BAILEY md EVANS: LOW-TEMPERATURE THRESHOLD 



 15 27 37 103 



EXPOSURE PERIOD (DAYS) 



Figure 3. — Four initial incubation temperatures to which 

 pink salmon eggs were subjected for four exposure per- 

 iods in laboratory experiments on effects of low tem- 

 perature on growth and development. Dashed lines in- 

 dicate when individual lots were transferred to ambient 

 temperature (see footnote 5). 



frequent variations as great as ±2° C for 1 day 

 were observed. Mean temperatures for the 37- 

 day period September 7 through October 14 

 were within ±0.2° C of the planned levels. Am- 

 bient temperature, initially 6.9° to 8.7° C, 

 dropped gradually to the usual winter level of 

 4.0° C by mid-November. 



We made no direct observations of the stages 

 of development of the embryos when they were 



transferred to ambient temperature (on days 

 15, 27, 37, and 103), but in a separate exper- 

 iment we observed development of pink salmon 

 embryos in relation to temperature. At tem- 

 peratures approximating ambient in the present 

 experiment, the eggs began gastrulation about 

 the 10th day and completed gastrulation about 

 the 26th day. Eggs incubated initially at 4.5° C 

 began gastrulation about the 21st day and com- 

 pleted gastrulation about the 45th day. Eggs 

 incubated initially at 3.0° C began gastrulation 

 about the 34th day and completed gastrulation 

 about the 62d day. 



We controlled water temperatures during in- 

 cubation by mixing chilled and unchilled water 

 in the intake line to each incubation funnel. A 

 continuous flow of fresh water chilled to 1.0° 

 ±0.3° C was obtained by operation of a 1/3-hp 

 refrigeration unit. The cooling coils and agi- 

 tator propeller were suspended in an insulated 

 20-gal fiberglass tank. Unchilled or ambient 

 water was introduced through Y fittings to pro- 

 duce the required temperature for experimental 

 lots of eggs. 



Eggs incubated entirely at ambient temper- 

 ature were first to hatch — the midpoint of hatch- 

 ing occurred December 9, 1966, 94 days after 

 fertilization. The last eggs to hatch were from 

 the group that was incubated initially at 3.0° C 

 for 103 days. The survivors of the prolonged 

 cold treatment hatched February 8, 1967, 154 

 days after fertilization. 



Mortalities were inversely related to initial 

 incubation temperatures. None of the eggs in- 

 cubated at 2.0° C survived (Table 2) ; at 3.0° C, 

 about 75% died; and at 4.5° C, about 10% died. 

 Average moi'tality of eggs incubated entirely at 

 ambient temperature was only 3%. 



The occurrence of developmental anomalies 

 was also associated with severity of the initial 

 cold treatments. No alevins were produced in 

 the 2° C treatment lots. Mild spinal defoi'mities, 

 various degrees of spinal curvature in the verti- 

 cal plane, occurred in the 3.0° C and 4.5° C lots. 

 The spinal flexures were not always severe 

 enough to be easily recognized as deformities, 

 but the deformity caused the lengths of alevins 

 in these lots to be less uniform than the lengths 

 of alevins in the ambient temperature lots. 



591 



