Performance of farmed, hybrid, and wild Atlantic salmon (Salmo salar) families in a natural river environment

Abstract

Survival, growth, and diet were compared for farmed, hybrid, and wild Atlantic salmon (Salmo salar) families from the eyed egg to the smolt stage in River Guddalselva, Hardangerfjord, Norway. All individuals that survived until the smolt stage were captured in a Wolf trap and identified to one of the 69 experimental families using microsatellite markers. Survival of farmed salmon progeny was significantly lower than that of hybrids and wild progeny. However, survival varied considerably, from 0.17% to 6.4%, among farmed families. Egg size had an important influence on survival. Half-sib hybrid families with a farmed mother had higher survival when fathered by wild salmon than by farmed salmon. The overall relative survival of farmed families compared with that of their hybrid half-sib families fell from 0.86 in the second cohort to 0.62 in the last cohort with increasing fish density. Smolts of farmed parents showed significantly higher growth rates than wild and hybrid smolts. The overlap in diet among types of crosses demonstrates competition, and farm and hybrid progeny therefore will reduce the river’s capacity for production of wild salmon.

Quantifying heritable variation in fitness-related traits of wild, farmed and hybrid Atlantic salmon families in a wild river environment

“…Intriguingly, found that within-family variation in body size of Atlantic salmon (measured in a hatchery setting) diminished with increasing generations of domestication (see also Solberg et al, 2013a). Under fully wild conditions, variance differences between wild, farmed and hybrid families may be largely unpredictable and context dependent, given that our findings did not match those of Skaala et al (2012) despite very similar study designs (but different genetic backgrounds). One possibility is that farmed fish may lose their environmental sensitivity (that is, degree to which their phenotypes or performance is buffeted by prevailing conditions) in hatchery environments, but not wild environments, as they are only selected in the former.…”

“…In a few samples, we did find variance heterogeneity but the patterns were inconsistent; for example, in the 1998 cohort electrofished 0+parr, survival variation was greatest among F 2 Hy families (perhaps due to rare advantageous recombinants), while for the 1993 and 1998 cohort trapped parr samples, variance in representation was highest for pure wild families and lowest for pure farm families (as one would predict if farm families are genetically depauperate), with hybrid families being generally intermediate. Although Skaala et al used Mowi strain salmon in one of their experimental cohorts, the Fanad Mowi strain used by us is likely to be divergent from theirs in its genetic make-up (because of lower broodstock numbers and a separate breeding programme); thus differences in genetic background and selection trajectories of the farm strains may explain the inconsistent results, in terms of variance in performance, between Skaala et al (2012) and this study. For offspring L F and mass, no heterogeneity in between-family variance was found (Supplementary Table S3), suggesting that each group had similar levels of additive genetic variance for these traits.…”

“…A priori, therefore, one may expect that offspring produced by farm parents should exhibit reduced phenotypic variation in the wild and therefore less variable survival rates compared with wild families. Skaala et al (2012), however, reported the opposite: a larger range in survival rates (a ratio of 38:1 between the lowest and highest survival rates) in farm families compared with hybrid (7:1) or wild (8:1) families in a natural stream setting. In our case, however, no variance heterogeneity in representation with respect to group was found for most of the samples considered (Supplementary Table S2).…”

“…Experimental studies involving artificial crosses between wild and farm fish have provided evidence that offspring with one or two farm parents display lower survival than those with two wild parents (McGinnity et al, 1997(McGinnity et al, , 2003Skaala et al, 2012). Larger, more aggressive farm and hybrid fish may also displace native fish or force them into suboptimal habitats, which increases average mortality (McGinnity et al, 1997(McGinnity et al, , 2003Fleming et al, 2000).…”

“…These studies suggest that repeated introductions of farm fish may depress the productivity of wild populations through both ecological and genetic mechanisms, in addition to fostering genetic homogenisation (Skaala et al, 2006;Glover et al, 2012) and potential loss of local adaptations. Most studies of the effects of artificial immigration of non-native fish (whether from farms or hatcheries), however, tend to emphasise group-level performance differences and typically overlook familylevel variation in performance (but see Skaala et al, 2012). Information on families can minimise analytical bias and yield important insights; for example, certain non-native or hybrid families may fortuitously perform much better than others in natural environments and therefore contribute disproportionately to introgression of nonnative alleles/traits into wild populations (Garant et al, 2003).…”