It is a nearly universal truth that larger fish eggs have increased chances of survival. However, theory predicts that as temperatures rise, larger eggs may suffer the costs of oxygen limitation and lose their advantage altering the optimal life history trade-off between fecundity and offspring size. Whether these effects may happen under the predicted warming of climate change scenarios is still unknown. Here we show that wild Esox lucius females lay smaller eggs later in the season when temperatures are higher and that this was associated with a seasonal decline in female body size. Experimentally, we found that larger eggs had higher survival rates at lower temperatures, but this advantage diminished in rising temperatures and disappeared entirely in temperatures (~ 18 °C) that could arise under climate change. The results demonstrate that the optimal egg size is temperature dependent, and that future warming may favor smaller females with smaller, more numerous, eggs that may profoundly affect population dynamics and the evolutionary optima of life-history traits.