OXYGEN CONCENTRATION CC/L 



Figure 8. --Change in rate of 



oxygen consumption 

 (cc. /gm. /hr. ) with a de- 

 crease in the oxygen 

 content of the water. 

 Ordinate represents 

 the decrease in rate 

 of oxygen consunnption 

 from control values. 



The experiments were performed in 

 February of 1952 and 1953, at temperatures of 23 - 

 24°C. {average 23.9 C. ). Fish used ranged in length 

 from 39 to 62 mm. and in weight from 0. 56 to 2. 46 gm, , 

 weight being taken to the nearest 0. 01 gm. on a Roller- 

 Smith torsion balance after 1 week's preservation in 

 10-percent formalin. 



Results . --From the eye-fitted curve (fig. 9) 

 it is seen that as the oxygen content of the water falls, 

 the respiratory rate rises, slowly at first, then more 

 rapidly, to a peak beyond which it slowly decreases. 

 The maximunn breathing rate is attained probably some- 

 where between 2. and 2.5 cc. /I. --the peak in the curve 

 is not well defined, principally because of the very 

 gradual subsequent decrease in breathing rate. The 

 points are widely scattered in the first part of the curve 

 and this is reflected in the behavior of the fish. As the 

 oxygen concentration is lowered the fish at first show 

 some agitation and occasional irregular breathing. 

 Later, at the very low oxygen values, breathing is slow- 

 er and nnore regular. No quantitative measurements of 

 breathing-depth were nnade, but the fish were observed 

 to breathe deeper, as well as faster, as the oxygen con- 

 tent was lowered. Deep breathing continued, in several 

 cases, after breathing frequency began to diminish. 



Discussion 



Comparison of the relationship of oxygen consumption (fig. 8) and opercular rhythm 

 (fig. 9) to the oxygen content of the water is experimentally difficult for at least two reasons. For 

 one thing, it was necessary to use different techniques in each series of tests. In the experiments 

 on oxygen consumption each group of fish could be tested at only one oxygen level, whereas in the 

 breathing rate experiments a fish could be run through a whole series of oxygen values. Further- 

 more, the points are somewhat scattered in both cases, especially at the higher oxygen concentra- 

 tions, and it is difficult to determine precise oxygen values below which rate of oxygen consump- 

 tion and breathing rate decline. With these precautions in mind, it is still perhaps safe to say 

 that both breathing rate and oxygen consumption begin to decrease at approximately the same level 

 of oxygen, somewhere between 1.5 and 2.5 cc. /I., the implication being that the fall in breathing 

 rate contributes, in part at least, to the fall in oxygen consumption. Frona the standpoint of the 

 livebait fishery, a "danger" point might be placed at 2. 5 cc. /I. If the oxygen content of the water 



in the live-wells feills much below this 

 value, the fish will more than likely 

 not be getting sufficient oxygen to main- 

 tain normal metabolism. 



LETHAL VALUES OF OXYGEN 



OXYGEN CONCENTRATION CC/L 



Figure 9. --Breathing rates (beats /minute) at 

 different oxygen concentrations. 



A major problem in the 

 confinement of baitfish is the possible 

 depletion of oxygen to a level below 

 that tolerated by the fish. Suehiro 

 (1951) found that newly introduced 

 "wild" bait congregated densely at the 

 surface, and a localized depletion of 

 oxygen to below the lethal value occur- 

 red, regardless of an increase in the 



16 



