The effects of long-term 20 mg/L carbon dioxide exposure on the health and performance of Atlantic salmon Salmo salar post-smolts in water recirculation aquaculture systems

Authors: GOOD, CHRISTOPHER - Freshwater Institute; DAVIDSON, JOHN - Freshwater Institute; TERJESEN, BENDIK FYHN - Nofima; TAKLE, HARALD - Nofima; KOLAREVIC, JELENA - Nofima; BAEVERFORD, GRETE - Nofima; SUMMERFELT, STEVEN - Freshwater Institute

Submitted to: Aquacultural Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/29/2018
Publication Date: 1/31/2018
Citation: Good, C., Davidson, J., Terjesen, B., Takle, H., Kolarevic, J., Baeverford, G., Summerfelt, S. 2018. The effects of long-term 20 mg/L carbon dioxide exposure on the health and performance of Atlantic salmon Salmo salar post-smolts in water recirculation aquaculture systems. Aquacultural Engineering. 81:1-9. https://doi.org/10.1016/j.aquaeng.2018.01.003.
DOI: https://doi.org/10.1016/j.aquaeng.2018.01.003

Interpretive Summary: Elevated dissolved carbon dioxide (CO2) can affect fish growth, feed conversion, and overall health. With increasing production of post-smolt Atlantic salmon being carried out in land-based recirculation aquaculture systems (RAS), it is necessary to investigate environmental variables, such as CO2, to define safe limits and to optimize rearing environments. In this study, post-smolt S0 Atlantic salmon were exposed to either high (20 plus or minus 1 mg/L) or low (8 plus or minus less than 1 mg/L) dissolved CO2 in six replicated freshwater RAS for 384 days. At study's end, no significant differences in fish weight, survival, feed conversion ratio, or thermal growth coefficient were observed. Blood chemistry and gill enzyme regulation analyses both revealed physiological adaptation to the higher CO2 environment. These results suggest that, under the conditions of this study, post-smolt Atlantic salmon can be raised in freshwater RAS to harvest size with up to 20 mg/L CO2 without significantly impacting fish health and performance.

Technical Abstract: Previous research and experience has linked elevated dissolved carbon dioxide (CO2) to reduced growth performance, poor feed conversion, and a variety of health issues in farm-raised fish, including Atlantic salmon Salmo salar. Supplemental control measures in water recirculation aquaculture systems (RAS) to reduce CO2 accumulation, however, such as increased water pumping to decrease tank hydraulic retention time, can represent significant costs for operators. We exposed post-smolt S0 Atlantic salmon (197 plus or minus 2 g, 423 days post-hatch) to either high (20 plus or minus 1 mg/L) or low (8 plus or minus less than 1 mg/L) dissolved CO2 in six replicated freshwater RAS for 384 days to investigate differences in performance and health as the salmon were grown to harvest size. All RAS were operated at moderate water exchange rates (1.0 % of the total recirculating flow), a 24-hr photoperiod was provided, fish were fed to satiation, and densities were maintained between 40 - 80 kg/m3. Over the study period, dissolved oxygen was kept at saturation, mean water temperature was 14.1 plus or minus 0.1 oC, and alkalinity averaged 237 mg/L as CaCO3. At study's end, no significant differences in fish weight (high CO2 mean weight = 2,879 plus or minus 35 g; low CO2 mean weight = 2,896 plus or minus 12 g), feed conversion ratio (1.14 plus or minus 0.12 vs. 1.22 plus or minus 0.13, respectively), or thermal growth coefficient (1.45 plus or minus 0.01 vs. 1.46 plus or minus 0.01, respectively), were observed. No significant differences in survival (high CO2 mean survival = 99.1 plus or minus 0.4 %; low CO2 mean survival = 98.9 plus or minus 0.3 %) or culls due to saprolegniasis (3.5 plus or minus 1 % vs. 3.0 plus or minus 1 %, respectively) were determined, and no nephrocalcinosis was observed through histopathological evaluation. Blood gas and chemistry evaluation revealed higher pCO2, bicarbonate, and total CO2, and lower chloride and glucose, in the high CO2 cohort. Molecular analyses of gill enzyme regulation showed significantly higher expression of Na+/K+ ATPase a1a in high CO2 fish at 3-weeks post-challenge, indicating physiological adaptation to the higher CO2 environment without any noticeable long-term impacts on health or performance. Overall, the results of this study suggest that, at 237 mg/L as CaCO3 mean alkalinity, post-smolt Atlantic salmon can be raised in freshwater RAS to harvest size with up to 20 mg/L CO2 without significantly impacting fish health and performance.