LANDER and HENRY: 1965-66 BROOD COHO SALMON 



Table 5. — Calculated values of m (proportion maturing as 

 jacks), Ml (natural mortality rate during first 6 mo after 

 release of smolts), M2 (natural mortality rate during last 

 12 mo of ocean life), F (fishing mortality during last 4.5 

 mo of ocean life) and Zr (average total mortality for 18 

 mo after release of smolts) for 1965-66 brood Columbia 

 River hatcherv coho salmon. 



M2 have a relatively lesser effect on the esti- 

 mated values of m than the effect on M2 by a 

 corresponding range of selected values of m 

 (Figure 3). For example, if the selected value 

 of M2 is increased four times, the estimated value 

 of m is reduced by about one-half. On the other 

 hand, if the selected values of m are increased 

 four times the estimated value of M2 is decreased 

 to about one-fourth. Finally, the use of selected 

 values of m (Equations 12-15) is based on a 

 logical, probablistic, portrayal of the life history 

 of the coho salmon whereas the use of selected 

 values of M2 is a more deterministic approach. 



ANALYSIS OF 1965-66 BROOD DATA 

 BASED ON SELECTION OF m 



Let us return briefly to the estimates in Table 

 4. It might be argued that since m and M2 are 

 inversely related (this will be discussed in 

 greater detail later), the declining estimates for 

 m as one moves upriver are merely the result of 

 using a fixed value for M2 when in fact M2 is 

 different for the various sections of the river. To 

 examine this possibility in greater detail, we 

 assumed the proportion maturing as jacks (m) 



ib .10 



1966 BROOD 



,02 ,04 ,06 



PROPORTION MATURING AS JACKS ( m ) 



Figure 3. — Relations of monthly natural mortality (Mi, 

 M2), fishing mortality (F), and total mortality (Zj) rates 

 to the fraction maturing as jacks (m), for 1965-66 brood 

 coho salmon from Columbia River hatcheries. For any 

 particular value of m, the corresponding rates operated as 

 follows: Ml -first 6.0 mo after release of smolts, M2-last 

 12.0 mo of ocean life, F-last 4.5 mo of ocean life, and 

 Zj-all 18.0 mo from release of smolts to end of ocean life. 



was the same for all river sections and again 

 calculated the pertinent estimates of the para- 

 meters relating to these data. The results are 

 listed in Table 6. In making these calculations 

 we discovered that m had to be less than 0.008 

 before it was applicable to the data from all four 

 river sections. Furthermore, again ignoring the 

 Middle River data for reasons previously dis- 

 cussed, the estimates of M2 (natural mortality 

 in the last 12 mo) decreased from the Lower 

 River to the Uppermost River. 



We find these results based on an assumption 

 of a constant m for all river sections difficult to 

 accept for a number of reasons. The decreasing 

 M2 (natural mortality) as one moves upriver is 

 suspect in view of the unobserved natural 

 mortality that probably occurs during the up- 

 river migration, as it does for chinook salmon 

 (Merrell, Collins, and Greenough, 1969). Also, 



685 



