Linking embryonic temperature with adult reproductive investment in Atlantic salmon Salmo salar

The expression of fitness-related traits, such as egg and gonad size, often varies among habitats and exhibits clinal variation along climatic and latitudinal gradients. However, the mechanisms allowing such variations are obscure and have been ascribed to both phenotypic plasticity and genetic adaptation. We experimentally tested whether variation in egg and gonad size of a poikilotherm vertebrate is influenced by the temperature individuals experienced during embryogenesis, possibly as an epigenetic effect. Atlantic salmon Salmo salar eggs were incubated under 3 embryonic thermal regimes: cold, mixed and warm treatments. The cold group received ambient river water (mean ± SD: 2.6 ± 0.4°C) and the warm group received water at 4.6°C above ambient temperature, the expected temperature in the river towards the end of this century, from fertilization until exogenous feeding commenced. The mixed group received ambient river water until hatching, whereupon the larvae received heated water until exogenous feeding commenced. When exogenous feeding was initiated, all fish were reared under identical, natural thermal conditions. At adulthood, fish that developed from warm-incubated eggs were largest, had the highest mass−length relationship and developed larger eggs and higher gonad mass relative to their own body size. There was no similar effect of thermal environment during larval development. The treatment did not affect age of maturity or fecundity. Thus, thermal conditions during embryo genesis affected the expression of adult life-history traits, a mechanism by which fish may rapidly change the size of their propagules to the anticipated thermal offspring environment. This is a novel result explaining variation in these core life-history traits.