The respiratory behaviour of Clarias lazera under different conditions was investigated.
In water saturated with air but with the fish allowed access to the surface (Fig. 2) the respiratory movements were seen to increase before it surfaced. Once the fish had settled at the bottom of the aquarium, however, a marked drop in the frequency occurred (Fig. 4): The rate gradually built up, reaching a certain maximum before a revisit to the surface was made.
At low oxygen concentrations but again with the fish allowed access to the surface (Fig. 2), diminution of the oxygen content of the water to critical value 4-3 mg/l. resulted in a marked increase in the frequency of surfacing (Fig. 7(a)). This lasted for only a short period, however, before the fish became acclimated and showed a regular pattern of surfacing at a relatively lower rate (Fig. 7(b)). The frequency again is influenced by the activity of the fish.
These results conflict with Moussa's (1957) findings and show that Clarias does not die if prevented from surfacing provided that the oxygen concentration in the surrounding water is sufficiently high. This condition, however, imposes on the fish a respiratory frequency higher than normal.
It was concluded that surfacing in Clarias is rhythmic. It is affected in the first place by the activity of the fish and secondly influenced by the oxygen concentration in water. The fish surfaces to inhale air which comes in contact with the accessory respiratory organs where gaseous exchange takes place.