Growth reaction norms of domesticated, wild and hybrid Atlantic salmon families in response to differing social and physical environments

Abstract

BackgroundDirectional selection for growth has resulted in the 9-10th generation of domesticated Atlantic salmon Salmo salar L. outgrowing wild salmon by a ratio of approximately 3:1 when reared under standard hatchery conditions. In the wild however, growth of domesticated and wild salmon is more similar, and seems to differ at the most by a ratio of 1.25:1. Comparative studies of quantitative traits in farmed and wild salmon are often performed by the use of common-garden experiments where salmon of all origins are reared together to avoid origin-specific environmental differences. As social interaction may influence growth, the large observed difference in growth between wild and domesticated salmon in the hatchery may not be entirely genetically based, but inflated by inter-strain competition. This study had two primary aims: (i) investigate the effect of social interaction and inter-strain competition in common-garden experiments, by comparing the relative growth of farmed, hybrid and wild salmon when reared together and separately; (ii) investigate the competitive balance between wild and farmed salmon by comparing their norm of reaction for survival and growth along an environmental gradient ranging from standard hatchery conditions to a semi-natural environment with restricted feed.ResultsThe main results of this study, which are based upon the analysis of more than 6000 juvenile salmon, can be summarised as; (i) there was no difference in relative growth between wild and farmed salmon when reared together and separately; (ii) the relative difference in body weight at termination between wild and farmed salmon decreased as mortality increased along the environmental gradient approaching natural conditions.ConclusionsThis study demonstrates that potential social interactions between wild and farmed salmon when reared communally are not likely to cause an overestimation of the genetic growth differences between them. Therefore, common-garden experiments represent a valid methodological approach to investigate genetic differences between wild and farmed salmon. As growth of surviving salmon of all origins became more similar as mortality increased along the environmental gradient approaching natural conditions, a hypothesis is presented suggesting that size-selective mortality is a possible factor reducing growth differences between these groups in the wild.

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

“…In general, we found that V A (and hence evolvability) was lower in the FF group compared with the WW group, which is in line with previous findings that genetic variation in farm salmon strains are often lower than in wild strains (Norris et al, 1999;Skaala et al, 2004). Interestingly, Solberg et al (2013b) reported reduced heritability of juvenile mass in farm-provenance Atlantic salmon, compared with progeny of wild parents, when both were reared under standard hatchery conditions with unrestricted access to food. This pattern was reversed, however, when access to food was restricted, possibly reflecting selective mortality against the slowest-growing wild genotypes (Solberg et al, 2013b).…”

“…Interestingly, Solberg et al (2013b) reported reduced heritability of juvenile mass in farm-provenance Atlantic salmon, compared with progeny of wild parents, when both were reared under standard hatchery conditions with unrestricted access to food. This pattern was reversed, however, when access to food was restricted, possibly reflecting selective mortality against the slowest-growing wild genotypes (Solberg et al, 2013b). We also found strong positive genetic correlations between L F and mass (40.85 in all samples), which is higher than the median r G of +0.71 reported by Carlson and Seamons (2008) for pairs of morphological traits.…”

Are farmed salmon more prone to risk than wild salmon? Susceptibility of juvenile farm, hybrid and wild Atlantic salmon Salmo salar L. to an artificial predator

“…In a previous study performed under hatchery and semi-natural conditions, we demonstrated that the relative differences in growth between wild and farmed juvenile salmon decreased along an environmental gradient approaching more natural conditions (Solberg et al, 2013b). However, the most natural conditions included in that study, which was conducted in the period between egg hatching and the fry stage, only included a semi-natural habitat with hiding places and competition for food.…”

“…The relative difference in weight between farmed and wild salmon at the fresh water stage has been documented to be small in the wild, e.g., in River Guddal offspring of farmed salmon outgrew offspring of wild salmon at a maximum of 1.25:1 at the smolt stage . Furthermore, we have previously demonstrated that the relative difference in weight between farmed and wild salmon decreased when competition and mortality levels increased along an environmental gradient approaching natural conditions (Solberg et al, 2013b). Our previous study, based upon the same genetic material as included in this study, were performed along an environmental gradient without predation.…”

Maternal inheritance influences homing and growth of hybrid offspring between wild and farmed Atlantic salmon

“…Because of genetic selection, the mean mass of AquaGen salmon in fish farms has increased several fold compared with similar-aged, wild conspecifics (Gjedrem 2010, Solberg et al 2013). Increased growth is also observed when farmed salmon are released into the wild (Fleming et al 2002, McGinnity et al 2003.…”