band together make up the annual growth, 

 and age in years is determined by counting the 

 number of dark bands. The otolith from a 

 yellowtail flounder shown in Figure 6 illus- 

 trates the features described. 



For many fishes otoliths show age more 

 clearly than scales. They are often considered 

 better than scales for determining age of older 

 fish. 



Preparation of otoliths for examination var- 

 ies with the fish species under study. In some 

 cases bands show clearly in dried otoliths, 

 which may be examined whole or sectioned and 

 polished. For other species, banding remains 

 clear only if the otoliths are stored in some 

 fluid such as glycerine or alcohol, upon removal 

 from the fish. 



Determination of fish age from otoliths is 

 generally not difficult since little preparation 

 time is required. Otolith extraction requires 

 killing the fish, however, a disadvantage for 

 some studies. 



Bones. — Cross sections through the bases of 

 fin rays, the thin bones that support fins, often 

 show concentric banding that is related to fish 

 age. The age of the 152-year-old sturgeon, 

 mentioned earlier, was determined from a 

 cross-section of the thick, spiny fin ray found 

 at the base of one of the paired breast fins of 

 this fish. Fin rays have been used for age 

 determination in a number of species besides 

 the sturgeon, such as catfish, bullheads, and 

 suckers. Zonation is similar to that found in 

 otoliths with a light band forming in the early 

 part of the growing season followed by a nar- 

 row, dark band in fall and winter. The had- 

 dock fin ray cross-section shown in Figure 7 

 shows seven pairs of such bands. Age in years 

 is determined by counting the dark bands. 



Certain other bones of fishes may show 

 seasonal zonation associated with age. Indi- 

 vidual vertebrae and opercular bones (gill cov- 

 ers) are often well suited for age determination 

 although other bones have been used in some 

 cases. Annual bands, where present, are ident- 

 ified by diff'erences in color shade between 

 early and late season growth, as in fin rays. 

 Zonation in vertebrae is concentric around the 

 center line of the backbone. Opercular bones 

 show banding along the growing edge. 



Figure 7. — The cross-section of a fin ray from a 23- 

 inch haddock, showing seven growth zones. 



Bones may be cleaned by boiling in water. 

 Most bones are then air dried to ready them 

 for study, but fin rays must be cut into thin 

 sections with a jeweler's saw or other suitable 

 instrument. The sections are sometimes 

 ground and polished for examination under a 

 microscope. 



References 



The foregoing treatment of fish age deter- 

 mination is necessarily brief. For the reader 

 who is interested in further pursuit of the 

 subject, a short list of reference books con- 

 taining sections on fish age and growth is given 

 below. Some of the larger public libraries may 

 be consulted for these as well as additional 

 works. 



CARLANDER, KENNETH D. 



1950. Handbook of freshwater fishery biology. 



Wm. C. Brown Co., Dubuque, 281 pp. 



Summaries of age and growth data for 



most North American freshwater species, 



extracted from published reports. 



CURTIS, BRIAN. 



1948. The life story of the fish. Harcourt, 

 Brace and Company, New York, 284 pp. 

 A general work on fishes. 



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