Spatial and Temporal Patterns of Pelagic Catfish Larvae Drifting in Lowland Rivers of Eastern Ecuador (Pisces: Siluriformes)
This study investigated larval catfish abundance and diversity in various fluvial habitats in the Amazon lowlands of eastern Ecuador. It assessed the relationship between spawning behavior and the hydrological cycle, identified a potential spawning site, and elucidated community assemblages based on larval diversity among river tributaries with differing water chemistry profiles. Sampling sites included the mainstreams of the Napo and Aguarico Rivers and lower reaches of three tributaries. Drift samples were taken before, during, and after strong, rain-induced flooding occurred. Results suggest a strong peak of larval abundances in April, coinciding with the start of the flood period. In the Napo mainstream, samples were dominated by larvae of large migratory pimelodid catfishes, Pseudoplatystoma spp., which were caught between the upriver site in the foothills (254 m a.s.l.) and the downstream Peruvian frontier (175 m a.s.l.). Back-calculations based on water velocity and larval growth rate suggest that most of these larvae originated in the lowlands between those two locations, where the riverbed transitions from rocky to sandy/silty substrate. Thus, it appears that most spawning occurred over sandy substrates rather than the rocky substrates that were dominant above the upriver site. Comparisons of samples taken by day and night at the same sites revealed significantly higher larval abundances at night; thus, larvae must either preferentially drift at night, actively avoid the nets by day, or may do diel vertical migrations (e.g., related to predation pressure or pursuit of prey items). Dominant larval taxa in the whitewater mainstreams differed from those dominating samples from a blackwater mainstream system, and catfish larval assemblages in each of those habitats were distinct from those drifting in small, blackwater tributaries. These results provide key life-history information for catfish species in the Napo River, including for important commercial species, such as Pseudoplatystoma spp. Identification and protection of discrete spawning sites, such as those identified here, are critical to conservation efforts for these species.
(A) Map of Ecuador highlighting the study region (inset, yellow box); and map showing localities of various sampling sites. Triangle, diamond, and square markers indicate whitewater river, blackwater river, and blackwater stream sites, respectively. 1) Napo mainstream at Coca (Puerto Francisco de Orellana)—white water, 254 m a.s.l.; 2) Napo mainstream at Añangu—white water, 224 m a.s.l.; 3) Pañacocha—blackwater tributary, 222 m a.s.l.; 4) Napo mainstream at Pañacocha—white water, 208 m a.s.l.; 5) Napo mainstream at Nuevo Rocafuerte—white water, 175 m a.s.l.; 6) Yasuni mainstream—black water, 186 m a.s.l.; 7) Rio Yanayacu—blackwater tributary, 185 m a.s.l.; and 8) Rio Aguarico mainstream at Puerto Loja—white water, 172 m a.s.l. (B) Napo basin study area with inferred catfish spawning zone indicated by yellow frame; that piedmont region is characterized by extensively braided channel, which transforms into a broad, shallow, turbulent lake-like habitat during the flood season (inset: aerial image of similar piedmont region in the upper Aguarico River near city of Nuevo Loja [Lago Agrio]; photo by DJS). Note encroachment of human disturbance indicated by light green network of roads to left. All map imagery apart from the inset of B was provided by Google Earth Pro software (v. 7.3.2.5491, Google LLC) using Landsat/Copernicus images and was edited using Microsoft PowerPoint and Adobe Photoshop.
Mean monthly rainfall in mm from December 1984 to December 1997 at Nuevo Rocafuerte (site 5, Fig. 1). Error bars are standard deviations. Adapted from Galacatos et al. (2004). Shaded, dotted, and white areas denote the approximate timeframes of the dry season, wet season, and rising/falling water hydrological periods, respectively.
The larval net (50 cm diameter mouth) with flow meter being lowered into the water (Pictured: REBS; photo by DJS).
Number of larvae caught at each site standardized to sampling effort (water volume sampled) at day vs. night. Sites without any larvae were not included. Data were log-transformed (log10(x + 1)) so that low daytime values would be distinguishable from 0. Nineteen larvae collected at Rocafuerte were not included in this figure, as water volumes were not recorded. *Napo at Nuevo Rocafuerte had no daytime sample.
(A) Morphotype richness of larval catfishes recorded at five sites in the Napo drainage of eastern Ecuador. (B) Expected morphotype richness for larval catfishes at each site based on rarefaction adjustments to an assumed sample size of 25 individuals. Only sites with ≥ 25 catfish larvae were included; Yanayacu and Pañacocha are small tributaries. *Napo at Pañacocha had too few samples to be included in rarefaction estimates. (C) Shannon diversity index for larval catfishes at each site in the Napo drainage of eastern Ecuador.
Non-metric multidimensional scaling ordination of larval catfish abundances in the Napo River, Aguarico River, Yasuni River, Pañacocha River, and Yanayacu River; symbols used here are the same as those used in Figure 1A.
Contributor Notes
Associate Editor: R. E. Reis.