EFFECT OF MINING SILT ON YIELD OF FRY FROM SALMON BEDS 33 



Yield of Fry — Gravel Control Nests 



The first fry appeared above the gravel in six of the control nests 

 of Trough 1 on February 27th. Assuming that the hatch time in the 

 gravel corresponded to the basket hatch, then 13 days elapsed to first 

 emergence of fry above the gravel. The temperature during this 

 period averaged 51.9° F., representing an additional 259 temperature 

 units. In the other nest (#3) the first fry appeared on March 4. 

 Fry emerged for 44 days, until April 11, or a total of 94 days from 

 the start of the experiment. The seven control nests yielded an average 

 of 16.2% and a maximum of 25.4% fry from eggs (see Table 1). 

 At the conclusion of the experiment the entire mass of gravel from 

 each control bed was searched and the remaining eggs and fry 

 counted. (See Table 1.) 



TABLE I 



Yield and Recoveries from Gravel Control Nests 

 (500 Salmon Eggs per Nest) 



During the incubation period several storms brought in natural 

 sediment which tended to settle in the upper nests of the trough. 

 Samples taken during one storm contained 50 p. p.m. of suspended 

 solids above nest 1 and 33 p.p.m. below nest 7, indicating a deposition 

 of 17 p.p.m. Visual observation indicated that most of this sediment, 

 which was very dark in color, settled on the first three nests while 

 the last four were relatively free from silt. The higher and fairly 

 constant yield of live fry from the last four nests is therefore repre- 

 sentative of development without appreciable silting, while the lower 

 values from nests 1-3 represent yield of fry for gravel beds that were 

 subject to natural silting. Previous experiments by Shapovalov (1937) 

 on development of steelhead eggs in artificial gravel nests resulted in 

 a yield of 29.8% during a period of storms and a yield of 79.9% in 

 tests at a later date when storms were not prevalent. Further experi- 

 ments by the same author (Shapovalov and Berrian, 1939) on develop- 

 ment of salmon eggs in gravel resulted in the low yield of 10.2% which 

 was attributed to silting from severe storms. 



