Acute hydrogen sulfide exposure in post-smolt Atlantic salmon (Salmo salar): Critical levels and recovery

Despite the importance of Atlantic salmon in marine aquaculture production systems, remarkably little is known about the effects of hydrogen sulfide (H2S) on the physiology of the species. In recent years, mass mortalities of Atlantic salmon have been reported in recirculating aquaculture systems (RAS) due to acute H2S exposure.

This highlights the importance of obtaining a better understanding of tolerance thresholds and metabolic responses to this toxic gas. The toxicity of H2S is exerted at the level of the mitochondria, where impairment of the enzyme cytochrome c oxidase inhibits cellular respiration. Because H2S depresses oxygen uptake (MO2), intermittent flow-through respirometry, a common method for assessing the metabolic response to various stressors in fishes, is a suitable method to determine concentration thresholds for when H2S affects the metabolism of Atlantic salmon.

During exposure trials, 3 size groups (range ⁓100-500 g) of fish were acclimated to control conditions to obtain baseline measurements, whereafter they were exposed to progressively increasing H2S concentrations (0.53 ± 0.14 μM h−1) until MO2 decreased below the standard metabolic rate or loss of equilibrium occurred, which we considered to be the critical H2S concentration (H2Scrit).

Fish were then allowed to recover in H2S free water to determine the excess oxygen consumption (EOC) following H2S exposure. The results show that Atlantic salmon have a lower tolerance to H2S than previously estimated, with a mean H2Scrit of 1.78 ± 0.39 μM H2S, which was independent of size. During recovery, the estimated EOC greatly exceeded the accumulated oxygen deficit (DO2) in all groups, and the small salmon had a significantly larger EOC.

While the magnitude of the EOC was greater for small salmon, it did not differ in duration (recovery time) among the different sizes of fish. The larger EOC showed that H2S exposure had a greater effect on the recovery phase of the small salmon, and exposure to H2S may leave the fish more vulnerable to other stressors post-exposure.

This study provides specific values that underline the sensitivity of Atlantic salmon to acute H2S exposure and emphasizes the importance of the aquaculture industry to implement mitigating strategies for the occurrence of H2S at production facilities.