James et al.: Validation of the use of vertebrae and dorsal-fin spines for age determination of Squalus acanthias 47 
x 
= 
© 
iS 
O 
lo 
O 
-_— 
22) 
© 
jou 
= 
=) 
3 
1S) 
x 
© 
2. 
se} 
c 
G 
aa} 
y = 0.7855x + 0.308 
R2=0.5304 
3 
Time at liberty (years) 
Figure 2 
Relationship of time at liberty and the number of band pairs past the oxytetra- 
cycline (OTC) mark in the (A) vertebrae and (B) dorsal-fin spines of spiny dog- 
fish (Squalus acanthias) tagged in the Gulf of Maine, on Georges Bank, and off 
southern New England during 2011 and 2012. The dotted line in panel A and 
the solid line in panel B indicate linear regressions. The 95% confidence inter- 
vals (CIs) of the slope of the linear regressions (gray lines) do not include a 1:1 
relationship (dashed line) in panel A (95% CI: -0.078—0.330) and do include a 
1:1 relationship (dashed line) in panel B (95% CI: 0.558-1.013). R*=coefficient 
of multiple determination. 
(e.g., McAuley et al., 2006). Fewer studies have exam- 
ined OTC deposition in dorsal-fin spines, but McFarlane 
and Beamish (1987) reported an OTC marking success 
rate of 34%. There has not been a direct comparison of 
OTC marking success rates between vertebral centra and 
dorsal-fin spines; therefore, it cannot be known if rates 
differ between structures. However, the success rate for 
OTC marking was lower in dorsal-fin spines than in ver- 
tebrae in our study and was similar to that reported by 
McFarlane and Beamish (1987). This result indicates that 
dorsal-fin spines may incorporate OTC less readily than 
vertebral centra. 
One explanation for the absence of 
an OTC mark is insufficient mineral- 
ization directly after injection (Smith, 
1984). Mineralization may slow season- 
ally (Jones and Geen, 1977) and with 
ontogeny (Francis et al., 2007; Natanson 
et al., 2018). In our study, spiny dog- 
fish were injected with OTC in several 
seasons, but there were few recaptured 
individuals with OTC marks that had 
been injected in February (Tables 1 
and 2). Similar results were reported 
by McFarlane and Beamish (1987), who 
found that OTC marks were visible at 
a higher rate in the dorsal-fin spines of 
spiny dogfish injected in the fall than in 
the spines of those injected in the spring. 
The low growth rates observed in our 
study and the generally large sizes of 
specimens (Tables 1 and 2) support the 
idea of slowed body growth and sub- 
sequently slowed mineralization, par- 
ticularly for vertebrae (Francis et al., 
2007, Natanson et al., 2018). Such small 
changes in growth compounded with 
inaccuracies from measuring live then 
frozen spiny dogfish could have resulted 
in the negative growth observed in our 
study. With the environmental and onto- 
genetic effects on mineralization and 
growth, low OTC marking success rates 
are not unexpected. 
Age information is important for stock 
assessments intended to inform fisheries 
management. We investigated valida- 
tion of annual deposition of band pairs 
in vertebral centra from spiny dogfish 
because of concerns over the effect on 
age estimates of wear and tear on dorsal- 
fin spines. Although band pairs on ver- 
tebral centra are distinct, the results 
of our study from the use of chemical 
marking indicate that band pairs are 
not deposited annually and, therefore, 
that use of band pairs on this structure 
underestimates age in spiny dogfish. In 
contrast, validation of the use of dorsal- 
fin spines is supported by our findings. Therefore, despite 
their limitations, fin spines are more representative of age 
than vertebrae in spiny dogfish. 
Acknowledgments 
We thank the Northeast Cooperative Research Program of 
the NOAA Northeast Fisheries Science Center, B. Gervelis, 
D. McElroy, and the fishermen that helped tag and recapture 
the spiny dogfish used in this study. We thank K. Wooley for 
helping process samples in the lab. 
