Can variation in standard metabolic rate explain context‐dependent performance of farmed Atlantic salmon offspring?

Abstract Escaped farmed Atlantic salmon interbreed with wild Atlantic salmon, leaving offspring that often have lower success in nature than pure wild salmon. On top of this, presence of farmed salmon descendants can impair production of wild‐type recruits. We hypothesize that both these effects connect with farmed salmon having acquired higher standard metabolic rates (SMR, the energetic cost of self‐maintenance) during domestication. Fitness‐related advantages of phenotypic traits associated with both high SMR and farmed salmon (e.g., social dominance) depend on environmental conditions, such as food availability. We hypothesize that farmed offspring have an advantage at high food availability due to, for example, dominance behavior but suffer increased risks of starvation when food is scarce because this behavior is energy‐demanding. To test these hypotheses, we first compare embryo SMR of pure farmed, farmed‐wild hybrids and pure wild offspring. Next, we test early‐life performance (in terms of survival and growth) of hybrids relative to that of their wild half‐siblings, as well as their competitive abilities, in semi‐natural conditions of high and low food availability. Finally, we test how SMR affects early‐life performance at high and low food availability. We find inconclusive support for the hypothesis that domestication has induced increased SMR. Further, wild and hybrid juveniles had similar survival and growth in the semi‐natural streams. Yet, the presence of hybrids led to decreased survival of their wild half‐siblings. Contrary to our hypothesis about context‐dependency, these effects were not modified by food availability. However, wild juveniles with high SMR had decreased survival when food was scarce, but there was no such effect at high food availability. This study provides further proof that farmed salmon introgression may compromise the viability of wild salmon populations. We cannot, however, conclude that this is connected to alterations in the metabolic phenotype of farmed salmon.


