DETERMINING AGE OF ATLANTIC SHAD FROM THEIR SCALES 



By James P. Gating, Fishery Research Biologist 



Determining the age of fish is important in 

 any study requiring knowledge of the age-class 

 composition of the population. An accurate read- 

 ing of shad scales was needed in the investigation 

 (if the shad fishery of the Atlantic coast, being 

 carried on by the United States Fish and Wildlife 

 Service t<> furnish information for fishery regula- 

 tions to the Atlantic Stales .Marine Fisheries 

 Commission. 



This paper presents criteria established for read- 

 ingtheageoi -had (Alosa sapidissima) from their 

 scales, in the hope that it may he of aid to other 

 investigators. It is possible that the criteria given 

 here, with minor changes, may be applied to ol her 

 clupeids having scale markings similar to the 

 shad'-. 



Fishery biologists have generally considered 

 shad scales difficult to read, although most of them 

 agree that some scales, especially from fish 4 years 

 or younger, are easily read because the annuli are 

 clear cut and proportionately spaced. 



Leim's (1925) paper on the life history of the 

 shad (Alosa sapidissima) in Canadian streams 

 discusses the work by European investigators on 

 age and growth of Alosa alosa and Clupea fvnta — 

 very similar to Alosa sapidissima — in which they 

 assumed the validity of the annuli as year marks. 

 By plotting the length of scale against the length 

 of the fish for a group of -had. Leim showed that 

 the growth in length of the scale is almost propor- 

 tional to the growth in length of the shad, lie 

 back-calculated the lengths of a series of shad at 

 each winter of their life, and t he calculated lengths 

 agreed closely with length of fish as he had meas- 

 ured them at a given age. His total lengths 

 plotted against age gave a curve that rose rapidly 

 till the fourth year, and then gradually leveled off. 

 In establishing the relation of length of scale to 

 length of fish, he fulfilled one of the important re- 

 quirements for establishing the validity of annuli 

 as year marks on shad scales. 



In hi- work on the fresh-water growth of shad. 

 Hammer (1942) found that shad scales are con- 

 stant in number and retain their identity through- 

 out the life span of the fish. He did not attempt 

 to age shad, but did show that hack-calculated 

 lengths determined for the fish at the time they 

 left fresh water agreed with actual measured 

 lengths of -had at that time. 



Borodin i 1925) presented a method of reading 

 -had scales by counting the number of transverse 

 grooves and dividing bj 2 to get the age. Greeley 

 ( L937), however, has pointed out that this method 

 gives erroneous results on Hudson River shad 

 scales and his age determinations from easily read 

 scales agreed closelj with Leim'-. Later workers, 

 for instance Moss (1946), read only spawning 

 marks on the shad scales and presented no data on 

 the ai_ r e ol the li-ii studied. 



These investigators did not establish criteria for 



separating t he false from the true annuli on scales 

 or for ascertaining the amount of absorption that 

 takes place at the scale edge during the spawning 

 migrat ion of older fish. 



I wish to acknowledge the assistance of mem- 

 hers of the si aff at the Beaufort Laboratorj of the 

 Fish and Wildlife Service who helped with this 



study through their constructive criticism and ad- 

 \ ice during the period of it- development. These 

 are C. E. Atkinson. ( i. B. Talbot, R. A. Fredin, 

 and John II. Finucane who worked with me dur- 

 ing the earlier part of tin' studv and assisted in 

 reading scales. 



DESCRIPTION OF A SCALE 



Figure 1 illustrates a scale from a Hudson River 

 female shad that was 11 years old. 22^ inches 

 lone-, and weighed 6^4 pounds when taken. The 

 anterior, sculptured portion of the scale com- 

 prises aboul three-fourths of the scale surface, 

 while the posterior area (exposed portion of 

 scale) consists of the remaining one-fourth of 



187 



