2.1 Study area
The Ren River is the largest tributaries of the upper Han River, which originates from the Dayanshan Mountain in the southern foot of the Dabashan Mountains. It flows for 211.4 km before drain into the Han River in Ziyang County, Shanxi Province, with a drainage basin of 4871 km2. The whole basin is located in the center of the Chongqing Dabashan National Nature Reserve, which aim to protect the subtropical forest ecosystems and their biodiversity.
In this study, fish fauna and habitat features were investigated in the upper Ren river, a 128 km stretch ranging from headwater to the confluence with Pingba River (Fig. 1). There are many medium to small streams (e.g., Kang river, Huangxi River, Shixi River, Lanxi River, Caiziba River, Longtan River, and Yanzi River) flow into this stretch. The climate is typical subtropical monsoon, with hot-wet summers and cold-dry winters. The annual average temperature is 13.8℃ and the monthly mean temperature changes from 24.8℃ (July) to approximately 2.4℃ (January). The average annual rainfall is 1261.4 mm, but is unevenly distributed in time, with 68% of total annual precipitation occurs in May to September. The river habitat is characterized by mainly deep valleys, shallow and winding channels, rapid water flow. As the headwaters and most tributaries originate from primeval forest with less human influence, water quality in this area preserves well, except for a few sites due to urban sewage and industrial wastewater.
2.2 Fish sampling
Fish surveys were conducted in May-July and December 2017, representing the rainy and dry season, respectively. A total of 18 sites were sampled, covering the length of the main channel of the Ren Rive and its main tributaries. Among them, 8 sites located at 1st-order streams, 6 at 2nd-order, 5 at 3rd-order, and 1 at 4th-order streams (Table 1). The sampling sites were selected based on accessible, similarity in habitat types, and to maximize the diversity of habitat types (Araújo et al. 2009). Fish specimens were caught by a backpack electro-fisher (Susan 1030S, China; 12 V import, 250 V export) by single pass. Each sampling was carried out at different habitat units (riffles, glides and pools) by two persons against the current. The length of each segment varied between 100 and 200 m, according to the accessibility. After samplings, all captured specimens were identified to species level, measured (mm) and weighted (g). Individuals that could be confidently identified were released downstream from the sites after recovery, while individuals that could not be identified in the field were fixed in buffered formaldehyde (7%) and then transported to the laboratory for further taxonomic determination.
For each site, habitat parameters were measured prior to fish sampling. Specially, water temperature (℃), pH, dissolved oxygen (mg/l) and conductivity (μs/cm) were measured using an YSI6680 Multi Probe. Average channel width (m) was calculated using a Ranger Laser Finder instrument in the beginning, middle and end of the sampling reach. Water depth was measured at three points (25, 50, and 75% of the transect width) along each transect. Current velocity (m/s) was determined in the middle of the sampling site with a LJD-10 flow-meter. Substratum composition was categorized to bedrock, cobble, pebble/gravel, sand and mud/silt, according to Hoeinghaus et al (2007). In addition, landscape parameters were recorded in situ or calculated from topographical maps (Table 1). Therein, altitude, longitude and altitude (m) were recorded by a Garmin GPS-76 system at the time of field sampling. Stream order was assigned according to Strahler (1957). Description and calculation of stream link magnitude (link, Shreve, 1966), C-link (Fairchild et al. 1998), and D-link (Osborne and Wiley 1992) were adapted from Smith and Kraft (2005). Measures of these spatial parameters referenced to a public map of Chengkou County (1:50 000).