The distribution and abundance of fish is influenced by multiple factors at temporal and spatial scales, and their community composition represent good river integrity indicators. The main objective of this study was to describe the composition and structure of fish assemblages along the Anchique river sub-basin (that drains to Magdalena river), and to test the hypothesis that these communities vary temporally and spatially. For this, four sites (E1, E2, E3 and E4) were sampled along the river (from river birthplace to Magdalena river), during the periods of high and low waters, in 2014. In each site, longitudinal transects (100 m long x 2 m wide, 60 min) were established and fishing was made with both electrofishing and trawl (10 m x 1 m, 0.05 m) nets. Fish samples were fixed and transported to the laboratory for posterior taxonomic identification with the use of keys; fish diversity and structure composition were calculated with the use of standard models. Besides, substrate composition and environmental variables were also considered. A total of 652 fish belonging to 36 species were recorded; high water showed higher values (325 fish with 32 species) than low water period (300 fish with 31 species). Siluriformes was dominant (51 %), followed by Characiformes (32 %), Gymnotiformes (5 %), Cyprinidontiformes and Cichliformes (6 %). Most abundant families were Characidae (28.3 %; 9 spp.), Loricariidae (23.4 %; 8 spp.) and Trichomycteridae (15.0 %; 2 spp.). Kruskal-Wallis test indicated that there is no significant difference among some environmental variables at the temporal level. The values of structural complexity and substrate composition indices showed that E4 had the highest values (0.8), followed by E1 (0.6), and E2 and E3 (0.4). Moreover, E4 had the highest richness and diversity, according to Hill’s number. The canonical correspondence analysis showed that the substrate, and the complexity and width of the river channel, had a significant influence on fish composition and distribution. The fish community richness and diversity did not change temporarily, but did spatially, which could be related to the different structural features of sampling sites.

Keywords: Ichthyofauna, aquatic ecosystem, environmental variables, Anchique river.