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Fishery Bulletin 102(4) 



grow little or not at all over the course of a year. Using 

 the equation for width at sectioning site from Table 2, 

 we found that the increase in bone diameter for these 11 

 animals was =0.13 mm or less, which places the LAGs 

 very close together. Because it not uncommon for sea 

 turtle to exhibit little or no growth over a year, LAGs 

 spaced closely together very likely represent distinct 

 years as also determined by de Buffrenil and Castanet 

 (2000). Although the sample sizes are still small for a 

 definitive answer, our results indicate that counting 

 the LAGs individually is the correct interpretation of 

 double or bifurcating LAGs in juvenile as well as adult 

 loggerhead sea turtles. 



Similarly, our results indicate the same interpretation 

 for double or bifurcating LAGs in juvenile Kemp's ridley 

 sea turtles. The CWT Kemp's ridley sea turtles, samples 

 LK-7 and LK-8, displayed LAGs near the outer edge 

 of the bone and a small amount of bone was deposited 

 after the LAGs. These animals were each 2.25 years 

 old and had one-year marks visible in the humeri but 

 no LAGs or annuli other than those at the periphery. 

 Other CWT samples clearly indicated that LAGs are 

 deposited at the end of the second GM. The indirect 

 validation results demonstrated that LAGs were visible 

 in bone tissue by late spring or early summer. It seemed 

 that the LAGs at the outer edge of the LK-7 and LK-8 

 bones were the LAGs ending the second GM and that 

 very little growth occurred over the subsequent growing 

 season. Both of these animals were recovered as dead 

 strandings resulting from a major cold stun event in 

 Cape Cod, Massachusetts, in 1999; hence their growth 

 rates may have been anomalous in their last year of 

 life. Had these animals survived the cold stun event, 

 they would have deposited a year-three LAG very close 

 to year two, giving the appearance of a double or bi- 

 furcating LAG. 



Another anomaly in skeletochronology, supplemen- 

 tal lines, may form as a result of temporary stressful 

 environmental events such as droughts. In support of 

 this, Rogers and Harvey (1994) noted a supplemental 

 line in 11 of 43 specimens of the toad Bufo cognatus, 

 and in 10 of these animals the supplemental line was 

 within a growth zone that corresponded to a drought 

 year. Most skeletochronology studies that have noted 

 the presence of supplemental lines have indicated that 

 supplemental lines are easily identified as such because 

 they are less distinct and do not appear around the 

 entire circumference of the bone. In general, the same 

 has been observed in sea turtles. Supplemental lines 

 do appear but are generally easily differentiated from 

 LAGs by appearance. An exception to this was the 

 presence of supplemental marks in one- to two-year-old 

 Kemp's ridley sea turtles. These marks were similar 

 in appearance to the first year annuli. We were able 

 to identify these marks as supplemental only by the 

 observation of known-age animals. In addition, there 

 appeared to be a supplemental line in CC-2 that rep- 

 resented when the animal was released; hence, highly 

 stressful events may cause the deposition of nonannual 

 lines, but these events are likely to be relatively rare 



in wild turtles and not likely to interfere significantly 

 with age estimations. 



Resorption of early growth marks 



The loss of the early GMs due to endosteal resorption 

 and remodeling of the interior region of the bone is a lim- 

 iting factor in the application of skeletochronology to sea 

 turtles. From our findings, it was possible to accurately 

 age juvenile Kemp's ridley sea turtles up to at least 5 

 years from GM counts and this may be true for other 

 sea turtle species (e.g., Bjorndal et al., 2003), with the 

 possible exception of the leatherback sea turtle (Zug and 

 Parham, 1996). Because sea turtles have distinct life- 

 cycle stages, we suggest that in order to age a population 

 of sea turtles, one must acquire an ontogenetic series of 

 samples spanning all sizes and stages. Average duration 

 can be determined for each ontogenetic stage and the 

 approximate age of older animals with extreme resorp- 

 tion can be estimated. Because GM patterns appear to 

 mimic somatic growth rates, once growth through each 

 life-cycle stage is understood, backcalculation techniques 

 can be used to estimate the number of layers resorbed. 



Conclusions 



For many species, skeletochronology is not a perfect 

 method for age estimation. As GMs are histological 

 expressions of variation in rates of osteogenesis (Casta- 

 net et al., 1993). external factors and individual varia- 

 tion will affect the appearance of the marks (Castanet 

 et al., 1993, Esteban et al., 1996, Wave and Gregory. 

 1998). Endosteal resorption also serves to confound this 

 technique and is the primary difficulty in the application 

 of the technique to sea turtles. However, the evidence 

 presented in the present study gives strong support 

 to the concept that GMs are deposited on an annual 

 basis in sea turtles and that the spatial pattern of the 

 GMs correspond to the growth rates of the animal. The 

 GMs therefore provide invaluable information on age 

 and growth that cannot otherwise be easily obtained, 

 and age determination by skeletocronology is valid and 

 appropriate for the study of sea turtles. 



Acknowledgments 



We thank L. Crowder, S. Heppell, A. Read, and D. 

 Rittschof for their valuable comments on earlier ver- 

 sions of this manuscript. A. Gorgone. B. Brown and J. 

 Weaver provided assistance with the preparation of the 

 humeri. Most of the humeri were received through the 

 Sea Turtle Stranding and Salvage Network, a coopera- 

 tive endeavor between the National Marine Fisheries 

 Service, other federal and state agencies, many academic 

 and private entities, and innumerable volunteers. We 

 especially thank R. Boettcher and W. Teas. In addition, 

 humeri were received from F. Swartz at the Univer- 

 sity of North Carolina-Chapel Hill Institute of Marine 



