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).