These data were collected at Shannon Point Marine Center in Washington, U.S.A. in 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 cardiovascular response of spring-spawned Pacific herring embryos to critical thermal maximum excursions (CTmax) up to either 20 or 25 celsius for 1, 2, or 3 hours of exposure. Heart contractions assessed cardiac stress in embryos post-CTmax trials. The results evaluate if acute temperature exposure affects the long-term cardiac function of Pacific herring embryos.