FISHERY BULLETIN: VOL. 76, NO. 1 



readers with chinook salmon scales of known age. 

 Scales from 400 marked fish of known age were 

 submitted to six readers: two from the Fish Com- 

 mission of Oregon and one each from the 

 Fisheries Research Board of Canada, Washington 

 Department of Fisheries, Oregon Game Commis- 

 sion, and Bureau of Commercial Fisheries. Length 

 offish and date of capture were available for each 

 scale. The six scale readers correctly aged 83*% of 

 the 400 test scales ( Worlund et al. 1969). Thus, we 

 believe that assumption 3 is reasonably well 

 satisfied. 



The equality of marked and unmarked survival 

 rates and maturity schedules, assumption 4, needs 

 some additional study. A lowering of the marked 

 to unmarked ratio at the hatchery from the time of 

 release to the time of return indicated possible 

 problems with this assumption. There are several 

 possible reasons for this change in marked to un- 

 marked ratio. They are: 1) errors in estimating the 

 number of marked and/or unmarked hatchery fish 

 at the time of release; 2) a difference in distribu- 

 tion or timing of marked and unmarked fish, re- 

 sulting in the marked fish being exposed to a more 

 intense fishery; 3 ) a selectivity of some fisheries for 

 marked fish; 4) a greater amount of straying for 

 marked fish than unmarked fish; 5) a difference in 

 maturation schedule for marked and unmarked 

 fish; 6) differential survival between marked and 

 unmarked fish because of marking; and 7) mis- 

 takes in aging unmarked hatchery returns. 



It is unlikely the difference in the marked to 

 unmarked ratios at the time of release and return 

 could have been caused entirely by mistakes in 

 estimating the ratio at release. The differences 

 were too great, considering the randomness of the 

 estimating procedures and the number of hatch- 

 eries involved. There is no way to determine nor 

 reason to believe differences in distribution, tim- 

 ing, or straying between marked and unmarked 

 fish caused the differences in the ratios at release 

 and return. Nor is there any way to determine or 

 reason to believe any fishery was selective for 

 marked fish. Thus we rejected these as possible 

 reasons for the change in marked to unmarked 

 ratios between the time of release and return. 



There is some indication a difference in time of 

 maturing did occur between marked and un- 

 marked fish (Cleaver 1969). Examination of the 

 marked to unmarked ratios at the hatcheries by 

 year of return shows a trend of increasing ratios. 

 This indicates the marked fish did not mature as 

 soon as the unmarked fish. The marked fish ap- 



188 



peared to stay in the ocean longer and thus were 

 subject to a higher natural and fishing mortality. 



It is also possible clipping fins and maxillary 

 bones caused mortality after the fish were released 

 from the hatchery. The unmarked fish would obvi- 

 ously not be subjected to this mortality. 



Mistakes in aging of unmarked hatchery re- 

 turns could easily have occurred because of the 

 poor condition of the fish when entering the hatch- 

 ery. The scales had been partially resorbed, mak- 

 ing them difficult to read. Since the same marks 

 were used in alternate brood years, the mark and 

 size of the fish would aid the aging procedure for 

 the marked fish. This would result in more accu- 

 rate aging of marked than unmarked fish. How- 

 ever, the errors in aging unmarked fish could have 

 been self cancelling. Possible errors in aging 

 seemed to be a very minor reason for the differ- 

 ences in the marked to unmarked ratios. 



Thus the two most probable reasons for the 

 change of marked to unmarked ratios from the 

 time of release to return were differences in mat- 

 uration schedule and differential survival of 

 marked and unmarked fish. These two problems 

 probably acted in combination. Since we have no 

 way of separating the effects of delayed maturity 

 and differential survival and since we are making 

 the best estimate of hatchery contribution, we are 

 assuming the change in marked to unmarked 

 ratio was due only to differences in survival of 

 marked and unmarked fish. Correction factors 

 were applied to adjust for the differential survival. 



The validity of assumption 5, equal ocean dis- 

 tribution and vulnerability to the fisheries for 

 marked and unmarked fish, is supported by ocean 

 tagging studies showing similar ocean distribu- 

 tion for marked and unmarked hatchery fish 

 (Cleaver 1969). 



Common marks were applied to 10 or 11% of the 

 production at the 13 study facilities for the four 

 brood years, 1961 through 1964 (Table 4). The 

 percentages ranged from 9 to 1 1 among the hatch- 

 eries for each brood. With these ranges we feel 

 assumption 6, application of common marks to the 

 same proportion of each hatchery's production, is 

 satisfied. 



FIELD OPERATIONS 



Marking and Release Procedures 



Artificial propagation procedures were similar 

 at all 13 study facilities. Adult fall chinook salmon 



