276 BULLETIN OF THE BUREAU OF FISHERIES 



edge of the ruler, all those that were separated being considered as complete circuli 

 even though they were connected with others a short distance from the ruler. 



Scale counts were made along the lateral line on the left side of the body. I 

 enimierated the perforated scales only, which excluded, therefore, the small, irreg- 

 ularly placed scales at the extreme caudal end of the fish. Scale pockets were 

 counted for the lateral-line scales that were lost. 



The age of a fish is usually indicated by a Roman numeral, representing the 

 year of life in which the fish was caught. Thus, a IV-year fish is one that, having 

 hatched in the spring, has passed its third winter following hatching; has, therefore, 

 three complete annuli on its scales and is somewhere in its fourth year. 



GENERAL HISTORICAL REVIEW 



The early scale investigators were concerned principally with the development' 

 the structure, and the chemical composition of scales and with their relation to 

 taxonomy. Thus, they paved the way to a correct appreciation of the relief struc- 

 tures in scales, upon which the scale method rests. The following historical review 

 sketches briefly the trend of thought among these early investigators relative to the 

 correlation of relief structures of scales and their growth. More comprehensive 

 reviews of the scale literature of this period may be found in the publications of 

 Baudelot (1873), Thomson (1904), and Taylor (1916). 



After the mvention of the microscope, fish scales became one of the interesting 

 objects for study. Fabricius d'Aquapendente (1618, 1621, 1625), Borellus (1656), 

 and Hooke (1667) wrote brief descriptions of the microscopic appearance of fish 

 scales. 



The first record relative to the growth of scales is found in one of the letters of 

 Leeuwenhoek (1686), dated July 25, 1684, in which he, describing the microscopic 

 appearance of an eel scale, writes "although all the Scales [of an individual] are not 

 of the same shape, I have yet observed, in many of them as I judged, the same 

 number of Circular lines. From whence I conclude that every year the Scale encreased 

 one Circular line; and by consequence, the number of these Circular lines, being seven ; 

 the Fish must have been seven years old." (Turrell, 1911.) His illustration shows 

 that Leeuwenhoek actually referred to the growth zones of the scale. In a letter 

 written May 22, 1716, and published hi 1719, Leeuwenhoek describes his method of 

 determining, from its scales, the age of a carp 42)^ inches long and 33 M inches in 

 circumference at its thickest. He cut the scale obliquely to count the age rings. 

 The scale having 40 rings, the author concluded that the carp was 40 years old. In 

 this letter Leeuwenhoek postulated scale growth as being due to the development of 

 a new scale underneath the old one, which it exceeds in size and to which it adheres 

 and is gradually closely welded. One such new scale is formed each year, so that by 

 enumerating the superimposed scales one can determine the age of the fish. It is 

 clear from his illustration and from his method of sectioning that in this case Leeuwen- 

 hoek did not refer to the growth zones but to the lamellae of the scale. 



Reaumur (1718) believed that the concentric lines indicated "different degrees 

 of growth in scales, just as the analogous markings indicate the growth of shells." 



Without giving the reference. Lea (1919) quotes Pastor Hederstrom, a Swede 

 (1759), as follows: "Anyone taking the trouble to examine a vertebra from a boiled 



