Salmonids are a socioeconomically and ecologically important group of fish that are often managed by stocking. Little is known about potential sex-specific effects of stocking, but recent studies found that the sexes differ in their stress tolerances already at late embryonic stage, i.e., before hatchery-born larvae are released into the wild and long before morphological gonad formation. It has also been speculated that sex-specific life histories can affect juvenile growth and mortality, and that a resulting sex-biassed demography can reduce population growth. Here we test whether juvenile brown trout (Salmo trutta) show sex-specific life histories and whether such sex effects differ in hatchery- and wild-born fish. We modified a genetic sexing protocol to reduce false assignment rates and used it to study the timing of sex differentiation in a laboratory setting, and in a large-scale field experiment to study growth and mortality of hatchery- and wild-born fish in different environments. We found no sex-specific mortality in any of the environments we studied. However, females started sex differentiation earlier than males, and while growth rates were similar in the laboratory, they differed significantly in the field depending on location and origin of fish. Overall, hatchery-born males grew larger than hatchery-born females while wild-born fish showed the reverse pattern. Whether males or females grew larger was location-specific. We conclude that juvenile brown trout show sex-specific growth that is affected by stocking and by other environmental factors that remain to be identified.

Since the dawn of life, transfers of metabolized material between individuals have led to great innovations of evolution. When metabolized material is transferred from one individual’s body to another (as with sperm, eggs, milk, symbionts), secondary manipulative molecules that induce a physiological response in the receiver are often transferred along with the primary cargo. The bioactive and transfer-supporting components in these socially transferred materials have evolved convergently to the point where they can be used in applications across taxa and type of transfer. Because these materials’ composition is typically highly dynamic and context-dependent, their focused study will allow deeper understanding of their transformative evolutionary and physiological role. We synthesize a conceptual framework for their study, and discuss future directions.

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