3.2 Spatial and temporal variations in fish assemblage structure
Fish assemblage structure varied significantly at the spatial scale. NMDS ordination plot showed that sampling sites at 3rd-order stream were mainly gathered to the lower left of the graph, while those of 4th-order sites were clustered to the right (Fig. 2). ANOSIM confirmed that the fish assemblages differed significantly among stream orders (Global R = 0.156, p=0.034<0.05). According to the SIMPER analysis, fish assemblages in the Ren River basin were highly dominated by cold-water or rheophilic species, such as R. oxycephalus ,S. macrolepi , M. omeiensis and G. herzensteini , except for the 4th-order sites (Table 3). Specially, fish assemblages in 1st-order sites were typified byR. oxycephalus , contributing 95.7% of the similarity. Fish assemblages in 2nd-order were typified byR. oxycephalus and G. herzensteini , contributing 78.3% and 7.1% of the similarity, respectively. Fish assemblages in 3rd-order sites were typified by R. oxycephalus , M. omeiensis andS. macrolepi , contributing 55.4%, 28.1% and 9.3% of the similarity, respectively. From 1st-order sites to 3th-order sites, the abundance of R. oxycephalus decreased gradually, while the abundance of M. omeiensis and S. macrolepi increased continuously. By contrast, fish assemblages in 4th-order sites were predominated mainly by warm-water and lentic or eurytopic species, such as Squalidus argentatus ,Hemiculter leucisculus and Z. platypus .
However, there was no significantly seasonal change in fish assemblages. NMDS ordination plot revealed substantial overlap between fish assemblages in wet and dry season (Fig. 3). ANOSIM further confirmed that the fish assemblages did not show significant changes across seasons (Global R = -0.022, p= 0.745>0.05).