The respiratory behaviour of an air-breathing catfish, Clarias macrocephalus (Clariidae)

Abstract

Clarias macrocephalus are continuous, facultative air breathers. Individuals (7.6–20.9 g) survived more than 25 days in normoxic water without surface access. Buoyancy decreased and water-breathing frequency increased when surface access was denied, but growth rate and the frequency of air-breathing attempts did not change. We examined air-breathing and water-breathing frequency in shallow (60 cm) and deep (235 cm) water under normoxic (8.0 mg O2∙L−1) and hypoxic (0.3, 0.7, 1.2, and 2.0 mg O2∙L−1) conditions to examine how changes in the travel costs of breathing affected the use of each respiratory mode. Air-breathing and water-breathing frequency increased as dissolved oxygen decreased from 8.0 to 2.0 mg O2∙L−1. Below this level air breathing continued to increase, but water breathing dropped sharply. At higher levels of dissolved oxygen (8.0 and 2.0 mg O2∙L−1), fish in deep water had lower air-breathing and higher water-breathing frequencies than fish in shallow water. Vertical distance travelled and time spent in air breathing increased with increasing depth and with decreasing level of dissolved oxygen. These results support the hypotheses that travel is a significant cost of aerial respiration and that fish respond to increases in this cost by decreasing their use of atmospheric oxygen when dissolved oxygen concentration permits them to do so.