Growth, appetite, and mineral deposition in rainbow trout reared in fresh- or seawater under different CO2 regimes

Abstract

Despite degassing efforts in recirculating aquaculture systems (RAS) and short hydraulic retention time in rearing units, carbon dioxide (CO₂) concentrations reach a hypercapnic steady state. Furthermore, as CO₂ excretion is correlated with the oxygen consumption of fish and bacteria, CO₂ levels in RAS may undergo fluctuations, of greater or smaller magnitude, depending on systems design and operation. Experimental approaches to assess the effects of CO₂ on fish usually subject fish to constant CO₂ concentrations, which might not reflect rearing conditions on an industrial production scale. Here, we compare the effects of oscillating against constantly elevated CO₂ levels on the appetite, growth, feed utilization, and mineral deposition in three separate growth trials. Two trials were conducted in freshwater (FW) and one in seawater (SW). In each trial, rainbow trout (Oncorhynchus mykiss) were subjected to four different CO₂ treatments: either constant levels of 10 mg/L CO₂ (pCO₂ = 3.86 mmHg in FW / 4.57 mmHg in SW), 25 mg/L CO₂ (pCO₂ = 9.65 mmHg in FW / 11.42 mmHg in SW), fluctuating between these two concentrations over 24 h, or normocapnic control conditions of ≤3 mg/L CO₂ (pCO₂ ≤ 1.16 mmHg in FW / 1.37 mmHg in SW) for at least 5 weeks on fixed daily rations of 1.3 % of the tank biomass. In one of the freshwater trials, the diet contained high levels of phosphorus (1.8 %) to assess if elevated dietary phosphorus concentrations promoted mineralization in kidney tissues (nephrocalcinosis) under hypercapnic conditions. In both freshwater trials, fish at all CO₂ levels accepted the offered feed, while in seawater daily feed intake was reduced by 35 % at exposure to 25 mg/L CO₂ (pCO₂ = 11.42 mmHg). Despite accepting the full, albeit restricted ration, fish reared in freshwater showed that CO₂ affected appetite, evidenced by changes in mRNA expression of appetite-regulating peptides in the hypothalamus and liver of the fish. This finding was confirmed by a maximum voluntary feed intake test, showing that fish consumed less food at higher CO₂ concentrations. Despite consuming similar daily ration sizes, the specific growth rate (SGR) and feed conversion ratio (FCR) were significantly affected in the 25 mg/L freshwater group (pCO₂ = 9.65 mmHg) but not at 10 mg/L (pCO₂ = 3.86 mmHg) or when oscillating between those concentrations. In the seawater trial, however, SGR and FCR of the trout were already significantly reduced at 10 mg/L of dissolved CO₂ (pCO₂ = 4.57 mmHg) compared to the control group, but even more so in the 25 mg/L CO₂ group (pCO₂ = 11.42 mmHg). While the CO₂ regimes applied in this study and high dietary phosphorous did not result in clear macroscopic pathologies indicative of nephrocalcinosis, there was calcium deposition in trout kidneys was highly elevated in the 25 mg/L group (pCO₂ = 9.65 mmHg), indicating the potential onset of this pathology, which was supported by histopathological examinations. Overall, the results of this study show that while fish were affected by CO₂ in all trials, the severity depends on water chemistry.