Table 5. --Rate of oxygen consumption (cc./gm./hr.) at various 

 oxygen concentrations 



Frequency of Opercular Movements in lao at Different 

 Oxygen Concentrations 



Procedure . --To obtain accurate gill movement counts, it was necessary to innmobi- 

 lize the fish, which was done by placing it in a clay mount on a nnicroscope slide with only the 

 head and gill cover protruding. With practice in the technique, handling could be reduced to a 

 minimum. The immobilized fish was placed in a finger bowl of sea water and, if it did not work 

 free and if it assumed a regular opercular beat, it was quickly transferred to the glass tube im- 

 mersed in the water bath. Normal sea water was aillowed to flow through the tube for about 30 

 minutes before counts were taken. In these experinnents it was possible to test aji individual fish 

 at nnore than one oxygen level, the different levels being obtained by mixing appropriate amounts 

 of water from the two carboys. Five determinations of the time required for 50 beats (from which 

 the beats /minute was calculated) were taken at each oxygen value. 



The reliability of the method was tested by taking counts in normal sea water over a 

 90-minute period (the maximum duration of any experiment) on two fish. The results indicated 

 little fluctuation in normal opercular rhythm. For each fish 15 separate counts were taken, the 

 means and standard deviations being 215 ± 4.2 (fish 1) and 213 i 3.7 (fish 2). 



