50 
Fishery Bulletin 1 13(1) 
Age (years) 
Age (years) 
Figure 2 
The age structure of the 462 American shad ( Alosa sapidissima) collected during May-June in 2008-2010 from 
the Merrimack River in Lawrence, MA, from annuli in (A) otoliths, (B) scales, (C) vertebrae, and (D) opercula. 
Annuli were counted by each of 2 readers. 
Results 
For this study, 545 American shad were collected. Only 
462 fish (283 males, 179 females) were deemed read- 
able for all 4 structures and, therefore, these fish were 
included in our analyses. Fish with TLs from 401 to 
639 mm (average=502 mm) were represented. Average 
TL was longer for females than for males (518 versus 
477 mm TL). 
All 4 structures that were examined contained an- 
nual marks (Fig. 1), but otoliths and vertebrae showed 
the most defined marks. American shad in this study 
ranged in age from 3 to 11 years. The majority of fish 
were age 5 and 6. No fish were assigned an age of 11 
years with the use of opercula or 3 years with scales. 
Both readers found age-6 fish at a larger percentage 
on the basis of scale ages than they did with any other 
structure (Fig. 2). 
Analysis of between-reader and within-reader results 
for each structure indicated that the most precise age 
readings came from otoliths: precision was measured 
between readers with agreement of 76.2% and a CV 
of 2.99% and among results from a single reader with 
agreement of 74% and a CV of 2.93%. The next most 
precise age estimates were determined from scales, 
with agreement of 66.9% and a CV of 4.28% between 
readers and agreement of 65%’ and a CV of 5.1% within 
results for a reader. Aging with vertebrae resulted in 
the third-most precise readings, followed by age analy- 
sis with opercula (Table 1). Within-reader comparisons 
were made between otolith ages and ages determined 
from the other structures. Vertebral ages matched the 
otolith ages most closely for reader 1 (55.6%). Vertebra 
and scale ages both agreed 53.2% of the time with oto- 
liths for reader 2. The CV for both readers was better 
between otoliths and vertebrae (5.95% and 6.39%) than 
between otoliths and scales (6.87% and 6.63%). Agree- 
ment between opercula and otoliths was the worst for 
both readers (Table 1). 
In comparisons of results between readers and with- 
in reader 2, at least 93% of all age disagreements were 
within 1 year. For both readers, agreement between ver- 