| INTRODUC TI ON
Feral domestic animals commonly constitute a threat to the viability of wild populations since their hybridization with wild individuals can disrupt local adaptations and reduce population fitness (Frankham, 2008;Laikre, Schwartz, Waples, & Ryman, 2010). Currently, there is concern for wild Atlantic salmon (Salmo salar) populations because of the expansion of the Atlantic salmon aquaculture industry and the accompanying increase in number of escapees breeding in nature (Forseth et al., 2017;Glover et al., 2017;Wringe et al., 2018). Wild salmon populations have evolved adaptations that are beneficial in their local environments (Fraser, Weir, Bernatchez, Hansen, & Taylor, 2011;Garcia de Leaniz et al., 2007;O'Toole et al., 2015). Hence, when interbreeding with escaped farmed salmon causes genetic introgression into wild populations Karlsson, Diserud, Fiske, & Hindar, 2016;Skaala, Wennevik, & Glover, 2006), this increases the number of mal-adapted individuals in nature (Bolstad et al., 2017). Farmed salmon and their descendants often have lower success in nature than wild salmon McGinnity et al., 2003), and genetic introgression of farmed salmon may therefore lead to a temporal or permanent decline in fitness of the wild populations affected. On top of this, the presence of individuals with farmed salmon ancestry can entail decreased production of seaward migrants of the wild type .
Both the reduced success of farmed salmon and their descendants in nature, and the negative effect of their presence for production of wild salmon, are likely related to genomic and phenotypic alterations that have occurred during the domestication process (Bolstad et al., 2017;Liu et al., 2017). Among the phenotypic alterations reported are increased growth rates (Gjedrem, 2000;Harvey et al., 2016;Solberg, Skaala, Nilsen, & Glover, 2013a) and changes in behavior, such as decreased response to predators, and increased aggression and social dominance (Einum & Fleming, 1997;Houde, Fraser, & Hutchings, 2010a;Johnsson, Höjesjö, & Fleming, 2001).
Decreased anti-predator response poses an obvious disadvantage in nature and could contribute to the reduced success of farmed salmon and their descendants. On the other hand, rapid growth and aggressive and dominant behavior could give farmed salmon a competitive advantage that enable them to displace wild salmon from territories with, for example, good feeding opportunities and may explain the finding of reduced production of wild salmon .
Standard metabolic rate (SMR, defined as the energetic cost of self-maintenance, reviewed in Burton, Killen, Armstrong, & Metcalfe, 2011;Metcalfe, Leeuwen, & Killen, 2016) is another trait that may have been affected by the domestication process.
Conditions characteristic of the farm environment, including high, predictable food availability and structurally simple habitats, concur with conditions reported to be advantageous for individuals with high SMR (Bozinovic & Sabat, 2010;Derting, 1989;Reid, Armstrong, & Metcalfe, 2012). Conversely, organisms in habitats with low food availability and/or predictability, conditions commonly found in nature, are hypothesized to evolve low SMR (reviewed in Chown & Gaston, 1999;Cruz-Neto & Bozinovic, 2004). Additionally, SMR is connected with a suite of phenotypes similar to some that are typical for farmed salmon, such as rapid growth rate, dominance and risk-prone behavior (Killen, Marras, Ryan, Domenici, & McKenzie, 2012;Metcalfe, Taylor, & Thorpe, 1995;Millidine, Metcalfe, & Armstrong, 2009). We therefore hypothesize that farmed Atlantic salmon have increased SMR compared to wild salmon. We test this by measuring SMR of embryos (eyed eggs) resulting from experimental crosses of farmed and wild salmon.
The advantages of phenotypic traits commonly associated with both farmed salmon and high SMR rely heavily on environmental conditions. For instance, farmed or growth hormone implanted salmon rapidly outgrow salmon of the wild type in captivity with high and reliable access to food (Solberg, Zwei, Nilsen, & Glover, 2013b;Sundt-Hansen et al., 2012). This trend is much less prominent and may even be reversed under natural conditions where food access is typically less reliable and scarce (Glover, Solberg, Besnier, & Skaala, 2018;Reed et al., 2015;Sundt-Hansen et al., 2012). Hence, we hypothesize that the performance of farmed salmon offspring, as well as their ability to outcompete wild salmon, depends on food availability. Specifically, we expect that farmed offspring have an advantage in conditions with high food availability due to their high growth potential and aggressive and dominant behavior. Conversely, we expect that farmed offspring have a disadvantage at low food availability when they cannot realize their growth potential and may suffer increased risks of starvation because of their energy-demanding behavior. How food regime affects benefits versus costs of having high SMR differs among published studies. When food is readily available, high SMR has been reported either to be advantageous or to have no performance effect. In contrast, under conditions of food shortage high SMR has been found to be advantageous, disadvantageous, or to have no performance effect (Auer et al., 2018;Bochdansky, Grønkjaer, Herra, & Leggett, 2005;Zeng et al., 2017).
In this study, we test how food availability affects the success of offspring of farmed and wild salmon, as well as their competitive ability, in an experiment with farmed-wild hybrids and pure wild salmon in 40 semi-natural streams. We manipulate food availability (high versus low) and competitive regime (allopatry: wild and hybrid juveniles alone versus in sympatry: wild and hybrid juveniles together) during the critical period for survival following juvenile emergence from the gravel and onset of exogenous feeding (Einum & Fleming, 2000;Einum, Sundt-Hansen, & Nislow, 2006;Elliott, 1984).
We compare the performance of juveniles (in terms of survival and growth) across the food availability and competition treatments. In doing so, we also test whether the previously found negative impact on early survival of wild offspring inflicted by the presence of pure farmed salmon offspring (Sundt-Hansen, Huisman, Skoglund, & Hindar, 2015) extends to hybrids. Finally, we test for effects of different levels of SMR on juvenile performance at high and low food availability by collating data on family-level embryo SMR with data on performance at the individual and family level.

| Parent fish
Atlantic salmon from two Norwegian populations were used as representatives for wild salmon. We sampled gametes from 20 adults (10 of each sex, Supporting Information Table S1) from the River Surna (Central Norway, 63.06°N, 9.14°E) that were caught during broodstock collection in autumn 2012. Gametes from the River Imsa (Southwestern Norway, 58.91°N, 5.95°E) were taken from 22 adults (11 of each sex, Supporting Information Table S1) caught during 2013 in a fish trap when they returned to the river to spawn.
Atlantic salmon from the Norwegian breeding company AquaGen were used as our farmed salmon representative. This population was originally founded from 41 Norwegian wild populations in 1971-1974 and had been subject to domestication and artificial selection for 10-11 generations when we made our crosses in 2012 and 2013. The River Surna was one of the founding populations of the AquaGen breeding strains (Gjøen & Bentsen, 1997).
The AquaGen material used to make crosses in 2012 originated from gametes stripped from 20 farmed adults (10 of each sex, Supporting Information Table S1), whereas the material used to make crosses in 2013 originated from 22 farmed adults (11 of each sex, Supporting Information Table S1).

