are clearly defined. Its application becomes 

 limited, however, when the boundaries of nu- 

 clear growth are not clear-cut. The chinook 

 salmon young of the Columbia River, for in- 

 stance, migrate seaward throughout most of 

 the year (Rich, 1922) ; consequently, the first 

 year's growth is subject to numerous varia- 

 tions that intergrade so completely that it is 

 impossible to draw any sharp line of distinction 

 (Rich and Holmes, 1929). Most Columbia River 

 chinooks, according to Rich and Holmes (1929), 

 have neither typical stream nor typical ocean 

 nuclei, but apparently have spent part of the 

 first year in fi'esh water and part in the ocean. 

 The result has been a nuclear area composed 

 in part of stream growth with narrowly spaced 

 circuli and in part of ocean growth with widely 

 spaced circuli to form what these authors term 

 "composite nucleus." 



The composite nucleus makes age determina- 

 tion difl^cult. In a composite nucleus, the 

 amount of stream growth varies inversely with 

 the amount of ocean growth. At one extreme is 

 the type with only a small amount of stream 

 growth accompanied by a large amount of 

 ocean growth. At the other extreme is the 

 type with a great amount of stream growth ac- 

 companied by a small amount of ocean growth. 

 The first type of nuclear growth approaches 

 the ocean nucleus, and the second type ap- 

 proaches the stream nucleus. Between these 

 two extremes there are complete intergrada- 

 tions. This poses the question : "Where should 

 the annuli be placed, and how many?" 



The question is further complicated by the 

 formation of the so-called "intermediate 

 growth," that is, growth of circuli in the 

 estuary while the fish is migrating seaward. 

 Circuli of this growth cannot be distinguished 

 with certainty from either the stream or the 

 ocean circuli, and they often form a check 

 which, in the words of Rich and Holmes, "might 

 easily be mistaken for an annulus by an inexpe- 

 rienced observer." These same authors main- 

 tain that with experience this kind of error may 

 be eliminated almost completely, and that their 

 own experience with the scales of fish of known 

 history has provided sufficient information for 

 correct age determination. 



The prerequisite of experience in scale read- 



ing cannot be denied, but the dependence upon 

 experience can be lessened and the accuracy of 

 age determination improved if some mechanical 

 method in scale work can be developed so that 

 the scale growth and marks can be interpreted 

 more objectively. The development of an 

 objective method is the major purpose of the 

 present work. From a large number of scales 

 collected from chinook .salmon of known ages 

 through recoveries of marked fish, we were able 

 to establish some definite criteria and methods 

 whereby one can objectively interpret scale 

 marks with a minimum amount of guess work. 



The present study comprises four parts. 

 First, scales from adult fall chinook that have 

 migrated seaward as unfed fry and as fed fin- 

 gerlings ' were compared in an attempt to find 

 characteristics that might serve to identify fish 

 of unknown origin; i.e., whether they come 

 from fry migrants or from fingerling migrants. 



Second, comparative studies were made be- 

 tween scales from fall chinook that had been 

 fin-clipped when released as fingerlings and 

 those that had not been marked. This was to 

 see if marking had any adverse effect on 

 growth that could be detected by scale measure- 

 ments. 



Third, marking experiments on young fall 

 chinook performed by U.S. Fish and Wildlife 

 Service personnel at the Little White Salmon 

 Hatchery provided an unusually valuable series 

 of adult scale samples for age and growth study. 

 Young chinook salmon were released over a 

 wide range of time (May to February), and 

 each release had a different mark. Scales from 

 returned adults originating from different re- 

 leases were studied to gain insight into the 

 formation of a fresh-water annulus and to 

 assess the relative amount of first and second 

 year's ocean growth due to different release 

 dates. This provided valuable information for 

 understanding scale growth patterns in fall 

 and spring chinooks. 



Fourth, the relative amount of the first 

 and second year's ocean growth on scales in 

 known stocks of fall and spring chinooks was 

 studied and compared. An objective method of 

 determining the presence or absence of an an- 



1 "Fed finKerlinKs" refers to young chinook salmon that have been 

 fed for about 3 months. 



166 



U.S. FISH AND WILDLIFE SERVICE 



