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In species with indeterminate growth, differential growth rates can lead to animals adopting alternative reproductive tactics such as sneak–guard phenotypes, which is partially predicted by variation in growth during the juvenile life-history stage. To investigate sources of growth variation, we examined the independent and joint effects of paternal reproductive tactic (G) and rearing environment (E) on juvenile growth in Chinook salmon (Oncorhynchus tshawytscha), hypothesizing G and E effects are partially mediated through differences in behaviour such as aggressive interactions and resulting foraging behaviours. We created maternal half-sibling families with one-half of the female's eggs fertilized by the milt of a sneaker “jack” and the other half by a guarder “hooknose”. At the exogenous feeding stage, each split-clutch family was then divided again and reared in a rationed diet or growth-promotion diet environment for approximately 6 months, during which growth parameters were measured. Before saltwater transfer at 9 months of age, social interactions were observed in groups of six fish of various competitor origins. We found ration restricts growth rate and juvenile mass, and evidence of genetic effects on growth depensation, where jack-sired individuals grew less uniformly over time. These growth-related differences influenced an individual's level of aggression, with individuals raised on a restricted diet and those whose families experienced greatest growth being most aggressive. These individuals were more likely to feed than not and feed most often. Jack-sired individuals were additionally aggressive in the absence of food, and when raised on a rationed diet outcompeted others to feed most. These results show how individuals may achieve higher growth rates via intrinsic (G) or induced (E) aggressive behavioural phenotypes, and eventually attain the threshold body size necessary during the saltwater phase to precociously sexually mature and adopt alternative reproductive phenotypes.



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