VISUAL APPEARANCE OF THE SCALES 



If a single scale is removed from the scale 

 pocket of a salmon or trout, cleaned, and ex- 

 annined under a microscope or projector, two 

 major areas will be seen: First, inbedded in 

 the scale pocket is the anterior field, the 

 larger portion of the scale, which contains a 

 series of concentric lines called circuli; and, 

 second, a smaller clear area, the posterior 

 field, which protrudes from the scale pocket 

 and shingles over the pockets of the adjacent 

 scales toward the tail of the fish. Both the 

 anterior and posterior fields are important in 

 identification of species. 



Scales of an individual fish vary considerably 

 in size and shape. Those from the back, along 

 the belly, near the fins, and operculunn are 

 often small and odd- shaped. Some of these 

 scales are unsuited for identification. The 

 scales used to illustrate this paper were all 

 taken from the middle of the side of the fish, 

 the "A" zone, Mosher (1963), The illustrations 

 {negative prints) of the various species are of 

 about the same nnagnification so they indicate 

 the relative size of normal scales. Examina- 

 tion of a few scales from a number of species 

 soon makes the relative sizes obvious. 



Anterior Field 



The features used in age determination of 

 the fish are found on the anterior field of the 

 scale and consist of the circuli and their pat- 

 terns of growth. The circuli are concentric 

 ridges on the outer surface of the scale 

 separated by valleys so that the scale roughly 

 resembles a fingerprint, or a cross section of 

 a tree where a light and dark ring show 1 

 year's growth. On a salmon or trout scale, 

 however, a year's growth is indicated by a 

 zone of widely spaced circuli, the summer 

 growth, plus a zone of closely spaced circuli, 

 the winter growth. When the scale is growing 

 rapidly, from May to August or September, 

 the circuli are wide with broad interspaces. 

 When growth slows in the winter, the circuli 

 are deposited closer together, broken, or 

 otherwise interrupted. This latter area is 

 called an annual mark, winter zone, or annulus. 

 Thus, it is possible to tell the age of the fish 

 by study of these scale features. 



Growth is much more rapid at sea than in 

 fresh water; thus circuli laid down in fresh 

 water are finer lined and more closely spaced 

 than those deposited in the ocean. These dif- 

 ferences make it possible to define the periods 

 a fish spends in each environment. 



The important scale features are identified 

 in figure 1. 



ANNUAL MARKS 



FRESH-WATER 

 GROWTH 



END OF 



FRESH- WATER 



GROWTH 



FRESH-WATER GROWTH 

 (NUCLEUS) 



On all the figures the winter growth zones, or annual 

 marks, in fresh water are Indicated by narrow arrows 

 and the ocean ones by wide arrows. Some of the features 

 In the figures can be seen more clearly with a magnifying 

 glass of 3- to 5-power. 



Figure 1. — Features of the anterior field (sculptured 

 area) of a salmon scale (a scale from a sockeye salmon, 

 age 2.2). 



In this paper I have used the European sys- 

 tem of age designation recommended by Koo 

 (1962a). The number of winters the fish spent 

 in fresh water (not counting the winter the egg 

 was in the gravel) is shown as an Arabic 

 numeral followed by a dot, then the number of 

 winters in the ocean. For instance, a salmon 

 of age 2.3 spent 2 winters in fresh water after 

 hatching and 3 winters in the ocean; the fish 

 is 5 years old and is in its sixth year. In many 

 studies of Pacific salmon, the age is recorded 

 by the system of Gilbert and Rich (1927). The 

 first digit is the number of winters from the 

 time the egg was deposited in the gravel to the 

 time of maturity (or capture); a number, 

 usually written as a subscript, shows the num- 

 ber of winters from the time the egg was de- 

 posited to time of migration to the sea. For 

 instance, an age designation of 63 corresponds 

 to age 2.3 in the European system. 



Posterior Field 



The posterior field or clear area'' of the 

 scale has certain features which help to identify 

 the species: (1) the number of circuli which 

 are complete below the focus or center of the 

 scale when viewed with the anterior field away 



Because this area of the scale is usually difficult to 

 examine under a microscope or microprojector, it may be 

 necessary to vary the lighting for each scale to find the 

 best Intensity and angle of illumination to reveal the 

 structures. In these negative prints the posterior field 

 tends to photograph black because it is relatively trans- 

 parent. 



