Acidification increases sensitivity to hypoxia in important forage fishes

Background/Question/Methods

Hypoxia and acidification are important stressors in aquatic environments, particularly in shallow coastal and estuarine waters, where cycles of respiration, photosynthesis, and tides result in daily fluctuations of dissolved oxygen (DO) and pH. Because the severity of hypoxic events is expected to increase with increasing global temperatures, and global pH is expected to decrease through ocean acidification, understanding how species respond to changes in hypoxia and pH is critical to understanding how climate change will affect estuarine ecosystems. In this study, we investigated how acidification affected the sensitivity to hypoxia of two important forage fishes, the silversides Menidia menidia and M. beryllina. Under hypoxic conditions, these fish are known to exhibit aquatic surface respiration (ASR), where they respire in the thin layer of more highly oxygenated surface water. We collected silversides in the field and observed them in the laboratory in groups of three for ASR and mortality. Fish were assigned to one of four treatments: a benign control; a low pH control; a low DO, high pH treatment; or a low DO, low pH treatment, where we lowered both stressors simultaneously.

 Results/Conclusions

Fish subjected to low pH and low DO exhibited ASR and died at higher DO than fish in trials where we only lowered DO, showing that acidification made them more sensitive to hypoxia. The combination of hypoxia and increasing acidification will likely cause direct mortality of fishes and have important indirect effects on their populations. Performing ASR at higher DO concentrations and for longer periods of time substantially increases a fish’s risk of predation, particularly by avian predators, causing increased mortality under hypoxic conditions that the fish might otherwise survive. In addition to indirectly causing mortality, increased acidification could render unusable some habitats that are currently considered marginal, reducing the amount of habitat available for these important forage fishes.