3-year-olds in the right-hand part. Beyond the 

 2-year-olds, no peaks are apparent because of 

 the blending together of age-groups of older 

 fish. 



Study of Seasonal Ring Formation 

 IN Hard Body Parts of Fishes 



Because fish are cold-blooded animals, their 

 body processes are regulated by the temper- 

 ature of the water in which they live. Growth 

 is rapid during the warm season and slows 

 greatly or stops in winter. The technique of 

 determining the age of a tree by counting an- 

 nual rings in a cross-section through the base 

 of the trunk is a familiar one. As in trees, 

 seasonal changes in growth rate of fishes are 

 often reflected in zones or bands in hard body 

 structures such as scales, otoliths (ear stones), 

 and bones. 



Fish Scales. — Of the hard body parts used 

 for age determination in fishes, scales are most 

 useful. They are easy to collect and prepare 

 for study. Of importance is the fact that a 

 few can be removed with little or no injury 

 to the animal since fishes have the ability to 

 regrow lost scales within a short time. A 

 further advantage, that of permitting an esti- 

 mation of the past growi^h history of a fish 

 from its scale, will be discussed later. 



Scales are of value for age determination 

 in many of our "bony fishes," a broad group- 

 ing which includes most fishes of importance 

 for food. Scales are formed when newly 

 hatched fish complete their larval stages, and 

 soon cover the entire body, with the exception 

 of head and fins. In most species they lie in 

 an overlapping pattern much like shingles on 

 a roof and serve as a protective coat. 



Scale growth begins with the formation of 

 the scale center or focus, and growth is out- 

 ward from this focus, though it is greatest 

 toward the forward margin of the scale. Fine 

 ridges called circuli are laid down in a circ- 

 ular pattern around the focus as growth pro- 

 ceeds. Many circuli are added to the scale each 

 year. 



Most food and game fishes have either 

 cycloid or ctenoid scales. These two scale 

 types are illustrated in Figure 3. Cycloid 



scales, found on trout, minnows, whitefish, pike, 

 cod, and most other soft-finned fishes, have 

 circuli which pass entirely around the scale 

 margin as growth is added. In ctenoid scales, 

 found on bass, perch, some flounders, and most 

 spiny-finned fishes, the focus is near the rear 

 edge of the scale, and circuli here are obscured 

 by the tiny spines or ctenii which give these 

 scales their name. It is the ctenii which give 

 the bodies of such fish a rough sandpapery 

 feeling. 



As stated earlier, fish growth is reflected in 

 scale growth. Circuli are widely spaced in 

 warm seasons whe" fish growth is rapid and 

 closely spaced in cold seasons when it is slow. 

 In some northern climates, especially in ice- 

 covered lakes, fish growth stops in winter. The 

 growth of a fish during 1 year, therefore, is 

 shown on its scale as a series of widely spaced 

 spring and summer circuli followed by a series 

 of closely spaced fall and winter circuli. Since 

 fishes continue to grow throughout their lives, 

 this pattern is repeated each year. The outer 

 edge of a series of closely spaced circuli is 

 generally taken to be the end of growth for 

 that year, and this point is referred to as the 

 year-mark or annulus (see whitefish and had- 

 dock scales in Figures 3 and 5). The age of 

 a fish is determined by counting the number of 

 annuli or year-marks. 



In some cycloid scales, such as those on trout, 

 there may be no clear seasonal diflTerence in 

 spacing between circuli. On these scales the 

 year-mark is sometimes shown as a discontin- 

 uous or broken circulus following a series of 

 complete circuli. 



In ctenoid scales, like those of sunfish and 

 bass, there is often no detectable diff'erence in 

 the seasonal spacing of circuli. Here another 

 feature of scale circuli is relied upon to identify 

 the end of growth for a given year. On these 

 scales the last few circuli laid down in a year 

 are often incomplete in that they do not con- 

 tinue all the way around to the spiny area of 

 the scale. When fast growth resumes in spring, 

 the circuli are again complete and cut across 

 the ends of the incomplete circuli inside of 

 them. The first complete circulus of the growth 

 for the new year is considered to be the year- 

 mark. This cutting over of circuli is illustrated 

 in the sunfish scale in Figure 3. 



— 3 



