B.\ILEV and EVANS: LOW-TEMPERATURE THRESHOLD 



5 10 15 20 



5 10 15 5 10 15 5 



TEMPERATURE ( *" C t 



Figure 2. — Temperature-depth profiles from Lake Grace 

 during months of the freshwater reproductive cycle of 

 pink salmon. 



January 1928 through December 1964.' The 

 basis of the model included records of streamflow 

 and climate from Grace Creek and several near- 

 by streams and also a computer study of the 

 monthly operation of the power plant. We have 

 chosen the year of lowest water level (1948) and 

 the year of highest water level (1934) from the 

 surface elevation model to illustrate the range 

 of temperature patterns that might be caused 

 by fluctuating water levels (Figure 1). The 

 simulated water levels and depth of water above 

 the entrance to the intake tunnel at 125.0 m used 

 to predict water temperatures are shown in 

 Table 1. 



In Grace Creek, the probable result of the 

 power plant would be lower than normal tem- 

 peratures during summer and fall when salmon 

 eggs are beginning their development and higher 

 than normal temperatures during the winter 

 when eggs and alevins are completing their de- 

 velopment. Temperatures of Grace Creek dur- 

 ing the normal spawning season currently range 

 from 15° to 10° C, but under the conditions of 

 power plant operation, temperatures would be 

 from 7° to 5° C. During the first month after 

 spawning, temperatures normally range from 



10° to 6° C, but under the altered conditions 

 they may be reduced to only 6° to 4.5° C. The 

 predicted temperatures for the winter incuba- 

 tion period, 3° to 4° C, would be consistently 

 higher than the 1° to 3° C in the unaltered 

 stream. 



LOW-TEMPERATURE THRESHOLD 



FOR NORMAL DEVELOPMENT 



OF EMBRYOS 



Several workers have studied the low-temper- 

 ature threshold for normal development of em- 

 bryos of fishes. In this section we review their 

 findings and describe our laboratory experiments 

 with pink salmon. 



EXPERIMENTS BY OTHER WORKERS 



An important aspect of the effects of low tem- 

 peratures is the stage of development at the time 



Table 1. — Estimated water temperatures that would 

 have prevailed at the 125. 0-m elevation (mean sea level) 

 of the proposed tunnel intake of Lake Grace if the pro- 

 posed power plant had been in operation during the year 

 of lowest water level (1948) and the year of highest 

 water level (1934) from 1928 to 1964. 



' Alaska Povifer Administration. 19G8. Lake Grace 

 Project, Alaska. On file, Alaska Power Administration, 

 Federal Building, Juneau, Alaska 99801. 



the critical temperature is imposed on the em- 

 bryo. Combs and Burrows (1957) associated 

 high mortalities and gross anomalies in embryos 

 of Chinook salmon with low temperatures during 

 the pregastrula stages; they used a significant 

 rise in mortality in defining the low-temperature 

 threshold. Combs (1965), Efimov (1962), and 

 Price (1940) found that salmonid eggs were 

 most sensitive to low temperature in the blastula 

 and early gastrula stages. These authors dem- 

 onstrated that once gastrulation is well under- 

 way, the embryos can tolerate temperatures close 

 to freezing. 



589 



