The effect of CO₂, total ammonia nitrogen and pH on growth of juvenile lumpfish (Cyclopterus lumpus)

The objective of this study was to examine the effects of CO₂, total ammonia nitrogen (TAN: nitrogen bound as either NH₃ or NH₄⁺) and pH on the growth and survival of juvenile lumpfish in a two-month growth study. The results demonstrate the complex interactions of these water quality variables. The specific growth rate (SGR) of the lumpfish was progressively reduced with increasing [CO₂] concentration above 5–10 mg·L⁻¹. However, growth may be reduced at even lower [CO₂] and the results provide no clear safe limits under which the growth of lumpfish is unaffected by CO₂ concentration. At the lowest [CO₂] tested (8 mg·L⁻¹) the SGR was reduced compared with controls. Moreover, in treatments where the [CO₂] concentration increased to 8 mg·L⁻¹ as pH was artificially reduced, the SGR was similarly reduced. These results indicate that lumpfish juveniles are very sensitive to increased [CO₂] concentration in water. The SGR of the lumpfish was progressively reduced as the NH₄⁺ concentration increased while maintaining the [NH₃] constant (70–80 μg·l⁻¹). However, in these treatments the [CO₂] also increased to 8–10 mg·L⁻¹ due to changes in pH and this likely contributed to reduced growth. Moreover, due to nitrification in the rearing systems, the nitrite concentration also increased to critical levels (1.5–4.2 mg·L⁻¹) that could affect growth. Therefore, the results do not provide clear evidence for effects of increased NH₄⁺ on the growth of lumpfish. Taken together, the results of the experiments show that lumpfish are sensitive to perturbations in water quality and provide a benchmark for operational welfare indicators in lumpfish aquaculture.