HENRY: FALL CHINOOK SALMON 



z> 



(T 

 O 

 tiJ 

 IT 



O 

 O 

 O 



6,000 



3,000 





 6,000 



3>000 



ft 





 6,000 



< 

 f- 

 o 



3,000 - 



1961 BROOD SPRING CREEK 







 



i.e 











1.8 



1.8 

 

 1.8 



-1.8- 

 

 18 



F,- 

 F4. 



1961 BROOO KALAMA 





 

 1.8 





 

 1.8 





 

 1.8 



-1.8' 



*l.8" 



1.8 



1962 BROOD KALAMA 



^5- 





 

 1.8 





 

 1.8 



yield 



= = 

 1.8 

 1.8 



stimatedj 'potentio 

 actual II 1 1 yield 



.MOKimum 



1.8 



'1.8- 

 1.8 



.24 



.45 60 .72 



NATURAL MORTALITY 



.96 



Figure 12. — Calculated maximum potential yields (kil- 

 ograms) per 1,000 recruits for the 1961 brood Spring 

 Creek and Kalama and the 1962 brood Kalama hatchery 

 fall Chinook salmon compared with yields calculated for 

 estimated rates of actual ocean fishing mortality (F 

 values shown are those producing maximum potential 

 yields) . 



those shown except that they depicted lower 

 yields, as would be expected with a higher na- 

 tural mortality. Similar analyses also were 

 made from the 1962 brood data, but those yield 

 diagrams are not depicted. 



In a similar manner to that shown for obtain- 

 ing the maximum yield, the calculated total yield 

 for any combination of Fs, Fa, and F-, could be 

 estimated from these graphs. However, the 

 purpose of these graphs is not to estimate single 

 yield values but rather to show the overall effect 

 on yield of varying values of Fs, Fa, and Fs. 

 Thus, Figure 13 shows that the total yields di- 

 minish as Fz and Fa increase but that Fs has 

 little effect on total yield. Under these condi- 

 tions, any troll fishing on 3- and 4-year-old fish 

 would reduce the total yield. 



It might be well to re-emphasize that these 

 calculations are based on the assumption of a 



constant M for the period analyzed. Variations 

 in M during this period would, of course, affect 

 the calculated total yield depending on when the 

 variations occurred and their magnitude. How- 

 ever, a differential M before entry into the troll 

 fishery, i.e., at age 2 and younger, would not 

 affect this analysis. 



There is considerable variation in the two 

 yield diagrams, and the reader will have to ex- 

 amine each figure to see in detail the effect of 

 varying levels of troll fishing for that particular 

 brood and level of natural mortality. In gen- 

 eral, at the lower levels of natural mortality 

 (Af = 0.24) , increased troll fishing on the young- 

 er age group (2 + and 3 + ) results in reduced 

 yields, whereas the reverse of this generally 

 occurs at the higher level of natural mortality 

 (Af = 0.96). In other words, at the younger 

 ages and the lower natural mortalities it appears 

 that the rate of growth exceeds the loss due to 

 natural mortality and, consequently, yield is in- 

 creased by letting the salmon reach an older age 

 before harvesting them. It should be pointed 

 out that even if no fish had been caught in the 

 ocean, the number returning to the river would 

 have been less than the sum of river entry and 

 ocean catch since some of the fish caught in the 

 ocean fishery would have died from natural 

 causes. 



The values of M and F that I used, although 

 arbitrary, are believed to cover the range of 

 realistic possible values. Obviously, any values 

 of M could be inserted into the program, but it 

 was felt that M = 0.24 and M = 0.96 would 

 depict extremes. Also, Ft values greater than 

 1.8 could be used, but I arbitrarily limited them 

 at this upper boundary because F values of 1.8 

 considerably exceed any of the F values calcu- 

 lated for either the 1961 or 1962 brood data 

 (Henry, 1971). 



The calculated total yields for 1961 and 1962 

 brood Spring Creek and Kalama fall chinook 

 salmon for various values of natural mortality 

 — if troll fishing could be restricted to only one 

 age group; i.e., troll fishing on only the 3-, 4-, 

 or 5-year-old fish — are shown in Figures 15 and 

 16. The values for river fishing intensity and 

 proportion spawning are the same as used in 

 the previous analyses. The yields shown are the 



441 



