Patterns of Fresh-water Mortality 



Before I describe the temporal patterns of 

 fresh-water mortality, I shall review the 

 chronological events pertaining to Sashin Creek 

 pink salmon of the 1 963 brood year. ** Spawners 

 first occupied the spawning ground August 10, 

 and the die-off of spawned fish had nearly 

 ended by September Z9. About 85 percent of 

 viable eggs had hatched before Decennber 12. 

 Fry migrated between March Z5 and June 1. 



Estimates of survival and mortality are given 

 for the following periods in fresh water: 



1. Egg deposition (mid-August to 



late September) 



2. Egg deposition to hatching (late 



September to mid-December) 



3. Hatching to fry emergence (mid- 



Decennber to late March) 



4. Fry ennergence and migration 



(late March to late May) 



Total 



Months 



1.5 

 2.5 

 3.5 

 2.0 



9.5 



Survival within the nth period is calculated 

 fronn the equation: 



Si 



-n S 



Sn - S 



-1 



-2 • 



-(n-1) 



(Z) 



(3) 



The estimates of survival shown in table 8 

 are used to connpute survival during periods 

 1, 2, and 3. An example from data for the 

 upper area will illustrate how the computa- 

 tions are nnade. Survival in period 1 is esti- 



A A 



mated by S to end of spawning, giving S, = 67 

 percent for the upper area. Survival in period 

 2 is: 



A 



0.33 

 0.67 



= 0.49. 



Survival in period 3 is: 



S3 = 



0.32 



= 0.97. 



(0.67) (0.49) 



Survival during period 4 (fry emergence 

 and migration) was high. The total abundance 

 of alevins was determined from samples 

 collected at the beginning of fry ennergence 



^ The term "brood year" describes the year of spawning 

 and is not necessarily synonymous with the term "year 

 class," which applies to either the year of hatching or the 

 year of emergence of the fry. 



(early March), An estimated 3,342,000 alevins 

 were in the streambed; 3,256,000 (97 percent 

 of the estinnate) were subsequently counted 

 at the weir when they were migrating to sea 

 as fry. Samples collected on May 2 in all 

 three areas and May 24 in the middle area 

 also indicated that mortality in spawning 

 beds was low during fry emergence. Survival 

 was assunned to equal 97 percent throughout 

 the streann during period 4. 



The estimates of survival in each of the 

 four periods and the total for all four periods 

 are summarized in table 10. The estimates 

 of total survival (from potential egg deposition 

 to nnigrating fry) weighted by size of the study 

 areas are used to compute total survival 

 for pink salmon of the 1963 brood year for 

 the stream as a whole. The estimate ob- 

 tained-- 19 percent--agrees closely with the 

 estimate of 20 percent obtained from potential 

 egg deposition and number of fry counted 

 at the weir. 



Instantaneous nnortality coefficients corre- 

 sponding to the survival percentages given 

 in table 10 were also calculated. The mortality 

 coefficients provide direct conaparisons of 

 rate of nnortality among the areas and pe- 

 riods considered. 



In computing mortality coefficients, I as- 

 sumed that the rate of change in nunabers of 

 live eggs and alevins with respect to time, 



dN 



dt 



-, was constant within each time period and 



area. Thus, for the jth area and the nth period: 



(4) 



dN . 



_J2.= -MN. 



dt — in 



where M^ is the mortality coefficient. 



The exponential equation: 

 N 



'jn 



Mt 



(5) 



N' 



■jn 



is obtained by integrating equation (4). In 

 equation (5), the denonninator (N'jn) is the 

 number of live eggs and alevins present at 

 the beginning of period n, and the numerator 

 (Njn) is the number at the end. Hence, the 

 following relation holds: 

 N. 



-in 

 = 3jn = e 



■ Mt 



The equation: 



M 



■li2(Sj^) 



(6) 



(7) 



is obtained by conversion to natural logarithms. 



