Apparatus 



To introduce water of different oxygen concentrations, several nnodifications in the 

 basic experimental assembly were necessary. Figure 7 shows the modified assembly set up for 

 experiments on breathing movements. When the apparatus was used to determine rate of oxygen 

 consumption, a wide-mouth gallon jar was substituted for the glass tube shown suspended in the 

 water bath. The carboy on the left contained water low in oxygen, obtained by bubbling through 

 nitrogen gas, and the one on the right contained normcil sea water. Water from both carboys was 

 joined by a "Y" tube into a single flow entering the overflow flask. Oxygen sjumples were taken in 

 the manner described earlier, with the exception that inlet samples were siphoned from the over- 

 flow flask. Control tests indicated only negligible variation in dissolved oxygen content between 

 carboy, overflow flask, and the outlet of the experimental chamber. 



Relation between Dissolved Oxygen Concentration and the Rate 

 of Oxygen Consumption of the lao 



Procedure. --Since the supply of low-oxygen water from the carboy was limited, the 

 plan was to test several lots of fish, each at a different oxygen level, from 7 to 14 fish being used 

 in each experiment. After the fish were in the container, normal sea water was circulated 

 through the jar for 1 hour. Two consecutive sets of oxygen samples were then taken from which 



the average rate of oxygen consumption 

 in normal sea water, i. e. , the " con- 

 trol" value, was calculated. The nor- 

 mal sea water line was then pinched off 

 and oxygen-poor water was permitted 

 to circulate through the system. After 

 30 minutes, a set of oxygen samples 

 was taken and the metabolic rate at the 

 new oxygen level was determined. 



The experiments were all 

 performed during morning hours at 

 temperatures generally between 24 

 and 25 C. A fairly wide range of sizes 

 was employed, the average length of 

 fish in each test ranging from 37 to 49 

 mm. 



Results. --The relationship 

 between oxygen consumption and oxygen 

 concentration (fig. 8) shows that the 

 rate of oxygen uptake fluctuates about a 

 Figure 7. --Apparatus used in measuring oxygen constant level down to low values of 



oxygen but eventually starts to fall, the 

 decline becoming more rapid as the 

 lethal value of oxygen is approached. 

 Complete data are reported in table 5. 

 Unfortunately, few points are available 

 in the region where the decline in oxy- 

 gen consumption begins and it is diffi- 

 cult to establish anything like a definite "critical" level of oxygen. At best, it can be stated that 

 the critical level lies between 1. 5 and 2. 5 cc. /I. Some of the scattering of points, particularly in 

 the first part of the curve, may be attributed in part to the rather large range of sizes used. On 

 the other hand, tests in which a great many small fish were used did not indicate, on the whole, a 

 greater or lesser decrease in oxygen consumption compared with tests in which many large fish 

 were used. (Compare, for example, experiments 7 and 14, table 5. ) Since the experinnents were 

 performed over a fairly narrow range of temperatures, it appears unlikely that the critical level 

 would be shifted due to differences in temperature between tests. 



-Apparatus used in measuring oxygen 

 consumption and breathing rate at 

 different oxygen concentrations. The 

 apparatus is shown assembled for 

 breathing rate experinnents. Detailed 

 description in text. 