| Crosses
Following stripping, gametes were kept on ice in plastic containers enriched with oxygen for shipment and storage until all crosses were done at the NINA Research Station Ims (hereafter referred to as "Ims"). All fertilizations were performed within the same day (8 November 2012 and 29 November 2013). Eggs from each wild and farmed female were fertilized with sperm from one wild and one farmed male so that each pure wild (ww) and pure farmed (ff) family were half-siblings with two hybrid families; one with a farmed mother and a wild father (fw), and one with a wild mother and a farmed father (wf). The crosses are hereafter called "types". The crosses performed using gametes from Surna and AquaGen in 2012 gave 40 full-sibling family groups. This part of the fish material will be referred to as "Surna-AquaGen". The crosses made with AquaGen and Imsa gametes in 2013 resulted in 44 full-sibling family groups, which are hereafter referred to as "Imsa-AquaGen".
Eggs and juveniles originating from these crosses were used to conduct two different sets of experiments: O 2 consumption was

| Measurement of embryo standard metabolic rate
Eyed embryos used in the SMR measurements were shipped from Ims to the Norwegian University of Science and Technology containing 10°C oxygenated water. The water had been transported from Ims and was filtered with Sterivex 0.2-µm filters. Syringes were sealed with warm wax and kept for 2.2-4 hr at 10°C. Sixteen syringes were not loaded with embryos, and these functioned as controls to account for microbial metabolism. For the Imsa-AquaGen material, five embryos per family were placed inside separate 13 ml glass vials that contained aerated 10°C synthetically prepared water (COMBO water, Kilham, Kreeger, Lynn, Goulden, & Herrera, 1998) and sealed with plastic caps under water and then with warm wax.
Sixteen glass vials were used as controls. Vials were placed on a rotation table to prevent formation of O 2 gradients and were kept at 10°C for 2-2.5 hr. For both the Surna-AquaGen and the Imsa-AquaGen material, the O 2 content was measured by inserting water into the chamber of the oxygen electrode using a syringe and recording values once readings had been stable for 5 s. Measurements were made both from a sample of the water from which the vials or syringes were filled at the time when the embryos were loaded into the syringes or glass vials, and again from the vials or syringes containing the eggs at the end of the measurement period.
Oxygen consumption was successfully measured on 6-9 individual embryos from each of 5 pure Surna families, 8 hybrid families with AquaGen mother and Surna father, 7 hybrid families with Surna mother and AquaGen father and 10 pure AquaGen families. The O 2 consumption of 11 pure Imsa families, 11 of the hybrid families with AquaGen mother and Imsa father, 10 of the hybrid families with Imsa mother and AquaGen father and 9 pure AquaGen families was measured in 3-5 replicates per family.
All eggs were weighed to the nearest 0.1 mg at the end of each day of O 2 consumption measurements for the Surna-AquaGen material and on the day following O 2 consumption measurements for the Imsa-AquaGen material. These measurements constitute the basis for the mean embryo masses reported in Supporting Information Table S1. A subsample of embryos was also photographed for later linear measurements to calculate the relationship between embryo mass and volume.
Total O 2 consumption (mg) of the embryos was calculated from the measured decline in O 2 concentration and the water volume (i.e., excluding the volume displaced by eggs) in the containers. Control measurements were used to correct for microbial metabolism. The results showed a decline in estimated O 2 consumption h −1 with increasing duration of the measurement period, likely caused by diffusion of O 2 into the containers. We could therefore not use O 2 consumption h −1 as a direct measure of SMR. To account for differences in duration of the measurement period among samples, and variation in embryo size, we estimated mass-specific SMR as residuals from linear models where total O 2 consumption (mg) was regressed against the duration of the measurement period and egg mass (Surna-AquaGen: r 2 = 0.36, F 2, 291 = 83.31, p < 0.001; Imsa-AquaGen: r 2 = 0.31, F 2, 184 = 41.64, p < 0.001). Oxygen consumption and egg mass were log 10 -transformed prior to these regressions to linearize the relationship between them (Supporting Information Figure S1).

