AGE OF ATLANTIC SHAD FROM SCALES 



195 



counts beyond the third annulus. This is not a 

 handicap, however, because the annuli beyond the 

 third arc usually legible. 



Table 1. — Frequency distributions of number of transversa 

 grooves entering area enclosed by fresh-water zoni and 

 first, second, timl third annuli on scales of !*'>', shad taken 

 in the Hudson River, spring 1950 



Constancy of transverse-groove counts 



To determine if the numbers of transverse 

 grooves were constant within the various .'111111111 



on scales taken IV various locations on the fish, 



selected samples of scales were mounted ami read 

 from live predetermined body locations on each 

 of 26 Hudson River shad brought to the labora 

 tory I'or special studies. The five locations chosen 



on the left side of each -hail were as follows: (1) 

 The midline of side just behind the frill opening; 

 (li) the midline of side under I he dorsal I'm : ( :'» ) 

 the midline of side near the tail : 1 I ) jusl under 

 the dorsal fin; (."1) j ust above the pelvic I'm-. 

 A count of the number of transverse grooves en- 

 tering the fresh-water zone and each of the first 3 

 annuli was made from a scale from each of the ."> 

 locations. This procedure was repeated for each 

 of the 26 shad. In most cases we found no varia- 

 tion in transverse-groove counts between scales 

 from the five locations on individual fish, and at 

 most they varied not more than ±1. The slight 

 variation in scales from different locations 

 could he a result of the difference in legibility 

 of markings and of possible error in counting, but 

 in all cases the counts fell within the ranges shown 

 in table 1. 



It should be pointed out that the most sym- 

 metrical and legible, and therefore most easily 

 read scales, are located on the midline of the side 

 below the dorsal fin. Since even in this location 

 many are regenerated scales (fig. '•>). it was found 

 necessary when sampling in the field to take about 

 2n scales in order to be sure of 2 good one- for 

 mounting. 



Spawning-mark criteria 



The transverse grooves cannot be -ecu clearly 

 on the part of the scale beyond the first spawning 

 mark. due to scarring of the scale surface and the 

 closeness of the spawning marks to each other. 

 Since the spawning marks in most case- are clearly 

 visible, there is no need for any index to them, 

 for usually after a shad has spawned once, the 

 spawning marks on the scales can be used with 

 the annuli to determine the age of the fish. < >nly 

 on -hail that have -pawned many times do the 

 spawning marks become so crowded near the edge 

 if the scale thai they are hard to separate. This 

 situation may be observed in figures 1 and in 

 which show five spawning marks each. A- many 

 as nine spawning marks have been found on scales 

 of individual fish taken in the 1950 Hudson River 

 sample. ( )n these scales it is usually possible to 

 find trace- of each spawning mark clearly visible 

 just below the baseline in the posterior field ol 

 the -rale- where apparently not so much absorp- 

 I ion takes place a- in the anterior field. 



Another problem encountered with spawning 

 marks is that during the lir-t spawning migra- 

 tion the previous year's scale growth around the 

 periphery of the anterior pinion 1- sometimes 

 partially or wholly eliminated. 



It is important when reading -had scale- to 

 examine the spawning marks to determine if an 

 annulus has been eroded away. On scales where 

 the annulus. laid down a year previous to the first 

 spawning, has disappeared in the anterior portion 

 of the scale due to erosion, a part of the annnhi- 

 may be found near or ju-t below the baseline of 

 the scale inside the spawning mark, indicating 

 that the absorption process does not affect the pos- 

 terior portion of the scale to the same extent as t lie 

 anterior portion. Figure 11 illustrates such a 

 situation, and in reading this scale the part of the 

 annulus remaining in the posterior region and the 



