Hypoxia tolerance of the mummichog: the role of access to the water surface

Abstract

Low dissolved oxygen (DO) had a significant effect on specific growth rate ( G S ), length increment ( I L ) and haematocrit ( Hct ) of the mummichog Fundulus heteroclitus . Regardless of access to the water surface, F. heteroclitus maintained high growth rates ( G S and I L ) at DO concentrations as low as 3 mg O 2 l −1 . With access to the water surface, both G S and I L of F. heteroclitus decreased by c. 60% at 1·0 mg O 2 l −1 compared to all higher DO treatments. When denied access to the water surface, a further decrease in G S ( c. 90%) and I L ( c. 75%) was observed at 1 mg O 2 l −1 . There was no effect of diel‐cycling DO (1–11 mg O 2 l −1 ) with or without surface access on G S , I L or Hct of F. heteroclitus . Similar trends between G S and faecal production across DO treatments suggest that decreased feeding contributed significantly to the observed decrease in growth rate. Haematocrit was significantly elevated at 1 mg O 2 l −1 for fish with and without access to the water surface. Increased Hct , however, was not sufficient to maintain high G S or I L at severely low DO. When permitted to respire in the surface layer, however, F. heteroclitus was capable of maintaining moderate growth rates at DO concentrations of 1 mg O 2 l −1 ( c. 15% saturation). Although aquatic surface respiration (ASR) was not quantified in this study, F. heteroclitus routinely swam in contact with the water surface and performed ASR at DO concentrations ≤3 mg O 2 l −1 . No hypoxia‐related mortality was observed in any DO or surface access treatment for as long as 9 days. This study demonstrates that surface access, and thus potential for ASR, plays an important role in providing F. heteroclitus substantial independence of growth rate over a wide range of low DO conditions commonly encountered in shallow estuarine environments.