that the depth and water velocity and geomorphological are major environmental variables that influence the fish distribution. Our findings are opposed to the tendency, for species composition, to increase from upstream to the mouth of the river, which is probably due to two major human activities (discharge of waste of pineapple fields and sediments in the main channel). These activities could constitute in the future a real threat for the fish population and other aquatic organisms. Growth rates, competition, predation pressures, community organization, biotic versus physical factors in relation to distribution, and many other aspects of fish biology remain to be studied. Future surveys will involve more quantitative surveys in different seasons and over time to monitor the long-term variation in the diversity and abundance of freshwater fish species in Térraba.
Highlights
Ictiológicamente el río Grande de Térraba pertenece a la Provincia Ístmica (Bussing 1998) y es uno de los ecosistemas riverinos más importantes del Pacífico Sur de Costa Rica, no por la compleja red acuática que reúne, sino porque desemboca en la zona deltáica del Humedal Nacional de Térraba-Sierpe, área protegida con la mayor superficie de bosque de manglar de Costa Rica y uno de los más grandes de Centro América (UICN-ORMA 1995)
Most of our results were similar to those of previous studies on freshwater fish distribution elsewhere, no significant correlation between species distribution and environmental variables was found, and we hypothesized that the depth and water velocity and geomorphological are major environmental variables that influence the fish distribution
Biomasa Total (g) de tallas pequeñas en toda la zona de estudio, sin embargo cuando se analizó la distribución de las especies a lo largo del gradiente de estudio se notó la tendencia a dos patrones, el primero conformado por táxones ampliamente distribuidos en todas las estaciones, por ejemplo A. aeneus, B. behreae, C. nigrescens y C. viridis y el segundo por un conjunto de especies restringidas a una sola estación, entre ellas A. monticola (1), A. transandeanus (4) y C. caninus (3) (Cuadro 3)
Summary
Descripción del área de estudio: Las estaciones de muestreo se ubican en la cuenca del río Grande de Térraba, en el sector sur de la vertiente del Pacífico (9°35’11’’-8°45’01’’N, 82o40’22’’- 83°50’11’’W), cantón de Osa, Provincia de Puntarenas (Fig. 1). El patrón climático corresponde a un régimen típico del Pacífico, influido en septiembre y octubre, por los huracanes y tormentas tropicales del Caribe. Las lluvias máximas se registra en octubre y las mínimas en febrero-marzo. La humedad relativa anual es mayor del 80%, febrero y marzo son menor humedad y octubre y noviembre los de mayor (ICE 1999). Trabajo de campo y análisis de datos: Previo al inicio del trabajo de campo se navegó la zona de estudio y mediante buceos con “snorkel” y mascarilla se seleccionaron los sitios de muestreo tratando de incluir la complejidad estructural típica de un río tropical, es
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