1. Introduction 1 Department of Microbiology and Immunology, Montana State University, MT, US. 2 Départment de Biologie, Université Laval, Québec, Canada An inconsistency in evolutionary ecology is that predictions of evolutionary change, which are mostly based on lab work, do not match our observations from nature. Organisms live in complex environments and such heterogeneity may strongly influence the amount of heritable variation in phenotypic traits. This could in turn affect responses to selection. However, the question of whether heritabilities change across environmental gradients has received little empirical attention, particularly for wild vertebrates. Heritable variation in body size is revealed only under specific conditions, such as those that allow for rapid growth. Although our results are specific to our experiment, G×E interactions may strongly affect underlying causes of phenotypic variation in nature Quanfying such interactions may thus provide more realistic cues into the adaptive potential of plastic traits in nature. Environmental-specific heritabilities and maternal effects for body size, morphology and survival in juvenile Atlantic salmon (Salmo salar): Evidence from a field experiment David J. Páez 1 and Julian J. Dodson 2 Fig 3 Phenotypic differences between fishes reared in rapid and slow water flows. A) Measurements between landmarks points: Body length (BL) = 1-2, head depth (HD) = 3-4, body depth (BD) = 5-6, caudal peduncle depth (CP) = 7-8, and caudal peduncle length (CAUD) = 9-2. B-F) Percent difference in mean traits across rapid and slow water by sire id (i.e. across all dams) for each trait. We test if heritable variation and maternal effects in body size, morphology and survival of juvenile Atlantic salmon differ between water flow regimes. For this, we exposed individuals of known genetic relationships (Table 1) to slow and rapid water flows in a field experiment (Fig 2). We found strong environmental effects on all measured traits (Fig 3). However, we also found considerable variation across paternal half sib families (Fig 3). Fig 4. Specific growth rates observed for the 5 measured traits in rapid (dark bars) and slow (light bars) water flow treatments. Error bars are 95 percentiles generated from a bootstrapping resampling procedure. Fig 1. Quantitative genetics experiment in the field. Full sib families were split into rapid (left) and slow (right) water flows across 72 channels like those shown above We observed elevated growth (Fig 4) but also high mortality rates (Table 1) in the rapid water treatment. In contrast, the slow water treatment was characterized by high survival (Table 1) but lower growth rates (Fig 4) Fig 5 Estimates of heritability and maternal effects for traits measured in rapid (dark grey) and slow (light grey) water flows from the field experiment. Top left shows environmental specific estimates of maternal effects on survival. Bottom left shows environmental- specific estimates of heritability of body size as measured by body lenght. Top right and bottom right panels show estimates of heritability for “raw” and size-corrected traits. Error bars are 95% high posterior density credible intervals In contrast, after correcting for body size, we found that trait heritabilities were not consistently higher in either water flow Objective Results: Phenotypic plasticity Results: heritability and maternal effects Conclusions Acknowledgements Heritable variation in body size was higher in rapid water flows (Fig 5), but survival variation in the rapid flow was explained by maternal effects (Fig 5) Finally, we found a strong correlation between survival and body size, suggesting size-selective mortality, but we cannot rule out density dependent effects Fig 6. Effects of differential mortality across rapid (closed symbols) and slow (open symbols) water flows on body length. Each point is the mean trait value for one full-sib family with error bars that are one standard deviation. This study contributes towards the research programs of CIRSA and Quebec Ocean. Funding was provided by an NSERC strategic grant awarded to L. Bernatchez., J.J.D., and H. Guderley and by an NSERC Discovery grant awarded to J.J.D. We thank A. Boivin, A. Moffett, O. Rossignol, M. Evans, C. Lavallee, Nicolas Allen- Demers, R. Pastis and LARSA for their valuable help.