Impact of high water carbon dioxide levels on Atlantic salmon smolts (Salmo salar L.): Effects on fish performance, vertebrae composition and structure

The role of high carbon dioxide (CO₂) levels on fish performance, bone structure/composition and as a potential cause of spinal deformities was studied. Two groups of fish were exposed to a low (control) and a high level of CO₂ for 135 days during the freshwater period. After smoltification, the fish were transferred to seawater and followed up for 517 days until they reached harvest weight (3.1 kg BW).Differences in body weight between the control and high CO₂ groups were observed. At the end of the freshwater period, average weight in the group exposed to high CO₂ levels was 20.9% lower than in the control group. Specific growth rates (SGR) from the start of the experiment (10 g BW) to smolt stage were 1.63 ± 0.04 and 1.36 ± 0.01 for the control group and the high CO₂ group, respectively. Differences in body weight were maintained during the initial stages of the seawater period, but were not observed at harvest weight.Nephrocalcinosis was not observed in any of the experimental groups at the end of the freshwater period and no external signs of spinal deformities were observed either at smolt stage or at harvest weight. X-rays revealed mild abnormalities in some vertebrae bodies, which could not be related to any experimental group. Despite the lack of signs of pathological bone alterations, the histological examination suggested that the exposure to high CO₂levels resulted in an increase in trabeculae volume and a higher rate of bone remodeling at the end of the freshwater period. Furthermore, fish exposed to a high CO₂ level presented a higher bone ash content at the end of the freshwater period. These differences could not be observed at the end of the grow-out period.