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Fishery Bulletin 99(2) 
Other studies have used von Bertalanffy models to eval- 
uate growth in immature loggerhead sea turtles in the At- 
lantic (Table 1) based on either mark and recapture data 
(Frazer and Ehrhart, 1985; Schmid, 1995) or skeletochro- 
nology data (Klinger and Musick, 1995; Parham and Zug, 
1997). Schmid ( 1995) and Parham and Zug ( 1997) present- 
ed data for a number of data sets and types of calculation. 
For both studies, the data presented in Table 1 are for the 
two data sets that Schmid (1995) and Parham and Zug 
(1997) considered the most representative. 
Our estimates of 20 year classes in the size range from 
46 to 87 cm CCL fall within the range of values of 9 to 29 
years that we calculated from the von Bertalanffy growth 
equations presented in other studies (Table 1). There is no 
geographic trend in these estimates. 
Von Bertalanffy growth model 
MULTIFAN uses a von Bertalanffy growth model as de- 
rived by Schnute and Fournier (1980). Concerns about 
the application of the von Bertalanffy model to sea turtle 
populations have been discussed elsewhere (Bjorndal and 
Bolten, 1988; Chaloupka and Musick, 1997). A major con- 
cern is the use of the von Bertalanffy model to extrapolate 
outside the size range of a study (Day and Taylor, 1997), 
which has been a common practice in estimating age at 
sexual maturity in sea turtle populations (reviewed in Cha- 
loupka and Musick, 1997). However, within a growth phase 
that exhibits monotonic decline — that is, for a given size 
range of a population with similar habitats and diet and 
that has declining growth rates with increasing body size — 
the von Bertalanffy model may provide reasonable esti- 
mates of growth rates and number of age classes. Green 
turtles in the southern Bahamas have a monotonic non- 
linear declining function for SCL-specific growth rates in 
a size range of 30 to 70 cm SCL as determined from non- 
linear regression of mark-recapture data (Bjorndal et ah, 
2000). Length-frequency analyses with MULTIFAN yielded 
the same estimates as the nonlinear regression analysis for 
growth rates and number of age classes between 30 and 70 
cm SCL for that population (Bjorndal et ah, 2000). Similar 
trends of decreasing growth rates with increasing size have 
been reported from mark-recapture studies in Atlantic log- 
gerhead sea turtles (summarized in Parham and Zug, 1997). 
In addition, use of the von Bertalanffy model within the 
studied size range is supported by the similarity between 
growth rates generated from MULTIFAN and those calcu- 
lated from recaptures of tagged pelagic-stage loggerhead 
sea turtles (Bjorndal et ah, 2000). As growth data accu- 
mulate for sea turtle populations, appropriate growth func- 
tions — which may well be polyphasic (Chaloupka and Zug, 
1997; Chaloupka, 1998) as a result of habitat shifts — can be 
incorporated into the length-frequency software. 
Age at sexual maturity 
An estimate of the number of years 
to grow to 87 cm CCL may be gen- 
erated by combining our results (20 
year classes between 46 and 87 cm 
CCL) with those from a growth study 
in pelagic-stage loggerhead sea tur- 
tles in the North Atlantic (Bjorndal 
et ah, 2000). In that study, based 
on length-frequency analyses and 
mark-recapture data, the age of log- 
gerheads with a CCL of 46 cm was 
estimated to be 6.5 years. Therefore, 
an 87-cm loggerhead sea turtle would 
be approximately 26.5 years old. 
This estimate of 26.5 years at 87 
cm CCL should not be used as an es- 
timate of age at sexual maturity. Al- 
though we used 87 cm CCL to rep- 
resent the largest subadult in our 
study, as explained above, we used a 
conservative estimate to exclude al- 
most all adults from the sample to 
avoid obscured modes in the length- 
frequency distributions. Many logger- 
head sea turtles will reach sexual ma- 
turity at lengths much greater than 
87 cm CCL, and because growth rates 
are slow in these large subadults, the 
average age at sexual maturity would 
be substantially greater than the av- 
erage age at 87 cm CCL. 
Figure 3 
Age-specific von Bertalanffy growth model generated by MULTIFAN for loggerhead 
sea turtles stranded along the Atlantic coast of Florida from 1988 through 1995 
within the size range of 46 to 87-cm curved carapace length. The growth curve 
for loggerhead sea turtles stranded in the Gulf of Mexico is not plotted because 
it is indistinguishable from the line plotted for the Atlantic coast of Florida. Age 
is defined as years since recruitment to neritic habitats at a size of 46 cm curved 
carapace length. 
