1 Introduction
Understanding how fish assemblages are structured in lotic ecosystems is one of the interesting aspects of community ecology and these knowledges can provide useful information in diversity conservation and ecological restoration (Araújo et al. 2009). The River Continuum Concept (RCC) suggested longitudinal variations in physicochemical environment (e.g., width, depth, velocity, flow volume and temperature) influence structural and functional characteristics of lotic aquatic communities (Vannote et al. 1980). Species diversity often increase with the upstream-downstream gradient, responding to the increased habitat diversity and food availability (Lowe-McConnell 1975). Although the original RCC developed for aquatic macroinvertebrates, similar patterns have been found for fish assemblages in both temperate and tropical regions (Hughes and Gammon 1987; Matthews 1998; Ferreira and Petrere Jr. 2009). Subsequent studies further found that large-scale landscape position, such as stream order, stream size, link magnitude, confluence link (C-link) and downstream link (D-link), are important factors in structuring stream fish assemblage (Fausch et al. 1984; Osborne and Wiley 1992; Fairchild et al. 1998; Smith and Kraft 2005; Mullen et al. 2011). For example, Fausch et al (1984) found fish assemblages changed gradually with stream order. Smith and Kraft (2005) demonstrated that confluence link and stream order were the stream network position measures with the greatest influence on fish assemblages. Osborne and Wiley (1992) observed downstream link explained the greatest portion of the variance in fish species richness and suggested downstream processes significantly influence the structure of fish assemblages. In addition, seasonal variations in environmental conditions caused by periodic flooding are expected to have significant impacts on stream fish assemblages (Ostrand and Wilde 2002; He et al. 2017). Flooding can increase the diversity and availability of habitat structures and food resources, and then lead to changes in fish assemblage between dry and wet seasons (Araújo et al. 2009). Therefore, the current consensus is that stream fish assemblages are structured by a series of local and regional factors operating at multiple spatial and temporal scales (Matthews 1998; Jackson et al. 2001; Vardakas et al. 2015). However, the relative importance of these factors varies with environmental variability, climate conditions, and nature of fish assemblage in the survey area (Grossman et al. 1998; Jackson et al. 2001).
To date, spatial and temporal variations of stream fish assemblages have been tested in numerous regions, including the North America (Ostrand and Wilde 2002; Mullen et al. 2011), Latin America (Silvano et al. 2000; Habit et al. 2007; Araújo et al. 2009; Fernandes et al. 2013), Europe (Pires et al. 1999; Vardakas et al. 2015) and South Asia (Bhat 2004). In recent years, similar studies have been also conducted in China. However, nearly all these studies were concentrated in the central and eastern regions (Yan et al. 2010; Li et al. 2012; He et al. 2017; Zhu et al. 2017). The southwest China, where raises the greatest number of streams and rivers and supports the highest degree of fish diversity, received little attention.
The Dabashan Mountains is located at the border area of Hubei province, Chongqing municipality, Sichuan province and Shanxi province in the southwest China. It is an important water source for many river systems. The Jialing River and the Han River, which represent the largest tributaries of the upper and middle Yangtze river respectively, both originate from this region. Furthermore, the Dabashan Mountains harbors a high level of biodiversity and species endemicity, which has been listed as one of the 35 biodiversity conservation priority area of China (Ministry of Ecology and Environment of People’s Republic of China 2010). Therefore, the Dabashan Mountains plays important roles in water security and biodiversity conservation. Even so, little information about the distribution patterns of fish fauna in this region is available until now. On the other hand, fish diversity in this region has been decreased seriously due to the increased human activities, such as over-fishing, hydropower station construction and water pollution (Zeng 1991). The development of effective conservation strategies needs urgently to understand the organization mechanisms of fish assemblages in this area.
Therefore, fish fauna and environmental features in mountain streams of the Ren River, the largest tributaries of the upper Han River, were investigated in this study. The main objective was to test whether and how fish assemblages in this mountain river vary at different spatial and temporal scales. Furthermore, we would like to identify the key environmental factors that contributed most to the observed spatial-temporal patterns of fish assemblage structure. We believe these studies can help us to understanding how fish assemblage structured in such mountain river systems and then provide valuable information in future fish conservation and management.
2 METHODS