FISHERY BULLETIN: VOL. 74, NO. 2 



Table 2. — Reproduction of Daphnia piilex introduced as 

 mature and young females and maintained at temperatures of 

 15° to 33°C (acclimated at 15°C, Seattle race). 



Table 3. — Mortality of Daphnia pulex (Seattle and Columbia 

 races) acclimated at 20°C and introduced as maturing fe- 

 males to temperatures of 20° to 32°C. 



'Total reproduction from 18 animals during 50 days of experiments. 

 ^Average reproduction based on number of days survivors remained. 

 ^Total reproduction from 10 animals for 52 days of experiments. 



regularly at 6-day intervals, whereas at higher 

 temperatures reproduction was erratic. The 

 greatest numbers of offspring were produced by 

 the older females at 15°C and generally decreased 

 with increasing temperatures. The highest rates 

 were at 27° and 24°C where the survivors repro- 

 duced rapidly before they all succumbed on the 

 13th and 27th day, respectively. High tempera- 

 tures increased the rate of reproduction for a 

 short period before total mortality, but the in- 

 creased rate was short lived and did not match 

 total production by animals at a more normal 

 temperature (15°C). 



Reproduction by females who were young at 

 the start of the experiment increased with in- 

 creasing temperature, contrary to the trend 

 shown by the older females (Table 2). No repro- 

 duction occurred at 15°C until the 34th day; at 

 18° and 21°C, initial reproduction took place on 

 the 3rd to 6th day but did not resume until the 

 44th and 22nd day, respectively. At 24° and 27°C, 

 the first reproduction occurred on the 3rd to 6th 

 day, stopped for 3 or 4 days, and then continued at 

 a high rate until the death of all females on the 

 34th and 27th day, respectively. The low repro- 

 duction by the younger females at 15° and 18°C is 

 not explained. 



Seattle and Columbia Daphnia Acclimated 

 to Water of 20°C 



Both Seattle and Columbia Daphnia reached 

 TDso within 24 h at 32°C (Table 3); 90% mortality 

 occurred in less than 24 h in the Columbia group 

 and within 48 h in the Seattle group. At 29°C, 

 both groups reached 50% mortality in 120 h. 

 There were significant differences in the length of 

 time to 50% mortality for each of the two groups 

 at 20°, 23°, 26°, and 29°C (Seattle— x^ = 37.9, 



P<0.01, 3 df; Columbia— x^ = 18.8,P<0.01, 3 df). 

 The Columbia group seemed to succumb more 

 rapidly than the Seattle group, but the more 

 rapid demise of the Columbia Daphnia at 20°C 

 (the acclimation temperature) casts doubt upon 

 these results. However, a test of homogeneity for 

 temperatures of 23°, 26°, and 29°C indicated sig- 

 nificant differences between the two groups in 

 days to 50% mortahty (x^ = 22.6, P<0.01, 2 df). 

 Comparatively little reproduction took place at 

 temperatures of 26°C and above (Table 4). The 

 greatest reproduction for the Seattle group was at 

 23°C and for the Columbia group at 20°C. The 

 Columbia animals remaining after the initial un- 

 explained mortality at 20°C outproduced the 

 Seattle animals at the same temperature. The 

 Seattle animals produced 62% of the total young 

 produced by the two groups. 



Table 4. — Reproduction of Daphnia pulex (Seattle and Colum- 

 bia races) acclimated at 20°C and introduced as maturing 

 females to temperatures of 20° to 32°C. 



'Total reproduction by 10 animals for 34 days of experiments. 

 ^Average reproduction based on the number of days survivors remained. 



Experiments Relating to Water 

 Passing Through Cooling Systems 



Exposure for 15 min at temperatures of 30°C or 

 less seemed to have little or no effect upon the 

 survival of D. pulex (Table 5). The only mor- 

 talities observed during the exposure period were 

 at 36°C: within 5 min, over 50% of the animals at 

 this temperature were dead; all but one died in 15 



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