In an atterapt t^ account for a greater share of the Rock Island 

 counted bluebackSj an estimate based on averages v/as applied. The esti- 

 mation (using avera{;e counts) of numbers of spawners in the Wenatchee 

 system is based on the i^ssumption that the total numbers of spawners 

 utilizing a given stream section is equal to the average number of live fish 

 in the stream during the spawning period multiplied by the ratio "time period 

 live fish are in the stream" over, "average length of life of the fish after 

 reaching the section." Algebraically^ 



R = M D / T 



R equals the total nuinber of spawners ^ M is the average number, D is 

 the duration of period that live fish are present in the sectionj and T 

 is the length of life of an individual fish after reaching the sectionT 

 To evaluate this equation, the symbols M, D, and T must be ^li'ven numerical 

 value. Tlie best estimate of M is the average of successive counts in each 

 section o D is not known i\dth precision^ but can be estimated by the occur- 

 rence curves of the number of fish found in a section upon successive visits o 



The 19l;7 occurrence curves from counts obtained in the Okanogan River 

 and fitted by inspection, indicated that the length of the period during 

 which fish were present was about 35 days (figure 7) . In the absence of 

 sufficiently complete data for the Lake Wenatchee drainage, this figure was 

 used for that area also. For T, the stream life of the average blueback, 

 no definite information is available for the Columbia River, but a period of 

 7 days has been estimated on the basis of tagging experiments at Karl-uk Lake, 

 Alaska, and is here used as the best available substitute,. 



-5 



The value of D/T or 35/7 then becomes 5. IVhenonly one count was made, 

 it was used as the average. This procedure produces counts of 3U..710, 

 36,185, 2,925, and 17,l95, for 19U7, 19U8, 1919, and 1951, which when added 

 to those fish trapped and spa;med by the hatchery, give totals of 38j,6l5 or 

 U8,U percent of the I9I1? Rock Island count, U0^350 or li7,7 percent of the 

 I9I18 count, l4,l56 or 22.3 percent of the 19k9 count, and 19,559 or 19.2 per- 

 cent of the 1951 count. This is probably a better estimate of the spawners 

 which utilized the Wenatchee area (table 3) than that based on maximum counts 

 alone. 



Thus estimates of numbers of spawners utilizing the Wenatchee region 

 have been made. Their calculation is bhe first step in calculating the ratio 

 of the number of spawners to the number of square yards of suitable spav/ning 

 area available in this system. Further useful information was obtained from 

 stream survey notes compiled by stream surveys of the Service, These surveys 

 give the to tal "n timber of square yards of available spawning gravel in the 

 Wenatchee River system. Finally, information on the square yard requirement 

 for each pair of spawning bluebacks was obtained from earlier work on this 

 subject (Burner, 1951, p, 110) , V 



U/ 80O square yards given in above publicatonj used 80U which was obtained by" 

 Burner after a more complete analysis of his data. 



13 



