Objective The broad objective of our study was to use a histological approach to examine possible tissue‐level effects of exposure to elevated CO2 in freshwater young‐of‐year salmonids. Methods To study these effects, young‐of‐year Arctic Char Salvelinus alpinus, Rainbow Trout Oncorhynchus mykiss, and Brook Trout S. fontinalis were exposed to either control levels of CO2 (1400 μatm) or elevated levels of CO2 (5236 μatm) for 15 days. Fish were then sampled for gill, liver, and heart tissues and histologically analyzed. Result A species effect was observed for the length of secondary lamellae, as Arctic Char had significantly shorter secondary lamellae than the other species. No notable changes within the gills and livers of Arctic Char, Brook Trout, or Rainbow Trout exposed to elevated CO2 were observed. Conclusion Generally, our results indicated that elevated CO2 levels over 15 days do not induce catastrophic tissue damage and it is unlikely that fish health would be seriously impacted. Ongoing research dedicated to examining how elevated CO2 long‐term may affect internal tissues of fish will allow for a more comprehensive understanding of how fish may fair with ongoing climate change and in aquaculture facilities.

This thesis explores the behavioural and physiological effects of elevated carbon dixoide (CO2) on freshwater, juvenile samonids. Arctic Charr, Rainbow Trout, and Brook Charr were each exposed to elevated CO2 over a two week period. During this time, metabolic rate, growth, and behaviour were monitored. Feeding and tissues were examined after the exposure period. Generally, few effects of CO2 were observed; however, Arctic Charr had higher standard metabolic rate as a result of the CO2 exposure. Additionally, Arctic Charr gills showed signs of tissue damage that may have been due to the CO2 exposure. Overall, my findings suggest that short-term elevated CO2 may not be harmful for juvenile salmonids in freshwater. Future studies should explore longer and higher CO2 exposures, and the relationship between elevated temperature and CO2.