ABSTRACT: Rising oceanic partial pressure of CO₂ (pCO₂) could affect many traits in fish early life stages, but only few species to date have shown direct CO₂-induced survival reductions. This might partly be because species from less CO₂-variable, offshore environments in higher latitudes are currently underrepresented in the literature. We conducted new experimental work on northern sand lance Ammodytes dubius, a key forage fish on offshore Northwest Atlantic sand banks, which was recently suggested to be highly CO₂-sensitive. In 2 complementary trials, we produced embryos from wild, Gulf of Maine spawners and reared them at several pCO₂ levels (~400-2000 µatm) in combination with static (6, 7, 10°C) and dynamic (10→5°C) temperature treatments. Again, we consistently observed large, CO₂-induced reductions in hatching success (-23% at 1000 µatm, -61% at ~2000 µatm), and the effects were temperature-independent. To distinguish pCO₂ effects during development from potential impacts on hatching itself, some embryos were switched between high and control pCO₂ treatments just prior to hatch. This indeed altered hatching patterns, consistent with the CO₂-impaired hatching hypothesis. High CO₂ also delayed the day of first hatch in one trial and peak hatch in the other, where later-hatched larvae were of similar size but with progressively less endogenous energy reserves. For context, we extracted seasonal pCO₂ projections for Stellwagen Bank (Gulf of Maine) from regional ensemble simulations, which indicated a CO₂-induced reduction in sand lance hatching success to 71% of contemporary levels by 2100. The species’ unusual CO₂ sensitivity has large ecological and scientific ramifications that warrant future in-depth research.