The Meta River is one of the largest tributaries of the Orinoco River, with a basin that covers 10830.4 km² (Machado-Allison et al., 2010), sheltering a richness of 577 fish species (Usma et al., 2016), equivalent to 79.3% of the native species of the Orinoco (DoNascimiento, 2021). Previous studies have documented the fish fauna of tributaries originating in the foothill part of this basin, with 115 species reported for the Cusiana, 96 for the Cravo Sur River, 49 for the Túa River, 45 for the Upía River and 10 for the Guachiría River (Urbano-Bonilla et al., 2009), and 113 species in the Orotoy River (Ramírez-Gil et al., 2011), 180 for the Pauto River (Maldonado-Ocampo et al., 2013), 241 for the Cusiana River (Urbano-Bonilla et al., 2018) and 180 for the Casanare River (Zamudio et al., 2017). However, there are still information gaps for some drainages and small streams in the upper Meta River (Zamudio & Maldonado-Ocampo, 2021), an area highly impacted by changes in land use (e.g., deforestation, agriculture, livestock, hydrocarbon exploitation, and urbanization), and biodiversity loss (Machado-Allison et al., 2010).

The Ocoa River, a tributary of the Guatiquia River, is one of the most important watersheds for the municipality of Villavicencio, providing water resources for human consumption, as well as for agricultural, forestry and mining uses of rural settlements (Osorio-Ramírez et al., 2015). In addition, it provides food resources (e.g., fishing for consumption) for the most vulnerable communities (Ramírez-Gil pers. obs.). However, the Ocoa River is the main recipient of more than 100 discharges from the urban center, serving as a sink for 80% of urban wastewater (Osorio-Ramírez et al., 2015), promoting degradation and ecological loss of the riverbed (Aguilera-Giraldo et al., 2019; Rojas-Peña et al., 2021).

Faced with this panorama of anthropic intervention, we conducted surveys to characterize the fish fauna subsisting in this river and document key areas for native species. With the knowledge of the ichthyic biodiversity of the Ocoa River, we also contribute to the knowledge of the ichthyofauna of the Orinoco River basin.

Study site

The Ocoa River is a lotic ecosystem of the second order, located in the piedmont of the Eastern Cordillera of the Andes, with a basin of 282.2 km² and 68.55 km in length, of which 17 km run through the urban area of the municipality of Villavicencio, a city with an estimated population of 451 212 inhabitants (Departamento Administrativo Nacional de Estadísticas - DANE, 2018). It originates in the southwest of the municipality, in the Samaria village at 1155 m a.s.l., and flows into the Guatiquia River between the Guamo and Indostán villages at 225 m a.s.l. (Osorio-Ramírez et al., 2015). The annual hydrological cycle of the Ocoa River is unimodal, with minimum rainfall between December-March and a rainy period between April and November with maximum precipitation during April-July (Instituto de Hidrología, Metereología y Estudios Ambientales - IDEAM, 2022). The average temperature range is 15.5–33.5 °C and average annual rainfall is 2700–5000 mm (Osorio-Ramírez et al., 2015).

Data collection and treatment

To obtain samples of the fish community assemblages, spatial and temporal collections were made in the Ocoa River during the 2014 annual hydrological cycle. A total of 15 locations were sampled, eight along the main channel and seven in the micro-basins that are tributary to the main channel (Fig. 1); four samples were taken in each site (Table 1). Stations 3 to 6 correspond to the river’s transit through the urban zone of Villavicencio. The fish were captured by electric fishing, following the methodology proposed by Ramírez-Gil et al. (2011). Simultaneously, some environmental variables were recorded in situ, including water temperature (ºC), pH, dissolved oxygen (mg/l) and electrical conductivity (μS/cm) using multiparametric equipment.

