Total weight of fish, container, and water was 

 determined, and the fish weigh) obtained hy 

 subtraction. 



ENVIRONMENTAL CONDITIONS 



Ahhough temperature was controlled as closely 

 as possible, there were some variations. These 

 were examineil in relation to possible effects on 

 growth or surxival. Oxygen determinations were 

 also made, to ascertain if tlie levels were within 

 those considered adequate for warm-water fishes. 

 TEMPERATURE 



Room air temperature was controlled by a 

 thermostatically regulated window heat pumj), 

 whicii could either heat or cool. Water tempera- 

 ture about S cm. below the surface of tanks A, 

 E, and I (fig. 1) was recorded tlaily at about 

 8 a.m., noon, and 4 p.m. (Only one reading i)er 

 day was taken on weekends.) The means indi- 

 cated reasonably stable temperatures (table 3). 

 No 30-day mean deviated more than (17° C. 

 from the grand mean. The means for all three 

 times of daj' gave some indication that tank A 

 averaged higher than the others, but the greatest 

 e.xcess of A over either E or T was 0.5° C. Like- 

 wise, the means for all three tanks indicated that 

 the 4 p.m. reading tended to be lower than llie 

 others, but again the greatest departure was 

 0.5° ('. The means of "AU 3 by All 3" revealed 

 no consistent trend in temperatures during the 

 e.\]ieriment. The total range of individual tem- 

 |)erature readings during the entire experiment 

 was from 21.1° to 27.2° (\ 



T.\BLE .3. — Mean temperatures for tanks A, E, and I ilurinij 

 three 30-day periods ' 



Temperature Mean temperature 



Period recording ^ 



time Tank A Tank E Tank I All 3 



tanks 



"C. °c. °c. °c. 



Mar. 5- 



Apr. 21, 1864 -- 8 a.m 24.4 24.1 24.1 24.2 



Noon 24.5 24.4 24.3 24.4 



4 p.m 23.9 24.0 24.1 24.0 



All 3 times 24.3 24.2 24.2 24.2 



.Sept. 16- 



Xov. 2, 1964 8 a.m 24.9 24.4 24,6 24.7 



Noon 24.4 24.6 24.6 24.7 



4 p.m 24.8 24.1 24.3 24.3 



All 3 times---- 24.8 24.3 24.6 24. ('. 



Apr. 23- 



JUJ1C1I,1965 8 a.m 24.6 24.3 24.3 24,3 



Noon 24.4 24.1 24,0 24-2 



4 p.m 23.9 23.8 23.6 23.8 



All 3 times---. 24.3 24.1 23,9 24,1 



.ill 3 periods All 3 times 24.3 



' The period means are based on 30 days in which three daily readings were 

 taken in all three tanks. The period is not hased on 30 consecutive days. 

 The days that were excluded from the periods were ones in which fewer than 

 three readings were made (these days usually were on weekends). 



The rather small deviations just i-eoorded sug- 

 gest that temperature was fairly well controlled. 

 Any effects ou growth or survival must have been 

 slight, and no further analj-sis of temperatures 

 seems justified. 



OXYGEN CONCENTRATION 



Oxygen determinations were made for each tank 

 during March 29 to April 14, 1965. Readings 

 ranged from 4.54 to 5. 58 p. p. in., all within or 

 above the 3 to 5 p. p.m. that Lewis (1963) con- 

 sidered adequate for warm-water fishes. 



Ozone was used in the tanks to ctmtrol algae 

 during weeks 56-72. This was supplied by a 

 "Sander Ozonizer" - at the rate of 5 mg. per 

 hour. Except for occasional treatments of indi- 

 vidual tanks, the 5 mg. per hour was delivered 

 to the main air supply, thus being divided among 

 the nine tanks. Previous tests with fisii not in- 

 cluded in the experiments produced no mortali- 

 ties when the entire 5 mg. per iiour was sujiplied 

 to a single 20-1. tank. \o relation was noted 

 between growth of algae and food supply or 

 amount of light. 



POPULATION CHANGES 



For [)urposes of tuialysis, the exi)eriinent was 

 arbitrarily divided into periods before (weeks 

 0-28), and after (weeks 29-72) exploitation began. 

 Changes during the first period reflected increases 

 in number and biomass resulting from rei)roduc.- 

 tion and growth. Exploitation was responsible for 

 the major changes in the second period, resulting 

 in initial declines followed by relative stabihty 

 in both ])oi)ulation size and yield. 



INITIAL GROWTH OF POPULATIONS 



The stocks entered a period of growtii in num- 

 bers and weight, each stock influenced by the 

 amount of food sup])lied. Mean numbers and 

 weights each week for the grou)) of tiiree tanks 

 at each food level (tables 4 and 5, and fig. 2) 

 clearly bring out the influence oi food supply 

 on growth. Total weights of the stocks were in 

 the same rank as, but not exactly proportional 

 to, amtaints of food supplied. During weeks 21 

 to 2s, mean weights for diet levels 0.5, 1.0, and 

 1.5 were 14.5, 26.0, and 36.6, respectively. These 



•' Trade names referred to in this puljlication do not imply endorsement 

 of commercial products. 



FOOD LEVEL AND EXPLOITATION IN FISH POPULATIONS 



429 



