to the focus or to the edge of the scale. Because 

 the circuli of the fresh-water zone are closer to- 

 gether and more finely lined and the growth 

 zones may be composed of fewer circuli than in 

 the ocean, the winters of fresh-water growth are 

 sometimes indistinct and difficult to identify. 



To assist in interpretation of the fresh-water 

 zone, winter growth zones are classified into 

 four types (numbers 1 to 4 in plate 4 corre- 

 spond with the following descriptions) : 



1. Those made up of two or three circuli close 

 together. This type of winter zone is the 

 most distinct and easiest to identify and 

 measure because it looks almost like a contin- 

 uous line around the anterior field. The width 

 of the arro\\'S indicates the relative width 

 of the winter zone. 



2. Those made up of three or more circuli lying 

 close together in a narrow band. This type 

 of zone is relatively easy to identify, but less 

 easy to measure or delimit than those of type 

 1. 



3. Those made up of a number of complete or 

 broken circuli lying close together in a broad- 

 er band than those of type 2. 



4. Those made up of a few circuli that are not 

 much closer together than the adjacent circuli 

 of the summer growth. These zones are diffuse 

 and difficult to identify and measure. 



FRESH-WATER AGES (Plate 5) 



Plate .5 shows the range of fresh-water ages 

 and the range of sizes of fresh-water zones of 

 each age that may be found in sockeye salmon 

 scales. The complete scale of the 0. age fish is 

 magnified about 20 X. All other photographs are 

 of the fresh-water zones of scales from adult 

 sockeye salmon and show the relative sizes of 

 the various zones as they appear when projected 

 at about 40 X. The winters in fresh water are 

 indicated by arrows. 



Age 0. A complete scale from a fish in its 

 third year that migrated to the ocean 

 soon after hatching, magnified about 

 20 X. The few sockeye salmon that 

 migrate seaward early in their first 

 year usually reach the ocean before 

 scales are formed; thus, their scales 

 lack a fresh-water gi-owth zone. Ad- 

 ditional examples are shown in plate 



12 and in Koo (1962c). 



Age 1. Scales from fish that migrated sea- 

 ward after one growing season in 

 fresh water (in the second year). 

 Small to large zones are illustrated. 

 The small zone is typical of fish from 

 Southeastern Alaska and some Brit- 

 ish Columbia areas. The large zone 

 is typical of fish from Fish Creek. 

 Cook Inlet, Alaska. (The stub pointer 

 near the top of the upper scale in this 

 series indicates the end of the tran- 

 sition zone; see plate 6.) 



Age 2. Scales from fish that migrated to the 

 sea after two full gi'owing seasons in 

 fresh water (in tlieir third year). 

 Small to very large zones are shown. 

 The small zone is common in fish from 

 Asian areas. The very large ones 

 have been found only in fish from Da- 

 linaia River, Kamchatka. 



Age 3. Scales from fish that migrated to the 

 sea after three full growing seasons 

 in fresh water (in their fourth year) . 

 Small to large zones illustrated. The 

 small zone is common in fish from 

 Asian areas. 



Age 4. A scale from a fish that migi-ated to 

 sea after four full gi'owing seasons 

 in fresh water (in its fifth year). 

 Fish of this fresh-water age are rare. 



TRANSITION ZONE BETWEEN 

 FRESH WATER AND OCEAN GROWTH 



(Plate 6) 



Correct interpretation of the zone of transi- 

 tion between fresh-water and ocean growth is 

 important in age determination. Many disagree- 

 ments on the age of individual sockeye salmon 

 can be traced directly to different interpreta- 

 tions of this portion of the scales. 



In most areas of western Alaska, new growth 

 beyond the zone of narrow winter circuli is evi- 

 dent on the margin of nearly all scales of young 

 fish in fresh water by mid-June (Koo, 1962a; and 

 Mosher") and earlier in areas to the south. 



" Mosher, Kenneth H. Time of fresh-water annulus formation on 

 the scales of young sockeye salmon from western Alaska. U.S. Fish 

 and Wildhfe Service. Bureau of Commercial Fisheries Biological 

 Laboratory, Seattle. Wash. [Manuscript.] 



PHOTOGRAPHIC ATLAS OF SOCKEYE SALiMON SCALES 



251 



