The experiment was divided into two different periods. The first period started on day −7 when the weight measurements were taken and lasted until the second weight sampling on day 23. From day −7 to day 0, the parr were not exposed to carbon dioxide, and from day 0 to day 7, all the experimental fish were exposed to about 7–10 mg L−1 carbon dioxide. The second period lasted from day 23 until day 38. The growth data have been presented both in relation to the carbon dioxide concentrations and in relation to the two separate trial periods. The concentration of the very high carbon dioxide group was significantly different between these two periods (p b 0.05). This was not the case for the other groups.