1. Climate warming imposes a severe threat to freshwater ecosystems, which are dominated by ectotherms such as fish and aquatic insects. To better predict the effects of climate warming on thermally sensitive ecosystems, information on how temperature affects individual traits within populations is fundamental.

2. Patterns of intraspecific variation in thermal reaction norms along geographic thermal gradients provide valuable information. Immediate temperature effects on individual traits can be inferred from the shape of the thermal reaction norm. The way that temperature and associated environmental conditions (to which populations have been exposed over generations) affect individual traits through transgenerational plasticity in, and/or natural selection on, these traits can also be inferred from patterns of trait variation along a geographic thermal gradient. 

3. Many studies have documented patterns of intraspecific variation in thermal reaction norms along geographic thermal gradients. However, most previous studies cannot exclude the possibility that the observed geographic patterns are solely explained by random processes, such as isolation by distance, due to the lack of replication of geographic gradients. Here, we show consistent patterns in intraspecific trait variation along geographic thermal gradients using Salmo trutta (brown trout), which is an ecologically and economically important fish in alpine streams. 

4. We kept trout embryos collected from 52 families from 14 populations along wide and replicated elevational gradients from three Alpine drainages (Danube, Po, and Rhine) in two temperature treatments (3.2 and 6.2 °C). Timing and body size at emergence from the nest, which are key early life-history traits of trout affecting their early growth and survival, were measured. 

5. Besides faster embryonic development at warmer temperatures, we found that offspring from low-elevation parents took very slightly more days to reach emergence from fertilisation and were larger than offspring from high-elevation parents. Importantly, this was evident for all three drainages. Further analyses found that the higher number of days until- and larger body length at- emergence of low-elevation trout were mainly due to the larger eggs laid by low-elevation females, which had larger body size than high-elevation females. 

6. Trout female body size, which is positively correlated with egg size, is susceptible to temperatures and associated environmental conditions. Consequently, climate warming may not only immediately accelerate development rate but also shift timing and size at emergence through egg-size-mediated maternal effects within a relatively short time scale.