A new study has observed increased heart rates and higher plasma chloride, hematocrit, and hemoglobin levels in fish exposed to NO3-N levels above 150 mg/L.
A recent study published in Aquacultural Engineering has assessed the impact of varying nitrate-nitrogen (NO3-N) levels on the growth, health, and welfare of post-smolt Atlantic salmon (Salmo salar) in freshwater recirculating aquaculture systems (RAS).
Recirculating aquaculture systems have become an increasingly popular method for producing Atlantic salmon. One challenge that RAS operators face is the accumulation of nitrate-nitrogen, a byproduct of the nitrification process where ammonia excreted by fish is converted into nitrate by beneficial bacteria. While nitrate is less toxic than ammonia, elevated levels can still pose risks to fish health.
The research, led by John Davidson and a team of aquaculture scientists, explored whether higher concentrations of nitrate could negatively impact the growth, health, and welfare of farmed salmon.
The study compared salmon raised in RAS with NO3-N levels ranging from 100 to 250 mg/L (“high”) against those in systems with levels between 50 and 100 mg/L (“low”).
The researchers used heart rate bio-loggers implanted in selected fish to monitor physiological responses to the nitrate levels. Despite the elevated NO3-N concentrations, the salmon’s growth rates, feed conversion ratios, survival, and welfare metrics showed no significant differences between the two groups.
Planning application submitted for new RAS facility with visitor centre in Ayrshire
Key Findings
At the conclusion of the study, the researchers found that the salmon exposed to both high and low nitrate conditions exhibited similar growth rates, with final weights of 1.84 ± 0.05 kg and 1.91 ± 0.02 kg, respectively. Other important production metrics, such as feed conversion ratios (FCR), survival rates, fin condition, and cataract scores, showed no significant differences between the two groups.
However, the heart rate data provided more nuanced insights. The fish in the high nitrate group began to exhibit elevated heart rates once nitrate concentrations exceeded 150 mg/L. This physiological response was corroborated by higher levels of plasma chloride, hematocrit, and hemoglobin in the high nitrate group, suggesting that the fish were experiencing stress as a result of the elevated nitrate levels.
Despite these findings, the overall impact of nitrate on the salmon was relatively minor. The study concluded that nitrate-nitrogen levels below 150 mg/L do not significantly affect the growth, health, or welfare of post-smolt Atlantic salmon in RAS under similar conditions. However, the researchers noted that concentrations above this threshold could lead to increased stress, potentially affecting long-term fish welfare.
The study’s authors recommend additional research to explore the effects of prolonged exposure to higher nitrate levels, particularly in systems with reduced water exchange. Such research could help to establish a more definitive threshold for nitrate-nitrogen concentrations in RAS, ensuring the optimal growth and welfare of Atlantic salmon as they reach market size.
