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Fishery Bulletin 111(2) 
rarely found below the thermocline (Holts and Bed- 
ford, 1993). Large fish approaching 200 cm fork length 
(FL) disappear from the catch and presumably move 
offshore to a more oceanic and highly migratory exis- 
tence, where most males are mature but females are 
not (Mollet et al., 2000; Joung and Hsu, 2005; Semba 
et ah, 2011). 
Researchers who study the age and growth of this 
species have used pairs of alternating bands of dif- 
ferent mineralization in vertebral centra to estimate 
age. These estimates differ depending on assumptions 
about the number of band pairs that represent a single 
year of growth. Bishop et ah (2006) noted that despite 
different conclusions in these studies, all of them ap- 
pear to have produced relatively similar growth curves 
if the same rate of band deposition was assumed. This 
finding indicates that techniques used by different 
research groups to identify countable band pairs are 
generally alike, with the greatest difference being as- 
sumptions about rates of band deposition. Smaller dis- 
parities may be due to ontogeny, geographic variability, 
divergent methods of preparing and interpreting verte- 
bral banding patterns, difficulties in interpreting band- 
ing patterns, differences in sample sizes, or limitations 
of the growth models used (Bishop et ah, 2006). 
Although considerable advances have been made 
in determination of age and growth of Shortfin Mako, 
questions remain as to why current length-at-age 
models underestimate growth of young fish, especial- 
ly fish <200 cm FL (Pratt and Casey, 1983; Bishop et 
ah 1 ; Maia et ah, 2007). Bishop et ah (2006) suggested 
that this discrepancy may be due to the inability of 
commonly used growth models to reconcile such rap- 
id juvenile growth with the slow growth predicted 
for subadults and adult sharks. Direct age-validation 
techniques, such as oxytetracycline (OTC) marking 
and recapture of an adequate sample size of Shortfin 
Mako <200 cm FL, therefore, are needed to help resolve 
the issue of band-deposition rates in juvenile Shortfin 
Mako vertebrae. 
In this study, we examined OTC-marked vertebrae 
of 29 juvenile Shortfin Mako (<200 cm FL) released off 
California and later recaptured from 2000 to 2010 (at 
liberty from 145 days to 4.4 years). OTC is deposited 
at sites of active calcification, such as vertebral centra, 
and is known to remain distinct for at least 20 years in 
certain finfish, such as the Sablefish ( Anoplopoma fim- 
bria) (Beamish and McFarlane, 2000), and sharks, such 
as the Leopard Shark ( Triakis semifasciata) (Smith et 
al., 2003). The combination of tag-recapture and chemi- 
cal marking is thought to be the most robust method of 
age validation (Campana, 2001; Goldman, 2005). These 
methods test the accuracy of the counts of vertebral 
band pairs as annual indicators through observation 
of the banding pattern deposited distally to the OTC 
mark during the known time at liberty. To supplement 
this information, we also conducted analyses of at-lib- 
erty growth of tagged-recaptured sharks and length- 
frequency data from 3 decades of length data from 
commercial and research sources. 
Materials and methods 
Tagging methods 
Sharks for tagging and OTC injection were captured 
in the Southern California Bight (SCB) (Fig. 1) with 
baited pelagic longlines and identified as Shortfin 
Mako through the method described by Compagno 
(2001). Leaders were unsnapped from the main line, 
and sharks were guided into a semisubmerged metal 
tagging cradle at the stern of the vessel. The cradle 
was then raised to facilitate tagging, measuring, and 
OTC injection, while the eyes of the shark were cov- 
ered with a wet chamois cloth and a saltwater ven- 
tilation hose continuously ran water over the shark’s 
gills. Each shark was tagged on the dorsal fin with a 
plastic Rototag 2 3 (Dalton ID, Henley-on-Thames, UK) la- 
beled with contact and reward information in English 
and Spanish and with instructions to measure the fish 
and save the vertebrae. Most sharks also were double- 
tagged with a spaghetti tag placed in the dorsal mus- 
culature beneath the first dorsal fin. 
At tagging, the sex of each shark was determined 
and each shark was measured (straight line FL or to- 
tal length [TL] ) to the nearest centimeter with a sta- 
tionary meter stick fitted to the shark tagging cradle. 
Sharks were given an intraperitoneal injection of OTC 
at a dose rate of 25 mg/kg of body weight; the dose was 
estimated with a length-weight dose table developed 
from length-weight measurements of Shortfin Mako 
measured by NOAA scientific observers for the Califor- 
nia drift gillnet fishery (Rasmussen 3 ). 
Laboratory methods for processing vertebrae 
Past studies have shown that band counts are consis- 
tent throughout the vertebral column in the Shortfin 
Mako, indicating that vertebrae from any region can be 
used in age analysis, although the larger central ver- 
tebrae are easier to read because of wider band spac- 
ing (Bishop et al. 1 ; Bishop et al., 2006; Natanson et 
al., 2006). OTC-marked vertebrae were obtained from 
Shortfin Mako recaptured on research cruises and com- 
mercial and recreational fishing vessels between 2000 
and 2010. The widest diameter vertebral centra in a 
given sample were chosen for sectioning. We used only 
OTC-marked vertebrae from tag recaptures at liberty 
longer than 124 days (or -4 months) to incorporate one 
full season into band-pair counts. 
2 Mention of trade names or commercial companies is for iden- 
tification purposes only and does not imply endorsement by 
the National Marine Fisheries Service, NOAA. 
3 Rasmussen, R. 1995. Unpubl. data. Southwest Fisheries 
Science Center, NOAA, La Jolla, CA. 
