3.3 Relationships between fish assemblage and environmental factors
Latitude of the 18 sampling sites ranged from 456 m to 1290 m. Stream orders ranged from 1st to 4th. Link, C-link and D-link ranged from 1 to 20, 1 to 13 and 2 to 30, respectively (Table 1). ANOVA analysis showed that water temperature, pH, dissolved oxygen, conductivity, velocity and the percent of cobble varied significantly with seasons (Table 4). Specially, water temperature in wet season was significantly higher than that in dry season, while pH, dissolved oxygen, conductivity, velocity and the percent of cobble showed the opposite trend. Other variables showed no significant seasonal changes.
After the forward selection procedure, 3 environmental variables (i.e., altitude, C-link and channel width) were finally retained for the wet season, while 5 environmental variables (i.e., altitude, C-link, channel width, water temperature and depth) were retained for the dry season.
For the wet season, altitude explained the most variance (44.5%), followed by C-link (33.6%) and width (15.9%). The species-environment correlations of the first two ordination axes were 0.924 and 0.526, respectively. The first ordination axis accounted for 45.6 % of the variance of the species data, which the second axis 50.6% of this variance. Altitude (-0.911) and C-link (-0.779) were negatively correlated with the first axis, while channel width (0.529) positively corrected.
For the dry season, altitude explained the most variance (37.1%), followed by C-link (24.0%), width (20.0%), water temperature (15.7%) and depth (14.7%). The species-environment correlations of the first two ordination axes were 0.946 and 0.591, respectively. The first ordination axis accounted for 39.6% of the variance of the species data, which the second axis 46.3% of this variance. Altitude (-0.915) and C-link (-0.709) were negatively correlated with the first axis, while channel width (0.620), water temperature (0.587) and depth (0.514) positively corrected.
The RDA ordination plots showed that R. oxycephalus was positively related to altitude and negatively related to channel width in both seasons (Fig. 4 and Fig. 5). S. macrolepis was positively related to C-link in both seasons. However, the species-environment association of other species, such as S. argentatus , Z. platypus , G. herzensteini , M. omeiensis , Cobitis sinensis and Rhinogobius giurinus , showed some changes across seasons. In wet season, all these species were positively related to channel width (Fig. 4). But in dry season, some species (e.g., Z. platypus , G. herzensteini , C. sinensis and R. giurinus ) preferred to habitats with high water temperature and some species (e.g., M. omeiensis ) tended to deep water (Fig. 5).