A descriptive study of carbon dioxide production and removal in full-scale RAS for Atlantic Salmon (Salmo salar L.) post-smolt: A comparison of two different measurement methods for CO2

Abstract

Reliable data on metabolic rates and CO₂ removal efficiency are essential for fish welfare in intensive production and RAS design. This descriptive study at a commercial post-smolt production facility evaluates two full-scale land-based RAS with different salinity levels: Module1 (M1) (2.5 ± 0.46‰ salinity) and Module2 (M2) (13.6 ± 0.46‰ salinity, with two separate RAS units, M2-RAS1, and M2-RAS2). Over eight weeks, water quality parameters were monitored in different places in the systems. CO₂ concentrations were measured with an OxyGuard probe and calculated from total inorganic carbon (TIC). As fish biomass and feed load increased in both modules over time, CO₂ concentrations increased and pH decreased, despite bicarbonate addition. In M1, CO₂ levels increased from 6.2 to 16.4 mg L⁻¹, while in M2, levels remained between 3.5 and 7.2 mg L⁻¹. Apparent CO₂ production, calculated from TIC, averaged at 5.2 ± 0.93 mg CO₂ kg fish⁻¹ min⁻¹ in M1, and 3.2 ± 1.25 and 4.0 ± 1.45 mg CO₂ kg fish⁻¹ min⁻¹ in M2-RAS1 and M2-RAS2 respectively. At the same time, the theoretical CO₂ production rates showed a value of 4.4 ± 0.73, 4.2 ± 0.38, and 3.9 ± 0.71 mg CO₂ kg fish⁻¹ min⁻¹ in M1, M2-RAS1, and M2-RAS2 respectively. The theoretical feed-to-CO₂ conversion weight-to-weight ratio was estimated to be 0.345. However, M1 exhibited a higher measured feed-to-CO₂ conversion ratio of 0.418, while M2-RAS1 and M2-RAS2 displayed ratios of 0.261 and 0.346, respectively. Observed CO₂ (%) removal changed from 40% to 54% in M1, from 17% to 46% in M2-RAS1, and 26–44%, in M2-RAS2 over the observation period. This study suggests a significant role of the moving bed biofilter in CO₂ removal, however, the effect size varied during the study and between modules, warranting further investigation. On average, 38% of CO₂ removal before the degasser occurred within the biofilter in M1, and 49% in both RAS systems in M2. The total carbonate levels fluctuated over time, making it challenging to determine a clear increase or decrease, likely due to manual and varying bicarbonate additions. The two methods for assessing CO₂ generally revealed minor differences and need more assessment. The prevalence of nephrocalcinosis was significantly more pronounced in M1 (73%), than in M2 (11%), which could be related to the difference in the CO₂ level between the fish tanks. This study provides valuable insights into CO₂ dynamics that may be useful in RAS design and operation for optimizing water quality and fish welfare.