The captured fish were anesthetized and sacrificed in a clove oil solution (Eugenol 300 mg/l), fixed in 10% formaldehyde, and preserved in 70% ethanol. For species identification, specialized taxonomic keys, original descriptions, and taxonomic revisions were used (e.g., Eigenmann & Fisher, 1916; Eigenmann, 1917, 1922; Myers, 1927; Schultz, 1944; Nijssen & Isbrücker, 1980; Mees, 1988; Vari, 1991; Lasso & Machado-Allison, 2000; Mago-Leccia et al., 2001; Taphorn, 2003; Londoño-Burbano et al., 2011, 2018; Ballen & Vari, 2012; Ballen et al., 2016; Marinho & Menezesm 2017; Armbruster et al., 2018; Kullander et al., 2018; Urbano-Bonilla et al., 2018; Urbano-Bonilla & Ballen, 2021; Lima, 2022). The taxonomic status of species was validated in Eschmeyer’s Catalog of Fishes (Fricke et al., 2022), and to verify thepresence of these species in the national territory, the list of freshwater fish species of Colombia (DoNascimiento et al., 2021), was reviewed. Later, the lots of collected specimens were deposited in the ichthyological collection of the Unillanos Natural History Museum - MHNU-I.

Statistical analyses

To calculate the interpolation and extrapolation of the species richness in the Ocoa River drainage, an analysis of the effective number of species was carried out based on the first number of the Hill series (q = 0) as suggested by Jost (2006) and Jost & González-Oreja (2012). In addition, the completeness of the sampling coverage (SC) was calculated, which allows the evaluation of the sampling effort (Chao & Jost, 2012; Chao et al., 2014). All these analyses were carried out in the iNEXT package (Hsieh et al., 2020) in R (R Development Core Team, 2022).

In total, 51 901 specimens were collected, grouped in five orders, 21 families, 61 genera, and 88 species (Table 2). The most representative orders in captures were Siluriformes, with eight families, 26 genera and 38 species, followed by Characiformes, with seven families, 24 genera and 37 species (Table 3). The families with the greatest richness and abundance were Characidae (26 species, 29.5%; n = 20058, 38.6%), Loricariidae (17 species, 19.3%; n = 1327, 2.6%), Heptapteridae (10 species, 11.4%; n = 836, 1.6%), and Cichlidae (7 species, 8.0%; n = 2087, 4.0%) (Fig. 2). The Poecilidae family was the most representative, with an abundance of 26 501 individuals (51.1%), with Poecilia reticulata (n = 3717) and Poecilia cf. caucana (n = 22 784) being the most abundant species of all samples.

Among the captured species, 12 are endemics to the Orinoco River basin. In addition, 33 species are used in ornamental fishing and ten in consumption fishing. Cetopsis orinoco, Moenkhausia oligolepis and Salminus hilarii are migratory species (DoNascimiento et al., 2021) and four species are introduced to the Orinoco River basin (Caquetaia kraussii, Oreochromis niloticus, Poecilia cf. caucana and Poecilia reticulata). Poecilia reticulata and Poecilia cf. caucana. were more abundant (n > 700 individuals) on stations 5 and 6, the sites with greater anthropogenic influence, where measures of dissolved oxygen ranged from 0.3–6.3 mg/l and conductivity between 135.3–222.5 µS/cm. It is worth noting that in station 6, the one with the lowest species diversity during the dry season, only four species (three of them alien) were reported: Poecilia cf. caucana, Poecilia reticulata, Oreochromis niloticus, and the native species Characidium zebra, with a strong dominance of Poecilia cf. caucana, that represented 83% of the individuals collected at that site, while the native species only represented 0.6%.

The completeness of the sampling coverage (SC) for the Ocoa River was 99.9%, indicating that the sampling is representative of the local fish community. Extrapolation analyses suggest that the expected richness for the Ocoa River is approximately 94 species (95% CI = 89.5–113.7), with the observed richness representing 93.6% of the expected richness (Fig. 3).

The physicochemical parameters recorded are summarized in Table 4. The temperature in the main channel ranged between 22.6 and 32.9 °C, and in the tributary channels between 16.9 and 27.2 °C. The pH gradient ranged from 4.2 to 7.8 in stations 1 to 5, and from 6.2 to 9.6 in the remaining stations, and in the tributaries between 4.2 and 10.8. Oxygen levels in general were between 0.3 and 7.8 mg/l, with the lowest levels found at stations 5 and 6 in the urban zone; in the tributaries, the variation was between 0.5 and 7.8 mg/l. The highest conductivity value was reported at station 12, which corresponds to a tributary.

