Water Resources

The study was conducted at a revegetation demonstration site of the Ruxi watershed of Zhong County, Chongqing Municipality (N30°24′16′′ and E108°08′03′′), near the riparian zone of the TGR. After the TGR reached an average water storage level of 175 m in 2010, a very large area of land–water ecotone formed, covering an area of nearly 350 km² (Yangtze River Water Resources Commission, 1999 ; Wang, 2003 ; Li et al., 2013 (link); Ren et al., 2016 (link)).
Considering the tolerance limit of trees (poor survival below 165 m) and the safety of the river channel, all trees are planted within 165–175 m of the upper part of the riparian zone in a mixed planting with row spacing of 1 m × 1 m. The trees of the study area were dominated by Taxodium ascendens Brongn, Taxodium distichum (L.) Rich., and Salix matsudana Koidz. The Ruxi River Basin belongs to the center of the TGR area and is also an important shipping channel. In order to ensure the safety, accuracy, and operability of the experiment, the decomposition experiment site was selected in an artificial reservoir connected to the Ruxi River (with an area of more than 700 m²). The reservoir and Ruxi River are separated by only one artificial dam, and the water of the reservoir flows directly to the Ruxi River. This subtropical southeast monsoon region experiences a humid (relative air humidity of 80%) and a mountainous climate with a mean annual temperature of 18.2°C. The annual sunshine time is 1,327.5 h, and the number of frost-free days is 341 days. Soils are dominated by purple soil.

Adult brood fish collected from a total of five wild populations and two commercial farmed strains were used to produce the experimental families (Fig. 1). The two commercial farmed strains used were Mowi and Salmobreed. Mowi is the Marine Harvest strain and is the oldest Norwegian commercial strain (Gjedrem et al. 1991). Salmobreed was established in 1999 and is based on genetic material from several older Norwegian farmed strains. Both strains are extensively used in commercial aquaculture in Norway and internationally. Strain ID was not the focus here, and both were thus anonymized randomly as Farm 1 and Farm 2 and are referred to as the farm populations throughout. Wild parental fish upon which the families were produced were either sampled directly in rivers (Vosso, Figgjo, Arna) and verified as wild based on reading scale characteristics (Lund and Hansen 1991), or alternatively collected from the Norwegian Gene Bank for wild Atlantic salmon (Driva and Skibotn). The sire of family 17 had a tag when caught in the River Figgjo, which indicated that the specific fish originated from the nearby River Ims. The Norwegian Gene Bank is a programme that conserves wild salmon populations regarded as under threat from disease or extinction. Individuals are taken from the rivers and are then reared in the Gene Bank where genetic structure is monitored. Gametes from first‐ and third‐generation Driva and first‐ and second‐generation Skibotn gene bank strains were collected at the Gene Bank hatchery and transported back to Matre. Wild salmon from the River Figgjo (58°81′N, 5°55′E) are predominantly one‐sea‐winter fish with some two‐ and three‐winter fish (Friedland et al. 2009). The River Vosso (60°64′N, 5°95′E) is characterized by its large multi‐sea‐winter salmon, and the Norwegian Gene Bank conserves this population; thus, fish from this strain have been reared in a local hatchery before release into the fjord at the smolt stage. The River Arna (60°24′N, 5°29′E) is a small river in western Norway, with a variable‐age spawning population. The River Skibotn (69°38′N, 20°26′E) population in northern Norway is conserved by the Norwegian Gene Bank due to repeated infestation by the parasite Gyrodactylus salaris. The River Driva (62°40′N, 8°34′E) population in mid‐Norway is also conserved by the Norwegian Gene Bank due to infestation by G. salaris. Hydrographical data pertaining to river water temperature were accessed through the Norwegian Water Resources and Energy Directorate (2015). The average monthly water temperatures for each river are presented in Fig. 2. There was no data available for Arna; thus, the nearby Oselva River was used as a temperature reference. The highest temperature recorded was 16.7°C in Oselva, and the lowest recorded temperature was 0.0°C in Skibotn.
All 35 experimental families were established at the Matre experimental field station located on the west of Norway in weeks 46–47 of 2012. The five wild populations and two farmed populations were used to create farmed, F1 hybrid and wild families as follows: 8 farmed families consisting of Farm 1 and Farm 2, 8 hybrid families consisting of two F1 hybrid populations and 19 wild families consisting of fish from five wild populations. Figgjo females were crossed with Farm 1 males to produce the Hybrid 1 families, and Farm 2 females were crossed with Vosso males to produce the Hybrid 2 families. The full crossing design is presented in Table S1. All nine experimental groups are herein referred to as the experimental populations. All nine populations were represented by 4 families each with the exception of Driva, which consisted of just 3 families (Table S1).