36 



CALIFORNIA FISH AND GAME 



received silt for varying periods of the latter portion of the incubation 

 and emergence time. 



TABLE 3 



Effect of Mining Silt During Later Stages of Incubation and Emergence — 



Trough 3 



•2-4 hours daily. 



In the first nest of this series only the first fry was removed, after 

 which the others were allowed to remain in the water above the nest, 

 as it was desired to determine the effect of turbid water on the fry. 

 Unfortunately these fish were lost during a flood on the last day of 

 the experiment and a count was not obtained. 



The six nests of Trough 3 on which complete data were secured 

 show a fair yield of fry when silt additions occurred only after the 

 normal basket hatch period (nests 2 and 3). The yield steadily 

 decreased for earlier initial dates of silt addition and nest 7, which 

 received silt from the day the eggs were first placed in the gravel, 

 yielded no fry. This series confirms series 2, that silt addition during 

 the incubation period causes severe damage and indicates that silting 

 during the last week of incubation (nest 5) may reduce the yield as 

 much as silting during the first week, and almost as much as silting 

 for the entire incubation period. Silting after the hatch date, but 

 during the period the fry are working up through the gravel may 

 reduce the yield, but the damage is not so extensive. 



The eggs recovered from these nests at the conclusion of the 

 experiment show steadily increasing numbers for earlier dates of silt 

 addition corresponding to decreased yield of fry and further show 

 that silt forms a coating which preserves the e^ and prevents develop- 

 ment of the fry. These eggs had faded in color but were not white and 

 the coating with fine particles from the mine slimes was obvious. 

 Figure 15 is a photograph showing this series of fry and eggs. As 

 previously explained, the single fry shown for nest one should be 

 disregarded as a total count was not obtained. 



The number of fry recovered from the gravel of Trough 3 was 

 small, apparently indicating cj:<j: damage rather than inability of the 

 fry to work through the gravel and silt. In this connection it was 

 observed that most fry appeared to emerge by coming through the \" 

 screen into the space between nests rather than working directly 

 upward through the deposited silt. For this reason the yields might 

 have been still lower if finer screens had been installed on the upstream 

 side of the gravel nests. For the total of 196 fry recovered at the 

 conclusion of the experiment from the gravel of all 21 nests, only five 



