Wider, darker circuli usually are laid down after 

 this zone in the year in which the fish migrates 

 seaAvard. If the fish reaches the ocean about the 

 time this zone of narrow circuli is completed, the 

 transition from narrow, light-textured fresh- 

 water circuli to wider, heavier textured ocean 

 circuli is abrupt. If the winter zone is completed 

 before the fish reaches salt water, however, a 

 varying number of circuli, intermediate in ap- 

 pearance between the fresh-water and ocean 

 circuli, may be formed. This type of growth is 

 called plus, transitional, intermediate, incidental, 

 or spring growth."* 



If the transition zone merges gradually into 

 the ocean gro\\i:h zone, it is usually easy to iden- 

 tify, although not to delimit. If, on the other 

 hand, an adventitious mark (called a "migration 

 check") forms and the zone is large, the migra- 

 tion check might be mistaken for another fresh- 

 water winter (Mosher®). 



Three general types of transition between 

 fresh-\\'ater and ocean growth zones can be iden- 

 tified : type A, sharp or abrupt; type B, gradual ; 

 and type C, diffuse. These general types can be 

 found in fish of all fresh-water ages. 



Examples of the three types of transition 

 zones are shown in plate 6. (The winter zones 

 are indicated by arrows, and the migration 

 checks in figures 2 and 3 by wider arrows.) 



Type A. Sharp or abrupt transition is shovra 

 by the following photographs: 



Figure 1. No plus growth. The 

 transition from the fine- 

 lined, closely spaced 

 fresh-water circuli to the 

 widely spaced ocean cir- 

 culi is sudden. This type 

 of transition poses no 

 problems for accurate 

 age determmation. 

 Figure 2. Little plus growth after 

 the vdnter zone formed. 

 A migration check is 

 between the transitional 

 growth and the ocean 



8 The term "spring growth" is usually applied to the intermediate 

 or plus growth observed in scales of seaward migrant fish, not adult 

 flsh. 



9 Mosher, Kenneth H. The plus or intermediate growth of sockeye 

 salmon scales. Bureau of Commercial Fisheries, Biological Laboratory, 

 Seattle. Wash. [Manuscript. J 



growth. That the portion 

 of the scale between the 

 two arrows is plus 

 growth is evident from 

 the narrowness of the 

 zone and the appearance 

 of the circuli, which are 

 intermediate in width 

 and spacing between the 

 circuli of the fresh-water 

 and ocean growth zones. 



Figure 3. Extensive plus growth 

 after the winter zone 

 formed. The weak migra- 

 tion check at the end of 

 the transition might be 

 mistaken for another 

 winter in fresh water. 

 Note, however, that the 

 seven or eight circuli im- 

 mediately preceding the 

 broad, widely spaced cir- 

 culi of the ocean zone are 

 intermediate in width 

 and spacing between the 

 circuli of the fresh-water 

 and ocean growth zones. 

 Type B. Gradual transition is sho\\ai by the 

 following photographs : 



Figure 4. No plus growth. The two 

 or three circuli following 

 the winter zone gradual- 

 ly expand into the broad- 

 er ocean-type circuli. 



Figure 5. Little plus growth. A 

 few circuli of intermedi- 

 ate type are present, and 

 the boundary of the tran- 

 sition is difficult to iden- 

 tify. 



Figure 6. Extensive plus growth. 

 The zone of intermediate 

 growth is wide. Because 

 there is no migration 

 check at the end of the 

 plus growth, the chance 

 is small that it will be 

 interpreted as another 

 year's gi'o\\i;h in fresh 

 water. 



PHOTOGRAPHIC ATLAS OF SOCKEYE SALMON SCALES 



25i 