From stations 3 to 6, corresponding to the section of the river in the urban area, the forest cover has been replaced by houses on both sides of the river, some of them with direct discharge of their domestic wastewater.

The present study constitutes the first approximation to the knowledge of the ichthyofauna of the Ocoa River, which crosses the municipality of Villavicencio, with a species richness representing 63.7% of the species listed for the Guatiquia River (Zamudio & Maldonado-Ocampo, 2021), 15.2% for the Meta River drainage (Usma et al., 2016), and 12% of those recorded in the Orinoco basin, a figure estimated at 722 species (DoNascimiento et al., 2021). The extrapolation suggests that the drainage could have about 94 species, with a high percentage of the expected richness (93.6%) recovered in our surveys.

The composition of the fish assemblage of the Ocoa River was mainly dominated by Characiformes and Siluriformes, contributing 85% of the total species recorded for this drainage (Fig. 2), similarly to other drainages of the Colombian Orinoco River Basin (e.g., Urbano-Bonilla et al., 2009, 2014, 2018; Ramírez-Gil et al., 2011; Villa-Navarro et al., 2011; Maldonado-Ocampo et al., 2013; Usma et al., 2016; Zamudio et al., 2017) and the Neotropics (e.g., Albert & Reis, 2011; Reis et al., 2016; Van der Sleen & Albert, 2018). Regarding higher taxonomic levels, our results suggest some impoverishment in the Upper Meta communities, with representatives of five of the thirteen orders registered for the Orinoquia, a figure that is usually higher (6-9) in other rivers of the region, and 20 native families out of the 50 registered for the area (Urbano-Bonilla et al., 2009; Ramírez-Gil et al., 2011; Maldonado-Ocampo et al., 2013; DoNascimiento et al., 2021).

The Poecilidae family was the most representative in terms of abundance, surpassing the Characidae and Loricariidae families (Table 2). This is probably due to the successful biological adaptations of Poecilia species (e.g., early reproductive maturity, sperm storage, superfetation, air-breathing ability, and high thermal tolerance), that make them highly tolerant eurytropic species (Jiménez-Prado et al., 2020; López-Fuentes et al., 2021), proliferating and dominating over native species, as reported by Casatti et al. (2009) for the Saõ Josẽ dos Dourados and Turvo-Grande rivers in Brazil.

Four introduced species Caquetaia kraussii, Poecilia cf. caucana, Poecilia reticulata and Oreochromis niloticus were collected in the sites with the greatest environmental degradation, influenced by urbanization and pollution, which favors the establishment of alien species, since habitat degradation negatively affects native species and facilitates the success of invasions (Hooper et al., 2005). According to Royero & Lasso (1992), C. kraussii was possibly introduced to the Orinoco River basin by escapes from fish farms, as reported for O. niloticus, which is widely distributed in 21 river basins worldwide (Agostinho et al., 2021). In addition, P. reticulata and P. cf. caucana were possibly introduced deliberately by aquarists (Claro-García et al., 2017). Previous fish assessments in foothill rivers in the region did not report introduced species (Urbano-Bonilla et al., 2009, 2018; Ramírez-Gil et al., 2011; Maldonado-Ocampo et al., 2013; Zamudio et al. 2017, 2021), this being the first records of their presence, highlighting the environmental deterioration of this river to the detriment of native species.

On the other hand, our surveys also recovered individuals from 11 morphospecies (e.g., Characidium sp., Ceratobranchia sp., Hemigrammus sp., Eigenmannia sp., Chaetostoma sp., Astroblepus spp., Heptapteridae gen. nov., Bujurquina sp.), that possibly represent new species, which requires further, in-depth taxonomic studies to elucidate their status and present a more complete picture of the actual species diversity in the basin.

The present study was carried out during a single hydrological cycle (2014), but there is a need to continue sampling efforts in the Ocoa River; since monitoring information is important to understand how different anthropogenic activities are affecting environmental conditions and freshwater communities, allowing the design and implementation of conservation strategies for the drainage.