100 



FISHERY BULLETIN OF FISH AND WILDLIFE SERVICE 



finishes depositing her eggs. Males are no longer 

 attentive. The female is gaunt and spent, but 

 she continues to dig at the gravel with ever- 

 weakening efforts until she dies. This post- 

 spawning digging, which may continue for 10 

 days, becomes shallow, ofiF-center, and ineffective. 

 The area of the nest is increased without (after 

 the first day at least) adding to the protection of 

 the eggs. 



In view of these facts, a mature redd is con- 

 sidered one in which all egg-laying activities have 

 been concluded, and some postspawning digging 

 has been accomplished. 



Several criteria were used to determine whether 

 an observed redd was matiu-e: (1) The gaunt and 

 spent appearance of a lone female salmon still 

 digging — considered the most reliable criterion; 

 (2) the infrequency of the mating act, which 

 becomes sporadic as the supply of eggs is ex- 

 hausted; and (3) the presence of freshly covered 

 eggs in a redd unoccupied by salmon. Most of 

 the redds measured during the study met the first 

 criterion. For redds observed daily, the second 

 criterion indicated near maturity. Some blue- 

 back-salmon redds in which no fish were present 

 were considered mature on the basis of the third 

 criterion. 



MEASUREMENTS AND METHODS 



Size of the redds was determined by wading into 

 the stream and taking the length, depth, and 

 several width measurements. The outline of the 

 redd was sketched to scale on engineer's notebook 

 paper, with all pronounced irregularities drawn in. 

 Later, in the laboratory, a planimeter was used to 

 compute the area. The entire excavated portion 

 of the redd back to the tailspill's highest point was 

 included in the measurement. The prespawning- 

 digging area, usually located directly under the 

 crown of the tailspill, was thereby included. The 

 long tapering downstream slope of the tailspill was 

 not considered an essential part of the nest meas- 

 urement, for several reasons: Live eggs were rarely 

 found there; the velocity of the current would 

 carry fine sand and lighter silt considerable dis- 

 tances, resulting in elongation of the slope; and 

 often the downstream slope of the tailspill of one 

 redd would be disrupted by salmon making redds 

 immediately below it. Such tailspill encroachment 



never proceeded far enough to endanger the eggs 

 laid in the neighboring redd. 



Depth measm-ements were taken from the sur- 

 face of the water at 1-foot intervals starting at 

 stream bed at the upstream edge of the redd, 

 down the central axis, through the deepest portion, 

 and over the tailspill. A cross section at the 

 greatest depth of the redd was taken in similar 

 fashion from the stream bed on one side to the 

 stream bed on the other. 



The gravel composition for each redd was 

 arrived at by estimating the amount of large, 

 medium, and small gravel that had been exposed. 

 The term "large" gravel is used to describe stones 

 more than 6 inches in diameter, but not necessarily 

 round. "Medium" applies to stones from 6 inches 

 down to 3 inches. "Small" applies to those less 

 than 3 inches, but larger than heavy sand. 



Stream velocities were taken by clocking a bit 

 of drift over a measured course. The currents and 

 other flow conditions in a redd were determined 

 with an aqueous solution of potassium permanga- 

 nate. 



The times of the salmons' first entry into the 

 streams, the times of first redd digging and of 

 peak of spawning, average stream and redd depths, 

 and water velocities and temperatures are shown 

 in the table. 



Except in small, sparsely populated creeks, no 

 attempt was made to measure all the redds in a 

 stream. In rivers thickly covered with redds 

 throughout their length, only representative 

 sections were observed, but all the redds in Such 

 sections were measured, in order to avoid selection. 

 In these sections each redd was marked by driving 

 a stake at the downstream slope of the tailspill, or 

 by placing a number on a tree opposite the redd. 

 The stakes facilitated observation, from day to 

 day, of the redd-digging progress of an individual 

 salmon. Figure 4 is a diagram of a redd that was 

 marked on the first day of digging and measured 

 daily thereafter. In this diagram, it will be seen 

 that redd digging extended from September 20 

 to October 8. The sign of Venus (9) below the 

 line indicates the female, present each day the 

 redd was measured. The sign of Mars (cf) above 

 the line shows for each day the number of male 

 salmon actively attending and fighting for the 

 privilege of fertilization. 



