ESTIMATING NATURAL AND FISHING MORTALITIES OF 



CHINOOK SALMON, ONCORHYNCHUS TSHAWYTSCHA, 



IN THE OCEAN, BASED ON RECOVERIES OF MARKED FISH 



Kenneth A. Henry ' 



ABSTRACT 



In this paper I demonstrate the method of calculating estimates of fishing mortality (F) and natural 

 mortality (M) occurring in the ocean for 1961 and 1962 brood Columbia River hatchery fall chinook 

 salmon, Oncorhynchus tshawytscha, based on assumed values of the proportion of fish that mature 

 annually {m) and on recoveries of marked fish. 



The advantages of this method over the method of assuming fixed natural mortality rates and back 

 calculating estimates are discussed. It was possible to develop estimates of 1962 Spring Creek data up 

 to the fourth year of life and to compare these estimates with values for the 1961 brood whereas no 

 estimates had been possible with the back calculation method. Thus, estimates of Af j are higher for the 

 1962 brood; estimates of Mj are very similar for the two broods and the estimates of M, are slightly 

 higher for the 1962 brood. A major difference between the two methods is that natural mortality was 

 assumed to be constant for the back calculation method whereas estimates of natural mortality were 

 obtained separately each year using assumed proportions maturing. Thus, for the 1962 brood general 

 marked fish, an A/ = 0.60 was used in the back calculation method while estimates of Mj = 5.814, Afj = 

 0.510, M3 = 0.653, and M^ = 0.727 were obtained by assuming varying proportions maturing. 



A series of graphs are developed that permit a quick analysis of any combination of proportions offish 

 maturing, fishing mortality, and natural mortality and which clearly depict the relationship between 

 these various factors. 



Cleaver (1969) developed a method for estimating 

 fishing mortalities and percentages of maturing 

 fish for each age group of fall chinook salmon, 

 Oncorhynchus tshawytscha,^ from the Columbia 

 River using selected values of natural mortality. 

 Cleaver's estimates were based on data obtained 

 from a cooperative marking experiment by fishery 

 agencies along the Pacific Coast. This experiment 

 started in 1962 and was designed to measure the 

 contribution of fall chinook salmon from Columbia 

 River hatcheries to the various fisheries. Cleaver's 

 analysis was specifically directed towards returns 

 for the 1961 brood year. The procedure used 

 catches and escapements, by age, along with 

 selected natural mortality values to back calcu- 

 late, from year 5 to year 2, annual estimates of 

 fishing mortality and proportion of fish that ma- 

 ture annually. 



Henry (1971) utilized Cleaver's method to ob- 

 tain similar estimates for the 1962 brood releases 

 of Columbia River hatchery fall chinook salmon. 



'Northwest and Alaska Fisheries Center, National Marine 

 Fisheries Service, NOAA, 2725 Montlake Boulevard East, Seat- 

 tle, WA 98112. 



^Seasonal races of chinook salmon in the Columbia River 

 system aie classified as spring, summer, or fall depending on the 

 time of year that the adults enter the river to spawn. 



Manuscript accepted June 1977 



FISHERY BULLETIN: VOL. 76, NO 1, 1978. 



Lander and Henry (1973), in analyzing returns 

 from marking experiments for Columbia River 

 coho salmon, O. kisutch, pointed out two methods 

 for estimating the various pertinent parameters 

 mentioned above from salmon mark/recovery 

 data: 1) assume selected values for M (natural 

 mortality) and 2) assume selected values for m 

 (proportion maturing). 



Although both methods gave identical esti- 

 mates of the parameters, their concepts differ. In 

 selecting a value for natural mortality, as was 

 done by Cleaver (1969) and Henry (1971), one has 

 to start at the end of the life cycle and work back- 

 wards since the calculated parameters are sequen- 

 tially dependent in that manner (Cleaver and 

 Henry also assumed a constant M for all ages to 

 simplify computations); by selecting values for the 

 proportion of fish that mature annually, one be- 

 gins at the younger age-groups and calculates the 

 various parameters sequentially towards the end 

 of the life cycle. This method more closely parallels 

 the actual life history of the salmon. Furthermore, 

 today's salmon management schemes are directed 

 at preserving existing runs and their fisheries, i.e., 

 changing diets, releasing fish at different times 

 and at different sizes, transporting fish to avoid 



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