Gametes from wild Atlantic salmon originating from the Etne River (59°40′N, 5°56′E), Hordaland, and farmed salmon originating from the Norwegian Mowi strain were used to generate three cross-types for this experiment in 2009; (i) ten pure wild families; (ii) ten pure farmed families; and (iii) ten F1 hybrid families, generated by crossing farmed females with wild males. Thus, the hybrid families were maternal and paternal half siblings of the farmed and wild families, respectively. These three experimental groups are from now referred to as farmed (Mowi), hybrid (Mowi x Etne) and wild (Etne).
The Etne River has the largest wild salmon stock in Hordaland [51] and salmon used as parents were collected directly from the river. The Mowi strain from Marin Harvest is the oldest Norwegian farmed strain [45] . This strain was established from large multi-sea winter fish collected from the River Bolstad in the Vosso watercourse and the River Åroy, in addition to wild salmon caught in the sea outside of western Norway, near Oster fjord and Sotra [6] , [52] . Phenotypic selection for growth, late maturation and fillet quality was conducted until 1999, when a family based selection program consisting of 250 females and 80 males was initiated [6] . In our study we used the offspring of the 9–10th generation of selected parents.
All families were established November 17, 2009, at the hatchery located on the river Etne. Unfertilized ova and milt from 10 male and 10 female farmed salmon were collected from the Mowi breeding station located at Askøy and transported to the Etne hatchery. Wild salmon were caught by rod in October – November, 2009, transported to the hatchery located on the Etne river, and stripped upon the arrival of farmed gametes (for family crosses, seeTable S1 ). Adipose fin clips were collected from all parental fish and scale samples from wild parents were collected and analyzed by the Norwegian gene bank for wild salmon (The Norwegian Directorate for Nature Management), to confirm that wild salmon were not escapees from farms [53] .
All 30 families were incubated in the dark in single-family units, at temperatures of approximately 3.5°C (range 2.0–6.6°C), until the eyed-egg stage. Dead eggs were picked daily and February 17–18, 2010, shocked to sort out dead eggs. One hybrid family was at this point excluded from the study due to high egg mortality; hence the wild, farmed and hybrid origins were represented by 10∶10:9 families, respectively. Weight and volume measurements of eggs from all families were taken on March 17, 2010. On the same day, equal numbers of fertilized eggs per family (n = 50) were counted out and sorted into four replicated mixed trays (n = 1450;Figure 1 ). Experimental groups were transported to the Matre research station March 18, 2010.
The four replicates continued their incubation at the Matre hatchery at approximately 5°C (range 4–5.6°C). April 19, 2010, all four replicates were transferred to 1.5 m3 tanks, continuously supplied with fresh water at an average temperature of 13.2°C (range 10.7–15.6°C). All experimental groups were kept under 24 hour daily light throughout the experiment. Fry were presented with a commercial diet starting on April 22, 2010. A standard feeding table for appropriate temperatures was used to calculate the feeding ration. The fish were feed with commercial pelleted fish feed (Biomar, Myre, Norway), 12 hours per day by automatic feeders, 09.00–21.00. Pellet sizes were adjusted to the mean fish weight (W, g) after weighing a sample of 50 individuals per tank. Due to visible differences in weight among individual fish within each tank, a combination of pellet sizes were used according to supplier’s protocol to ensure that all fish were given suitable feed. Mortality was recorded daily, however dead individuals were not assigned to family.
The Etne River has the largest wild salmon stock in Hordaland [51] and salmon used as parents were collected directly from the river. The Mowi strain from Marin Harvest is the oldest Norwegian farmed strain [45] . This strain was established from large multi-sea winter fish collected from the River Bolstad in the Vosso watercourse and the River Åroy, in addition to wild salmon caught in the sea outside of western Norway, near Oster fjord and Sotra [6] , [52] . Phenotypic selection for growth, late maturation and fillet quality was conducted until 1999, when a family based selection program consisting of 250 females and 80 males was initiated [6] . In our study we used the offspring of the 9–10th generation of selected parents.
All families were established November 17, 2009, at the hatchery located on the river Etne. Unfertilized ova and milt from 10 male and 10 female farmed salmon were collected from the Mowi breeding station located at Askøy and transported to the Etne hatchery. Wild salmon were caught by rod in October – November, 2009, transported to the hatchery located on the Etne river, and stripped upon the arrival of farmed gametes (for family crosses, see
All 30 families were incubated in the dark in single-family units, at temperatures of approximately 3.5°C (range 2.0–6.6°C), until the eyed-egg stage. Dead eggs were picked daily and February 17–18, 2010, shocked to sort out dead eggs. One hybrid family was at this point excluded from the study due to high egg mortality; hence the wild, farmed and hybrid origins were represented by 10∶10:9 families, respectively. Weight and volume measurements of eggs from all families were taken on March 17, 2010. On the same day, equal numbers of fertilized eggs per family (n = 50) were counted out and sorted into four replicated mixed trays (n = 1450;
The four replicates continued their incubation at the Matre hatchery at approximately 5°C (range 4–5.6°C). April 19, 2010, all four replicates were transferred to 1.5 m3 tanks, continuously supplied with fresh water at an average temperature of 13.2°C (range 10.7–15.6°C). All experimental groups were kept under 24 hour daily light throughout the experiment. Fry were presented with a commercial diet starting on April 22, 2010. A standard feeding table for appropriate temperatures was used to calculate the feeding ration. The fish were feed with commercial pelleted fish feed (Biomar, Myre, Norway), 12 hours per day by automatic feeders, 09.00–21.00. Pellet sizes were adjusted to the mean fish weight (W, g) after weighing a sample of 50 individuals per tank. Due to visible differences in weight among individual fish within each tank, a combination of pellet sizes were used according to supplier’s protocol to ensure that all fish were given suitable feed. Mortality was recorded daily, however dead individuals were not assigned to family.
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