| Survival and growth of juveniles in a seminatural environment
To examine survival effects of competition between wild offspring (Imsa × Imsa) and both types of hybrids (Imsa × AquaGen, either with farmed or wild mother) at different levels of food availability, we set up an experiment in 40 semi-natural stream channels. The stream channels were 4.5 m long, 24 cm wide, had a water level of 10-15 cm, and gravel substratum suitable for salmon juveniles (see e.g., Sundt-Hansen et al., 2015). Each channel had a mesh at both ends providing a confined environment. Food availability was manipulated by keeping 20 of the channels dry for five weeks immediately prior to initiation of the experiment, whereas water was allowed to run through the other channels during the same period to allow benthos establishment (cf. Einum & Fleming, 1999). To further ensure a contrast in food availability between the low and high food treatment, chironomid larvae were provided at 50% of the maintenance diet (the energy sufficient to maintain a juvenile without any change in its energy content) for the low food treatment and at 100% of the maximum diet in the high food treatment (calculated following Elliott, 1976). Chironomid larvae were introduced as semi-defrosted blocks of approximately 0.3 cm 3 (1 block per day per stream channel for the low food treatment vs. 3 for the high food treatment) that were spread by the water current so that food was available throughout the full length of each stream channel. Pre-trial tests confirmed that this procedure ensured a consistent distribution of food across the stream channels. Salmon juveniles from six families of the wild type as well as six from each of the two hybrid types were used (Supporting Information Table S2). A total of 36 individual juveniles were stocked in each stream channel on 5 May 2014 (Supporting Information Table S2), a few days before predicted median timing of emergence from the gravel (Crisp, 1981(Crisp, , 1988. Surviving fish were recaptured using dip nets on 10 June 2014 and assigned to their respective families using SNP analyses (see description below). The sampling was continued until depletion to ensure recapture of all fish.
Upon experiment termination, holes were found in the mesh of eight stream channels, all from the low food treatment. These replicates were excluded. The number of replicates omitted at the low food treatment was as follows: one for each of the hybrid types in allopatry, two for wild fish in allopatry, two for the sympatric treatment with wf hybrids and two for the sympatric treatment with fw hybrids (Supporting Information Table S2). In six additional replicates, one individual had been able to move to the neighboring stream channel (identified by genetic analyses). These six individuals were excluded from the analysis but the replicates were retained as the loss or gain of one individual would have limited effect on the remaining individuals.

| Genotyping and parental assignment
We extracted total genomic DNA from the 24 broodfish used to make the Imsa-AquaGen material, as well as from 942 surviving juveniles using the DNeasy kit from Qiagen (Hombrechtikon, Switzerland). Ninety-six SNPs (Bourret et al., 2013)  Parental assignment was conducted by a genotype exclusion approach allowing for mismatches (Vandeputte, Mauger, & Dupont-Nivet, 2006) and crosses between broodfish regardless of sex and registered crossings (Karlsson, Saillant, Bumguardner, Vega, & Gold, 2008). The latter was done as a check of the assignment power. Because mitochondrial DNA (mtDNA) is maternally inherited, we checked for possible assignment errors by comparing the haplotype of the offspring with that of the assigned mother using the 15 mitochondrial SNPs.
All 937 offspring were unambiguously assigned to a parental pair when allowing for all possible crossings between broodfish regardless of sex, and the assigned parental pairs were in agreement with the actual crossings. We identified five different haplotypes in the mtDNA, and females and their assigned offspring had the same haplotypes.
Starting with a full model fitted with maximum likelihood (ML), fixed factors were sequentially removed, and the resulting simpler models were compared with the preceding models using Akaike's information criterion (AIC). The removal of non-significant terms was done consecutively until the removal of further terms resulted in an increase in AIC ≥ 2.

| Standard metabolic rate
Differences in mass-specific standard metabolic rate (SMR) of embryos resulting from different crosses between farmed and wild adults (farmed × farmed, wild × wild, and reciprocal hybrids) at the eyed embryo stage were tested using LMMs. Different LMMs were fitted for the Surna-AquaGen and the Imsa-AquaGen data. Both models included the main fixed effects of female type (FT) and male type (MT) and their interaction. Since each female and male was represented by several offspring (full and/or half-siblings), we added female identification (femID) and male identification (maleID) as random intercepts. Thus, the initial LMM models for embryo SMR can be represented as: where α is the intercept, β are fixed factors, b are random factors and ε is a random error.

