FISHERY BULLETIN: VOL. 76, NO. 1 



excessive mortalities (related to passage at dams 

 and unfavorable environmental conditions caused 

 by dams and reservoirs), or transporting fish to 

 make a more direct input to a certain fishery. All of 

 these efforts may affect the maturity, growth, 

 fishing mortality, and the natural mortality for a 

 particular stock offish. In this paper, I describe a 

 method by which such changes can be accounted 

 for in the estimating procedure as soon as they are 

 determined. Thus, the present method reduces the 

 need for assumptions regarding constancy of 

 natural mortality in salmon stocks, and the re- 

 sults may be more realistic, particularly if the 

 maturity values selected are reasonable. 



In discussing their method of selecting values 

 for the proportion offish that mature annually and 

 then calculating the remaining parameters for 

 coho salmon. Lander and Henry ( 1973) pointed out 

 that the procedure also could be applied to chinook 

 salmon, although they also noted that ". . . this 

 gets to be very complicated to display graphically 

 . . . .", since coho salmon have a much simpler life 

 history than fall chinook salmon — m (proportion 

 offish that mature annually), M (natural mortal- 

 ity), and F (fishing mortality) need to be estimated 

 for 1 yr only for each brood of coho salmon, but 

 these parameters need to be estimated for three 

 separate years for each brood of chinook salmon. 

 Furthermore, the estimated values from this 

 method are quite complicated to apply to chinook 

 salmon. In fact for each m, (the subscript repre- 

 sents the different years of life covered by the 

 calculations) value selected, there is a series of 

 possible Wj values, and for each of the possible m2 

 values there is again a series of possible mg values. 

 Thus, if n separate calculations are made for each 

 m,, and there are three of them, as for the chinook 



salmon, the total calculations potentially needed 

 for a brood year would he n^ -\- n^"^ + n^^. 



METHOD OF ESTIMATING 

 PARAMETERS 



In this paper I demonstrate the method of cal- 

 culating estimates of fishing mortality (F) and 

 natural mortality (M) based on assumed values of 

 the proportion offish that mature annually (m ) for 

 the 1961 and 1962 brood Columbia River fall 

 chinook salmon. In particular, I compare data for 

 the 1961 and 1962 broods of Spring Creek fish. 



To aid in understanding the various parameters 

 I estimate, in Figure 1 1 have portrayed graphically 

 certain features of the fall chinook salmon's life 

 history, particularly the various parameters for 

 the period from the release of the fish as smolts 

 until final return to the Columbia River as 

 adults — approximately 54 mo. 



Figure 1 shows that as a result of this series of 

 events, I end up with eight items of observed 

 data: 1) number of smolts released (A'^o^; 2) 

 number maturing as 2-yr-olds (E-^); 3) number 

 caught by the ocean troll and sport fisheries as 

 3-yr-olds (Cj); 4) number maturing and return- 

 ing to the river as 3-yr-olds {E^Y, 5) number 

 caught by the ocean troll and sport fisheries as 

 4-yr-olds (C2); 6) number maturing and return- 

 ing to the river as 4-yr-olds (E^); 7) number 

 caught by the ocean troll and sport fisheries as 

 5-yr-olds (Cg); and 8) number maturing and re- 

 turning to the river as 5-yr-old fish (£^4). From 

 these eight known values I want to estimate: 1) 

 monthly fishing mortality rate on 3-, 4-, and 5-yr- 

 old fish (Fj, F2, and F^, respectively) over the last 

 6-mo period of each year; 2) monthly natural 



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D, 



t 



e-i8M, 



g-6Mj e-6<P.'M2]/'<!^'' D4 D5 





Figure 1. — Diagram depicting the life 

 history of Columbia River fall chinook 

 salmon for the period from release as 

 smolts until their return to the Colum- 

 bia River as adults — approximately 54 

 mo. Circled items indicate observed 

 data. See text for identification of let- 

 tered symbols. 



18.0 30 42 



TIME SINCE RELEASE OF SMOLTS (MONTHS) 



46 



