These data were collected at Shannon Point Marine Center in Washington, U.S.A. in March and May, 2021. Embryos were reared at either 10C (ambient) or 16C (heightened) and at either ~500 pCO2 uatm (ambient) or ~2000 pCO2 uatm (heightened). These data measure the response of winter-spawned Pacific herring embryos to critical thermal maximum excursions (CTmax) up to either 20C or 25C for 1, 2, or 3 hours of exposure. Percent survival was evaluated between rearing temperature and pCO2 level, maximum CTmax temperature, and duration of exposure to CTmax. The results evaluate if rearing conditions affect survival during a marine heatwave and the CTmax temperature and duration at which survival diminishes.