Wells et al.: Age validation of juvenile Isurus oxyrinchus tagged off southern California 
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Days at liberty 
Figure 2 
Average number of band-pairs (translucent and opaque) for the 
number of days at liberty, determined from the oxytetracycline 
(OTC) mark to the outer centrum edge on vertebrae of juvenile 
Shortfin Mako ( Isurus oxyrinchus) (n= 29) tagged from 2000 to 
2010 in the Southern California Bight for this study. Readings 
were based on 3 independent readers (±1 standard error (SE1). 
The solid line shows the linear regression of number of band 
pairs relative to days at liberty, and the lines with short and 
long dashes show predicted number of band pairs for 1 band 
pair/year and 2 band pairs/year, respectively. 
distal to the OTC mark and with an APE of 
5.65% and CV of 7.73% for counts distal to the 
birth band. Among readers, 93% (27 of 29) of 
the final band-pair estimates distal to the OTC 
were within 1 band pair of each other, and 86% 
(25 of 29) of the estimates distal to the birth 
band were within 1 band pair. 
The readings distal to the OTC that differed 
by more than one band pair were from sharks 
that had been at liberty for the longest time, in- 
dicating that variability in band-pair counts in- 
creases with age, likely because of disagreement 
among readers caused by structural artifacts. 
For example, sample A037789 was from a shark 
that was at liberty for 1594 days and band-pair 
estimates distal to the OTC varied from 6 to 9 
among the 3 readers for this shark (Table 1). 
Length-frequency analysis 
A total of 14,720 individual Shortfin Mako were 
used for the length-frequency analysis that was 
completed with fishery-dependent data from the 
California drift gillnet fishery (1981-2009) and 
fishery-independent data from juvenile Shortfin 
Mako surveys (1993-2009) (Fig. 5). No differ- 
ences in length-frequency modes were detected 
when analyzed by season or sex; therefore, pa- 
rameter estimates were generated by grouping 
across factors (season and sex) with age-depen- 
dent standard deviation incorporated into the 
model. Average size of Shortfin Mako used in 
growth modeling was 121.3 cm FL (±0.25 SE). 
The majority of fish (85%) ranged in size from 
80 to 160 cm FL with 3 identifiable modes con- 
sistently present in annual length frequencies 
regardless of survey type or sex. 
Results from MULTIFAN showed these 3 
modes at 86, 112, and 134 cm FL, and average modal 
sizes from MIXDIST were 83, 118, and 147 cm FL. Tak- 
ing the difference between modal sizes from MULTI- 
FAN analysis provided average annual growth rates of 
27 and 23 cm FL for the period from the first to sec- 
ond mode and for the period from the second to third 
modes, respectively (Table 2). Similarly, annual growth 
rates from MIXDIST averaged 36 and 29 cm FL for the 
same 2 periods (Table 2). 
Growth of tagged and recaptured sharks 
No difference was observed between growth rates cal- 
culated from at-liberty OTC-injected sharks (28 and 
21 cm per year at 85 and 130 cm FL, respectively) 
and tag-recaptured sharks not injected (29 and 19 cm 
per year at 85 and 130 cm FL, respectively); there- 
fore, these data were pooled. The growth rates that 
resulted from GROTAG analysis of lengths of tagged 
and recaptured sharks were similar to the growth 
rates from length-frequency calculations, averaging 29 
and 20 cm per year at 85 and 130 cm FL, respectively 
(Table 2). 
Discussion 
Results of this study indicate that 2 band pairs are 
deposited each year in juvenile Shortfin Mako tagged 
and released in southern California. The fast growth 
rates collectively obtained through the use of OTC- 
marked vertebrae, MULTIFAN and MIXDIST length- 
frequency analyses, and tag-recapture growth models 
were consistent, providing strength to our results be- 
yond the usefulness possible with the employment of 
a single method alone. Further, a sample size of 29 
OTC-marked vertebrae of Shortfin Mako at liberty 
from 4 months to more than 4 years is the most com- 
prehensive OTC tag-return data set reported for this 
species. We also compared our total band-pair counts 
at length with those of Cailliet et al (1983), the only 
other Shortfin Mako age and growth study completed 
