Ontogenetic shifts in thermal windows of six anadromous fish species (Alosa spp.).

Twaite shad (Alosa fallax) is a protected anadromous fish of European estuaries with regionally threatened populations, yet temperature requirements for embryo and early larval development remain poorly resolved. We present new experimental data on embryonic and yolk-sac larval temperature sensitivity of A. fallax from the Elbe River and complement these results with a structured literature compilation of life-stage-specific thermal windows across A. fallax and five congeneric Alosa species (A. alosa, A. sapidissima, A. pseudoharengus, A. aestivalis, A. mediocris). Development of A. fallax embryos was examined across a controlled temperature gradient (9.7-26.1°C) spanning and extending spawning-season conditions. Embryos hatched successfully between 12.1 and 26.1°C, with an optimum range for hatching success of 13.9-22.4°C. Embryonic development rate increased exponentially with temperature, corresponding to an apparent activation energy of 0.46 eV and a Q10 of 1.9. Warmer incubation produced larger larvae at hatch with reduced yolk reserves, but these larvae reached a smaller size at 90% yolk depletion than larvae reared at colder temperatures. Integrating daily growth rate and yolk consumption indicated an upper threshold for post-hatch yolk utilization efficiency (YUEDay) at 24.2°C. The thermal windows of A. fallax embryos and yolk-sac larvae were broadly consistent with stage-specific data from other Alosa species in Europe and North America, supporting the pattern that thermal windows shift toward warmer conditions and broaden during ontogeny. Together, the experimental data and genus-level compilation provide a basis for assessing spawning and nursery habitat suitability and incorporating climate risks into conservation planning for vulnerable anadromous fishes.