MEPS 245:299-304 (2002)  -  doi:10.3354/meps245299

Using annual body size fluctuations to explore potential causes for the decline in a nesting population of the loggerhead turtle Caretta caretta at Senri Beach, Japan

Hideo Hatase1,*, Kiyoshi Goto2, Katsufumi Sato1,**, Takeharu Bando1,***, Yoshimasa Matsuzawa1,****, Wataru Sakamoto1

1Laboratory of Fisheries and Environmental Oceanography, Graduate School of Agriculture, Kyoto University, Sakyo, Kyoto 606-8502, Japan
2Minabe Sea Turtle Research Group, 278 Higashiyoshida, Minabe, Wakayama 645-0001, Japan
*Email: Present addresses: **National Institute of Polar Research, 1-9-10 Kaga, Itabashi, Tokyo 173-8515, Japan ***Institute of Cetacean Research, 4-18 Toyomi, Chuo, Tokyo 104-0055, Japan ****Sea Turtle Association of Japan, 5-17-18-302 Nagao-motomachi, Hirakata, Osaka 573-0163, Japan

ABSTRACT: The annual recruitment dynamics and body size fluctuations in adult female loggerhead turtles Caretta caretta nesting on Senri Beach in Minabe, Japan from 1990 to 2001 were investigated along with the number of nests, so that the causes for the recent population decline could be assessed. The number of nests decreased from 335 in 1990 to 29 in 1998, and slightly increased thereafter. The recruit percentage (i. e. the percentage of recruits out of total identified females in each year) varied from 39 to 75%. The lack of a significant correlation between the rates of change in the recruit percentage and in the number of nests suggested that the population decline had been caused by factors affecting both recruits and remigrants. The mean straight carapace length (SCL) varied from 825 to 855 mm. The significant negative correlation between the rates of change in the mean SCL and in the number of nests suggested that the decline had been caused by the decrease in the number of small females. The lack of a significant difference in SCL between recruits and remigrants supported the above scenarios. Since small females tend to inhabit the pelagic Pacific irrespective of their recruit or remigrant status, we proposed that the population decline at Senri Beach was attributable to factors operating in the pelagic waters, such as incidental mortality by longline fisheries.

KEY WORDS: Loggerhead turtle · Caretta caretta · Nesting population decline · Body size · Recruitment · Senri Beach · Japan

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