| Juvenile performance in a semi-natural environment-effects of competition, food availability and standard metabolic rate
Juvenile survival was modeled in two different ways. First, we tested whether there were differences in survival among the three types (ww, fw, and wf) and if any such effect depended on food availability (high and low), within each of the two competition treatments (allopatry and sympatry). At the same time, we tested if the mean family mass-specific SMR (family-level SMR) or mean family embryo mass had an effect on survival, and whether any such effect depended on food availability. This was done by modeling the survival (S) of families from the different types in allopatry and in sympatry in two separate binomial GLMMs that included the main effects of type (T), family-level SMR (fSMR), family-level embryo mass (fEM, mean centered), food availability treatment (F), and the interaction between F and the other main effects. Family (fam) and stream channel (ch) were included as random intercepts to take into account possible block effects and that each family was represented in several replicates. Thus, the structure of the starting GLMMs was as follows: Secondly, we tested if the competition (ww, wf and fw in allopatry, ww and wf in sympatry, ww and fw in sympatry), food treatments, family-level SMR or any of the interactions between these main effects had an effect on survival within each type. This was done by modeling the effect of competition and food treatment on survival (S) of wild offspring and each of the two types of hybrids in three separate binomial GLMMs, one for each type. All initial models included the main effect of the food availability treatment (F), competition (C), family-level SMR (fSMR) and the interaction between these, as well as the main effect of family-level embryo mass (fEM, mean centered) and its interaction with F and C. Random factors for family (fam) and stream channel (ch) were also included. The structure of these models was: Variation in final mass among juveniles of different types was modeled in a LMM where final individual mass (FM, ln-transformed) was the response variable. Family-level embryo mass (fEM, ln-transformed) was included as a co-variate to take into account variation in start weight of juveniles from different families. Also included were the main effects of type (T), food availability treatment (F), competition (C: sympatry or allopatry), the final number of surviving juveniles (N) in each stream channel and family-level SMR (fSMR), as well as the interactions between T and F and T and C, as well as their interactions with fSMR. Similar to the GLMM models, stream channel (ch) and family (fam) were included as random factors. Thus, the initial LMM model can be represented as: ln FM = + 1 ln fEM + 2 N + 3 T + 4 F + 5 C + 6 fSMR + 7 TF + 8 TC + 9 fSMRT + 10 fSMRC + 11 fSMRF + 12 fSMRTC + 13 fSMRFC + 14 fSMRTF + b fam + b ch +

| Standard metabolic rate
The main effect of male type was retained in the model that best described variation in mass-specific SMR in embryos from the SMR = + 1 FT + 2 MT + 3 FTMT + b femID + b maleID + logit S = + 1 T + 2 fSMR + 3 fEM + 4 F + 5 TF + 6 FfSMR + 7 FfEM + b fam + b ch + logit S = + 1 F + 2 C + 3 fSMR + 4 fEM + 5 FC + 6 FfSMR + 7 CfSMR + 8 FCfSMR + 9 FfEM + 10 CfEM + 11 FCfEM + b fam + b ch + F I G U R E 1 Estimated residual mean SMR ± SE of Atlantic salmon embryos of farmed and wild males resulting from crosses between the farmed AquaGen strain and (a) the river Imsa population and (b) the river Surna population. Embryos of farmed and wild females are merged because their SMRs did not differ according to model selection. In the Imsa-AquaGen crosses (a) embryos of farmed males had higher SMRs than those of wild males (p = 0.012). According to the model selection, there were no significant differences in the Surna-AquaGen crosses (b) Imsa-AquaGen crosses, whereas female type and the interaction between female and male type could be excluded. Thus, mass-specific SMR did not depend significantly on whether an egg was produced by a farmed or wild female. However, it was lower in embryos fertilized with sperm from a wild male than from a farmed male (difference in intercept estimate = −0.016, SE = 0.006, t = −2.54, p = 0.012; Figure 1a).
In the Surna-AquaGen crosses, none of the main effects or interactions were retained in the model that best explained variation in the mass-specific SMR of embryos. Thus, mass-specific SMR of the Surna-AquaGen embryos was not significantly influenced by whether the parents were of farmed or wild origin (Figure 1b). there was no effect of family-level SMR.

