Comparison of Deep-River and Adjacent Sandy-Beach Fish Assemblages in the Napo River Basin, Eastern Ecuador
We sampled fishes with a small trawl in the deep midchannel and with beach seines on nearby sandy beaches at five sites along two parallel headwater tributaries of the upper Amazon in Eastern Ecuador. We quantified ecological distance between fish assemblages of deep-river and beach habitats and compared assemblage composition, species richness, and diversity. Results of detrended correspondence analyses (DCA) and two-way indicator species analyses (TWINSPAN) clearly revealed the presence of a striking faunal change (i.e., about 92% dissimilarity) between beach-zone and off-shore fishes. The boundary between these assemblages occurred at about 2 m depth. Beach samples collected as far apart as 325 km were more similar to each other than to trawl samples taken only 10s of meters away. The beach-zone fish assemblage was strongly dominated by small characins, whereas the deep-river habitat was dominated by catfishes and weakly electric gymnotiforms. Apparent adaptations to deep-water habitats included small size, flattened or elongated body, ventral mouths, reduced eyes, and sensory specializations for life in what may be a nearly light-less environment (e.g., chemo- and electroreceptors). Visually oriented fishes and those dependent on algal or detrital foods were mostly absent from the deep-river samples. We caught many more species along the beaches, but after correcting for higher numbers of individuals collected in beach habitats, species richness was not noticeably different between the two habitats. Species diversity also averaged slightly higher for beach samples, but the difference was not significant. We infer that the deep-river habitat was undersampled and that further effort in the deep river would reveal the presence of many more species. Muestreamos peces con una malla de rastra en el canal profundo del río y con redes playeras en las playas vecinas, en cinco sitios a lo largo de dos triburarios paralelos del Alto Amazonas en el Este del Ecuador. Quantificamos la distancia ecológica entre ensamblajes de peces de las profundidades del río y de los habitats de playa, y comparamos la composición de esos ensamblajes, la riqueza de especies y la diversidad. Los resultados de ‘detrended correspondence analyses (DCA)’ y ‘two-way indicator species analyses (TWINSPAN)’ claramente revelaron la precencia de una markado cambio faunístico (i.e., cerca de 92% disimilarity) entre la zona de playa y los peces del centro del río. La barrera entre estos ensamblajes ocurrió a cerca de 2 m de profundidad. Muestras de playa colectadas a distancias de hasta de 325 km fueron más similares de unas a otras que a muestras de rastra tomadas a solo unas decenas de metros de distancia. El ensamblaje de peces de playa fue markadamente dominado por carácidos, mientras que el hábitat de aguas profundas fue dominado por bagres y gimnotiformes débilmente eléctricos. Adaptaciones aparentes para los hábitats de aguas profundas incluyeron tamaño pequeño, cuerpo aplastado o alargado, bocas ventrales, ojos reducidos, y especializaciones sensoriales para vivir el lo que problamente es un ambiente casi completamente obscuro (e.g., quimo- y electroreceptores). Peces visualmete orientados y aquellos que dependen de alga o detritus como alimento estuvieron en su mayoría ausentes en las muestras de aguas profundas. Colectamos más especies a lo largo de las playas, pero después de corregir por el número alto de individuos, la riqueza específica no fue notoriamente diferente entre estos dos hábitats. El promedio da la diversidad específica tambien fue un poco más alto en las muestras de playa, pero la diferencia no fue significante. Inferimos que el hábitat de aguas profundas no fue muestreado suficientement y que esfuezos futuros en aguas profundas podrían revelar la presencia de muchas más especies.Abstract
Resumen
Location of sampling sites in the Napo River basin of Eastern Ecuador for paired trawl and beach seine samples. Cuyabeno site (6) was sampled only with beach seine
DCA ordination of 11 samples of fishes collected by trawling in midriver and by seining on adjacent sandy beaches in the Napo and Aguarico River basins. This ordination was based on a matrix of 74 species, excluding those present in only one sample because the latter were uninformative about between-site relationships (Appendix 1). Axis scales are in units of average standard deviations of faunal turnover (Hill and Gauch, 1980). Site numbers correspond to those in Figure 1
TWINSPAN dendrogram of the same 11 samples ordinated in Figure 2. The horizontal axis is the average Euclidian distance in the DCA sample ordination (Gauch and Whittaker, 1981). The percentages indicate persistence of a cluster after jackknifing
Comparison of species richness between deep-river (grey bars) and sandy-beach (white bars) habitats in the Napo and Aguarico River basins. Richness was expressed as total number of species collected at a site; expected number of species (striped bars) was based on rarefaction standardization to the smallest sample size (n = 36 individuals). Site numbers on the bars correspond to those in Figure 1
Comparison of Simpson's diversity indices between deep-river (black bars) and sandy-beach (white bars) habitats in the Napo River basin. Site numbers correspond to those in Figure 1