We developed the HoOS simulator (Hasty or Omniscient Shads) to assess how upstream migration and reproductive timings influence allis shad Alosa alosa (Linnaeus, 1758) temperature-dependent survival of early-life stages. We tested the influence of two reproductive timings depending on whether spawners can synchronize their spawning activity (i.e., referred to as “omniscient fish”) with optimal temperature for early-life stage survival, or simply spawn immediately after arriving on the spawning grounds (i.e., referred to as “hasty fish”). We computed the survival of early-life stages using an existing relationship that predicts the survival of allis shad from hatch to up to 14 days post-hatching according to the time series of daily water temperature at the spawning grounds. The early-stage survival rate was then calculated for each reproductive timing using a range of upstream migration speed while also considering physiological constraints to spawning adults on reproductive timing. Results indicated that the temperature-dependent survival of early-life stages relative to the migration timing by the adults was dome-shaped, confirming the hypothesis of an optimal period for reproduction across the reproductive season. The expected gain of the early-stage survival under the “omniscient” reproductive timing was highest for early arrivals of adults on the spawning grounds, lowest for intermediate arrivals, and was increasing for late arrivals. The HoOS simulator demonstrated that early and late arrivals of adults calculated as less favorable could be compensated by an “omniscient” reproductive timing. In contrast, for intermediate transit times, the two spawning timings performed the same. Our results provided modelling support that a good match of spawning activity with optimal thermal conditions is particularly crucial for the early arriving adults to maximize allis shad fitness-related outcomes (here early-stage temperature dependent survival), a causal relationship frequently claimed but rarely explored.