| Differences in survival within wild and hybrid crosses among competition treatments
For the survival of wild juveniles, the best model included the main effects of the food availability treatment, competition, family-level embryo mass, family-level SMR, and the interaction between food availability and family-level SMR. According to this model, wild offspring had significantly lower survival when they competed with either of the hybrid types (Table 1, Figure 2) than when they were in allopatry. Survival of wild juveniles increased significantly with increasing mean family embryo mass (Table 1, Figure 3a). Also, the relationship between family-level SMR and survival differed among the food treatments for the wild families (Table 1, Figure 3b).
Specifically, families with high SMR had lower survival than families with low SMR when food was limited. When food was abundant, SMR had no significant effect on the survival of individuals from the wild families.
For both types of hybrids (both those with farmed mother and wild father, and those with wild mother and farmed father), the corresponding best models included only the main effect of food availability. According to these models, survival was significantly lower at the low food treatment (60.7%; 50.5%) than at the high TA B L E 1 Parameter estimates from three statistical models that best describe survival of Atlantic salmon juveniles with wild parents (n = 108, no. stream channels = 18), farmed mother and wild father (n = 78, no. of stream channels = 13) and wild mother and farmed father (n = 78, no. of stream channels = 13) in allopatry and sympatry and at high and low food availability in semi-natural channels. For juveniles with two wild parents, the estimated slopes for survival effects of family-level embryo SMR at high and low food availability treatments and for family-level embryo mass (mean centered) are also given. All values are on logit scale and given as treatment contrasts F I G U R E 2 Survival of wild Atlantic salmon juveniles of the Imsa population in semi-natural streams at high (closed) and low (open) food availability when in allopatry and sympatry with hybrids with farmed mother and wild father (fw) or hybrids with wild mother and farmed father (wf). The values are back-transformed estimates from the binomial GLMM that was best according to model selection (85.6%; 84%) for wf and fw families, respectively (Table 1). Thus, the survival of the hybrids was not significantly influenced by the presence of wild offspring and did not depend on the family-level SMR or embryo mass.

