Title Ecological, Behavioral, and Physiological Evidence of the Hawksbill Turtle Reproduction in Captivity
Authers Masato KOBAYASHI
Citation Bull. Fish. Res. Agen. No.36, 107-142, 2012
The hawksbill turtle Eretmochelys imbricata is a species of sea turtle belonging to the family of Cheloniidae. They inhabit throughout tropical and subtropical coral reef regions of the world. In Japan, this species is seen in areas south from the Izu Peninsula on the Pacific side and Noto Peninsula on the Japan Sea side, while the northern limit of nesting occurs in the Nansei Islands. Hawksbill turtles have been captured not only as a source of protein but also as raw materials for ornaments, because their carapace scutes are richly colored and their aesthetic beauty. Their populations have decreased worldwide in recent year, and the hawksbill turtle has been listed as Critically Endangered. Also, the capture of sea turtles and their eggs is restricted to protect them in many countries around the world, and the international trade of sea turtles is prohibited entirely by CITES (Convention on International Trade in Endangered Species of Wild Fauna and Flora).
 Research Center for Subtropical Fisheries, Seikai National Fisheries Research Institute, Fisheries Research Agency has been studying the propagation of the hawksbill turtle for the recovery of the stock by“head-starting”since 1999. The development of the captive breeding technology is essential to recover the stock of this species by head-starting, and therefore it is important to elucidate reproductive biology of hawksbill turtles. In this study, I tried to elucidate the reproductive ecology and physiology of the hawksbill turtle in captivity.
 In the first chapter, I investigated plasma testosterone concentrations of 14 male hawksbill turtles held in captivity to determine the sexual maturity of them. I also tried to determine male sexual maturity based on the indices of morphological characteristics, which were the straight carapace length (SCL), tail length (TL) and the ratio of TL to SCL (TE, tail elongation). As a result, plasma testosterone concentrations of mature males gradually increased during the pre-mating season, and then they sharply declined to low levels in the mating season and remained low thereafter. In contrast, the concentrations of immature males remained low throughout the year. Furthermore, I found that sexual maturity of male hawksbill turtles could be estimated by their TE; mature individuals showed the TE of 0.35 or above while immature ones exhibited the indices of 0.33 or less.
 In the second chapter, I investigated plasma estoradiol-17βconcentrations and follicle in the ovary of 11 female hawksbill turtles held in captivity to elucidate the relationship be tween seasonal change of plasma estoradiol-17βconcentrations and the development of follicles. As a result, whenever plasma testosterone concentrations of mature females increased before the mating season, their follicles developed. In contrast, the concentrations of immature females remained low throughout the year, and their follicles did not develop. Reproductive cycle of mature females is two or three years the same as wild hawksbill turtles. However, follicles of one female developed annually in the breeding season, suggesting that female hawksbill turtles have the ability to mature annually.
 In the third chapter, I investigated the mating and nesting behavior, clutch size, number of clutches and hatching rate to reveal the reproductive ecology of captive hawksbill caught from the wild. I used a video camera system to determine the date and time of mating and nesting. Mean mating duration ranged from 50 to 150 minutes, and the period from mating to the 1st nesting was 29.6 ± 3.4 days. As for nesting, 4 females nested a total of 16 times between 2006 and 2009. Mean clutch size and number of clutches were 135.9 ± 25.2 eggs and 3.5 ± 0.7 clutches, respectively. These results closely correlated with data from wild populations. Hatching rate of captive eggs was markedly lower compared to wild eggs. However, the straight carapace length and body weight of hatchlings in captivity were comparable to the wild ones.
 Finally, I was able to clarify the reproductive ecology and physiology of the hawksbill turtle in captivity. The achievement of this study makes a huge step forward for the ability to produce significant number of juveniles for stock enhancement and aquaculture of the hawksbill turtle. To further stock enhancement and aquaculture of them, there is an urgent need to promote studies on improving the level of the hatching rate of the hawksbill turtle in captivity.
URI http://www.fra.affrc.go.jp/bulletin/bull/bull36/36-3.pdf