44 
Fishery Bulletin 119(1) 
Figure 1 
Overlay of an oxytetracycline mark (A) onto a histology 
section of vertebral centra (dotted green line) and (B) 
onto a dorsal-fin spine (green arrow) of a spiny dogfish 
(Squalus acanthias) (specimen ID code: SPD21) tagged 
off the coast of New England in 2011. White dots indicate 
all band pairs in panel A and the last 7 band pairs in 
panel B. This individual was at liberty for 1.2 years. Only 
half of a band pair formed on the vertebrae (panel A), and 
one and a half band pairs formed on the dorsal-fin spine 
(panel B). 
the OTC mark for spiny dogfish at liberty for 1.14.8 years 
(Table 1). Fewer band pairs were consistently seen than 
would be expected if band pairs were annual (Fig. 2). The 
slope of the line at 0.126 (95% CI: —0.078-0.330) is sig- 
nificantly different from the 1:1 reference line, confirming 
that, when vertebral band pairs were used, TAL was con- 
sistently underestimated. 
Dorsal-fin spines had 0.5-3.5 band pairs past the 
OTC mark for spiny dogfish at liberty for 0.1—2.6 years 
(Table 2). In contrast to that for vertebrae, the number of 
band pairs past the OTC mark in dorsal-fin spines did con- 
firm annual periodicity of band-pair formation (Table 2, 
Fig. 2). Annual periodicity is supported, and the regression 
slope of 0.786 (95% CI: 0.558-1.013) is not significantly 
different from the 1:1 reference line. 
Vertebral column analysis 
For the vertebral column analysis of one adult female, both 
centrum measurements and band-pair counts varied along 
the vertebral column. Vertebral band-pair counts roughly 
followed vertebral centra morphology (Fig. 3). The lowest 
band-pair count was observed in the vertebra directly 
behind the neurocranium (vertebra 1), and the highest 
count was found at vertebra 40. Six of the 17 centra exam- 
ined (35.3%) had band-pair counts that fell outside of the 
95% CI (11.566—-13.022) of the average band-pair count 
(12.294) along the entire vertebral column; therefore, the 
band-pair counts differed significantly along the vertebral 
column (Fig. 3). 
Discussion 
In this study, the results of OTC validation indicate that 
band pairs in vertebrae are not deposited annually in 
spiny dogfish. Use of counts of band pairs past the OTC 
mark consistently underestimated TAL for this species. 
Additionally, our results support previously published 
results indicating that band-pair deposition in fin spines 
is annual (Campana et al., 2006). On the basis of our 
results, we conclude that vertebrae are not an appropriate 
structure to use to estimate age for spiny dogfish. 
Using a more detailed method, we were able to also 
obtain distinct band pairs in the vertebral centra of 
spiny dogfish. However, because of the prevalent under- 
estimation of age observed from the location of the OTC 
mark within vertebral centra, we cannot recommend 
this structure for age determination of this species. 
These results are consistent with a growing number of 
studies with findings that indicate that vertebral band 
pairs do not accurately reflect age in sharks throughout 
their lives (see Harry, 2018; Natanson et al., 2018). In 
an attempt to understand the deposition of band pairs, 
Natanson et al. (2018) investigated the morphology and 
band-pair counts along the vertebral columns of several 
shark species and found that, in each species, band-pair 
counts varied along the vertebral column in relation- 
ship to vertebral size, not time, indicating a structural 
component to the band pairs. In our study, we found 
the same variation in band-pair counts along the ver- 
tebral column of a spiny dogfish. As per the species in 
the Natanson et al. (2018) study, lower band-pair counts 
were associated with smaller centra, indicating that 
band pairs are also related to the structure of the ver- 
tebra rather than to time in this species. Additionally, if 
band pairs were deposited annually, the same band-pair 
counts would be expected for every vertebra within an 
individual (Natanson et al., 2018). Although this finding 
for spiny dogfish is interesting, it is based on one speci- 
men, and further work on more specimens of this species 
