faint and concentric with the 

 scale margin, but are not like 

 circuli.). 



. . . Ocean-run sockeye sal- 

 mon Oncorhynchus nerka 

 (page 4 and figure 6). 

 IB. Nucleus (fresh-water zone) present. 

 5A. Radial striations absent. 



6A. No more than six circuli complete 

 below the focus. 



. . . Sockeye salmon Oncorhynchus 

 nerka (page 4 and figures 1, 5, 

 and 20). 

 6B. More than six circuli complete be- 

 low the focus. 



7A. Reticulations generally pres - 

 ent, granular; large scales. 

 One or more spawning checks 

 may be present. 

 .... Steelhead trout Salmo 

 gairdneri (page 8 and fig- 

 ures 14, 15, 16, 17, and 23). 

 73. Reticulations generally ab- 

 sent, but if present, netlike; 

 small scales, about the size of 

 pink salmon scales. No spawn- 

 ing checks are present. 

 .... Masu salmon Oncorhyn- 

 chus masou (page 7 and fig- 

 ures 12 and 13). 

 5B. Radial striations present. 



8A. Generally no circuli in the pos- 

 terior field; if present, they are 

 weak and discontinuous as circu- 

 lar striations. 



. . . Chinook salmon Oncorhynchus 

 tshawytscha (page 6 and figures 9, 

 10, 18, 19, and 22). 

 8B. Circuli generally present in the 

 posterior field. 



, . . Coho salmon Oncorhynchus 

 kisutch (page 5 and figures 7, 8, 

 18, 19, and 21). 



SCALES OF JUVENILE SALMON 



Usually it is more difficult to identify the 

 species of juvenile salmon than of adults. This 

 is true whether scales or characters of the 

 whole fish are used. Often the determination 

 of species from scales, especially of young 

 fish, depends on the evaluation of small dif- 

 ferences among the scales of the various 

 species. 



This difficulty of identification extends even 

 into some of the postjuvenile stages, for in- 

 stance: 



1. The scales of some pink salnnon and 

 chum salmon with only one winter mark 

 are very difficult, if not impossible, to 

 differentiate because of the variable char- 

 acter of the posterior field of the pink sal- 

 mon scale; and 



2. The scales of coho and young chinook 

 salmon are extremely difficult to separate; 

 the younger the fish, the more difficult 

 it is to distinguish between them. Figure 

 18 shows scales from a "jack" chinook 

 salmon and a coho salmon of similar age. 

 Figure 19 shows scales of a "jack" chinook 

 and a "jack" coho salmon, each with only 

 one summer at sea. 



Although it is sometimes not possible to sep- 

 arate positively the young of some species of 

 salmon by their scales, at other times they 

 can be identified at a relatively early age. 

 The small size of the scales of young fish 

 makes it difficult to clean all guanin and 

 tissue from the posterior area. If they can be 

 cleaned, however, the characteristics of the 

 circuli that are complete below the focus and 

 the features of the posterior field generally 

 will enable one to identify the species. The 

 features that distinguish the species are those 

 that are formed early in the life of the fish: 



1. The presence of fresh-water growth. 



2. The number of complete circuli that 

 encircle the focus. 



3. Whether circuli are present in the 

 posterior field, or invade the posterior 

 field from the bases of the incomplete 

 circuli. 



4. The presence of radial striations in 

 the posterior field, if it is large enough to 

 have any. 



5. The number of annual marks in the 

 fresh-water zone, if the scales are from 

 fish over 1-year-old. 



In contrast, features which are present in 

 adult scales only, such as the total age of the 

 fish and the appearance of the ocean growth 

 zones, are not applicable to identification of 

 young salmon by species. To assist in species 

 identification of young fish, series of photo- 

 graphs of the scales of the four species with 

 normal fresh-water residence (except the 

 masu salmon) are shown in figures 20, 21, 22, 

 and 23. 



10 