| Growth
According

| D ISCUSS I ON
This study advances the concern that feral domestic animals impact wild populations negatively since we found that the presence of farmed-wild Atlantic salmon hybrids led to decreased early survival of their wild half-siblings under controlled, semi-natural conditions.
This negative influence of hybrid offspring on survival of wild juveniles was, at least partly, caused by genes associated with farmed salmon since it prevailed irrespective of whether the wild juveniles competed with half-siblings from wild or farmed mothers. That is, irrespective of maternal effects due to, for example, egg size differences between farmed and wild mothers. Combined with results from earlier studies Sundt-Hansen et al., 2015), this demonstrates that genetic introgression of farmed salmon may represent a direct cost to wild populations by imposing increased mortality on genetically wild individuals at the critical early life stage.
As expected, the survival of both wild and hybrid fry was lower at low food availability than at high food availability. Contrary to our predictions, food availability did not influence the effect that hybrids had on the survival of wild fry, thereby indicating that hybrids were as strong competitors at low food availability as they were at high.
Furthermore, the relative survival of wild and hybrid fry in sympatry and in allopatry did not differ between the food availability treatments. Thus, despite the farmed salmon strain used in this experiment having adapted to high food availability for 11 generations, their hybrid offspring were able to perform as well as, and even outcompete wild salmon, under low food availability.
The impact of SMR on survival among wild families depended on food availability. At high food availability, family-level SMR had no effect on survival, while at low food availability there was a negative relationship between family-level SMR and survival. Thus, in accordance with findings in Bochdansky et al. (2005), there was a F I G U R E 3 Relationships for the wild salmon juveniles of the Imsa population between (a) survival and family-level embryo mass at high (solid, filled) and low (dotted, open) food availability plotted together with mean ± SE values for each family, and (b) survival ± CI (gray) and family-level embryo SMR at high (solid) and low (dotted) food availability after correcting for survival effects of family-level embryo mass. Estimated survival effects of embryo mass are centered and plotted on mean values of the raw data. All relationships are back-transformed estimates from the best GLMM for the treatment where wild salmon were in sympatry with hybrids with farmed fathers. The pattern was consistent across treatments (wild salmon in allopatry and in sympatry with both types of hybrids, shown in Table 1) TA B L E 2 Summary of the statistical model that best describe the relationship between mass (g, ln-transformed) of Atlantic salmon juveniles in semi-natural stream channels (n = 799, no. of stream channels = 32, no. of families = 18) at the end of the experiment and mean family embryo mass (g, ln-transformed), and the number of juveniles left in each stream channel upon experiment termination  (Auer et al., 2018;Auer, Salin, Rudolf, Anderson, & Metcalfe, 2015Závorka et al., 2017). Thus, future studies of metabolic rates in farmed versus wild salmon should be extended to include other traits of the metabolic phenotype.
Under conditions with high competition, the large juveniles that hatch from large eggs often have a competitive advantage (Hutchings, 1991;Robertsen, Skoglund, & Einum, 2013). Thus, to make sure that variation in egg sizes did not underlie our results, we tested and controlled for effects of egg size in all our statistical models. As expected based on the relatively high fish densities in the stream channels, a general positive relationship between egg mass and survival prevailed across food availabil- Studies reporting lower anti-predatory response of farmed compared to wild offspring (Einum & Fleming, 1997;Houde, Fraser, & Hutchings, 2010b) suggest that they could be more vulnerable to predation than their wild counterparts. Thus, if there had been predators present in our study we may have seen lower survival of the hybrids relative to that of the wild individuals.
We found no difference in growth among the farmed-wild hybrids and the wild juveniles. It appears that hybrid offspring were not able to utilize the higher growth potential from their farmed ancestry (Gjedrem, 2000) under the conditions of this experiment, similar to that seen for growth hormone implanted Atlantic salmon in nature (Sundt-Hansen et al., 2012). There was also no difference in growth rate between our high and low food treatment, which is likely ascribed to higher mortality in the low food treatment. The final number of individuals left in each stream channel had a significant negative effect on body mass. Thus, the per capita food availability could have ended up similar in the two treatments. The lack of difference in growth across food treatments suggests that the increased mortality in the low food versus high food treatment probably manifested itself early during the course of the experiment (cf. Einum et al., 2006).
Our laboratory tests of the hypothesis that farmed Atlantic salmon have acquired increased mean levels of SMR compared to that of wild salmon gave inconclusive results. Embryos of farmed males had significantly higher SMR than embryos of wild males in the crosses between farmed salmon and wild salmon of the Imsa population, but there was not a similar finding in the Surna population. Moreover, SMR did not differ significantly between embryos of farmed females and females of the wild populations. This is, however, in line with the previously reported finding that phenotypic F I G U R E 4 Estimated relationship between individual mass (g, lntransformed) of Atlantic salmon juveniles in semi-natural stream channels at the termination of the experiment and (a) family-level embryo mass (g, lntransformed), and (b) number of surviving juveniles in each stream channel plotted on raw data effects of farmed introgression vary among wild Atlantic salmon populations (Bolstad et al., 2017).
We emphasize that the methodology we employed to measure SMR is coarse. Yet, it should provide conservative results. For example, since the containers used to measure SMR were not totally impermeable to O 2 , a decline in the O 2 concentration due to embryo metabolism would continuously be counteracted by O 2 diffusing in, leading to an overall underestimation of the O 2 consumption. This tendency would be more pronounced in containers containing embryos with high SMR since the O 2 concentration in these would decrease at a faster rate than in containers containing embryos with low SMR. Thus, if a more precise methodology had been employed, larger differences among the Atlantic salmon types in this study could possibly have been detected.
Our results show that descendants of domesticated organisms can induce increased mortality of genetically wild individuals in early life. The resulting decrease in production of recruits may obviously impact the viability of wild populations negatively. In addition, presence of domesticated descendants and an accompanying increase in mortality of wild-type juveniles could affect the adaptive landscape, potentially resulting in unforeseen changes to the wild genotype.

ACK N OWLED G M ENTS
We thank the staff at NINA Research Station Ims, Martina Galciotto for technical assistance, Ida Beate Øverjordet for filtering water, and Nina Santi for valuable comments on an earlier version of the manuscript. AquaGen AS and Statkraft Energi AS kindly provided Atlantic salmon eggs and sperm used to produce fish material for this study.

CO N FLI C T O F I NTE R E S T
None declared.

AUTH O R S CO NTR I B UTI O N
GR, DR, SE, IAF, OU, EK, and KH contributed significantly to the conception and design of the work. GR, DR, TA, and LSH performed different parts of the experiments. SK planned and led the work with parentage assignment. GR, SE, and IAF contributed significantly to the analyses and interpretation of the data. GR, SE, and IAF drafted the manuscript. All authors revised the intellectual contents of the manuscript critically and agree to be accountable for all aspects of this work.