Growth 87 



fishes in Michigan. They believed that the outside member of the double 

 ring was the true annulus, whereas this outer ring in actuality probably 

 was a mark laid down during the first early peak of spawning. 



The 1938-brood bluegills at Fork Lake (Illinois) '-^ were growing very 

 rapidly in 1939 and during the summer growing period of that year, many 

 members of this brood laid down two or three distinct "annuli" on their 

 scales ( Figure 4.6a ) : formation of the true annulus was completed by the 

 last of May; the second annulus (false) first became visible during the 

 latter part of June and the third annulus (false) appeared on the scales 

 of a small per cent of the population in July and August. There was no 

 satisfactory method of separating false from true annuli.^- It is my belief 

 that the false annuli appeared on the scales of the Fork Lake bluegills 

 because they went "off their feed" for short periods. 



Some abnormalities of growth, which are reflected on the scales of a 

 fish and cause difficulty in the correct interpretation of age and growth, 

 are: 



(1) False annuli— ialse rings having all of the characteristics of true 

 annuli, but which form during the middle of die growing period and 

 after the true annulus has formed for the current year ( Figure 4.6a ) . 



(2) Skipped annuli— wheve the position of the annulus for one year 

 coincides with that of the preceding year, e.g. the fish does not grow 

 during one growing season ( Figure 4.7b ) . 



(3) Overlapping annuli— where growth in length through one growing 

 season is very small with no corresponding increase in plumpness. The 

 annulus for one year coincides in part with that of the next, but in part 

 (usually in the anterior field) is separated by 4 or 5 circuli (Figure 4.6b). 

 Without detailed growtli information, a scale reader is likely to consider 

 die second component of the double ring as a false annulus. 



(4) Close spacing of annuli— where growth for one season is small and 

 two annuli are separated entirely, but by only a few circuli. Without 

 growth information, a scale reader is liable to consider the outer annulus 

 as false. 



Scales, spines, bones, and otoliths of fishes have been used successfully 

 in age determinations. Studies of these parts from fishes of known ages 

 prove that most species usually lay down a single growth ring each year. 

 However, there are some exceptions, such as European carp, which fre- 

 quently form extra "annuli." But even the scales of carp may be inter- 

 preted on the basis of "growth patterns" for successive growing seasons, 

 provided a specific population is being studied intensively over a period 

 of several years. 



In the wake of the many excellent studies on the age and growth of 

 fishes, the methodology for using scales ( and other structures such as ear 

 stones, vertebrae, spines, and opercular bones that show annual rings) 



