© 2017 The Authors. A mortality model was developed to predict the impact of high CO2 concentrations on marine fish. To allow for acclimation to a gradual increase of PCO2, the model includes acid-base regulation processes, such as respiration and ion-exchange at the gills. Because the physiological mechanism connecting a decrease in blood pH and mortality is not known, a statistical model was adopted. The mortality calculated for Japanese sillago experiencing a transient change of PCO2 compares moderately well with observations.