4.2 Environmental effects on fish assemblage
The present study showed fish assemblages in streams of the Ren River
basin were structured by a
combination of large-scale landscape factors (e.g., altitude and C-link)
and local habitat features (e.g., channel width, water temperature and
depth). Among which, landscape factors acted as the most important
contributor in both wet and dry season. The importance of landscape
position in structuring fish assemblages have been demonstrated by
numerous studies (e.g., Fairchild et al. 1998; Smith and Kraft 2005; Li
et al. 2012; He et al. 2017; Zhu et al. 2017). According to landscape
perspective, streams should be regarded as connected networks with a
definable “network geometry”, rather than a linear hierarchy most
represented by stream order (Benda et al. 2004; Smith and Kraft 2005).
Abiotic and biotic stream characteristics changed from low-order
headwater streams to high-order downstream locations (Fausch et al.
1984; Smith and Kraft 2005). Specially, high altitude and low-order
stream often refer to low water temperature, narrow channel, shallow
water and low immigration (Murugavel and Pandian 2000.). Therefore,
landscape position in stream network geometry can influence the
distribution patterns of fish species and thus assemblage structure
(Smith and Kraft 2005; Li et al. 2012). In this study, R .oxycephalus , a typical cold-water species, was restricted to
headwaters and small tributaries with higher elevation, and absent from
the lower locations. By contrast, species that adapted to warm-water and
lentic environment (e.g., S. argentatus and H.
leucisculus ) were distributed exclusively in the lower locations. The
barrier caused by the extreme and harsh environmental conditions in high
altitudes seems effectively prevented the dispersion and colonization of
these fish species from lowland (Li
et al. 2012).
Compared to above-mentioned landscape spatial factors, local habitat
factors (e.g., channel width, water temperature and depth) played less
important roles in determining fish assemblage in the Ren River basin.
However, the influence of these factors increased somewhat in the dry
season. In the wet season, water temperature and resource availabilities
were not the limiting factors for warm-water fish species, such asS. argentatus , H. leucisculus and Z. platypus ,
because they were high enough to satisfy the ecological requirements of
these species. However, As the water level and water temperature
declined in winter, diversity and availability of habitat and food
resources became limited (Liu et al. 2019). In this situation, these
species tended to occupy larger and deeper habitats where possess warmer
water temperature and richer food resources (Li et al. 2012).