Utility of the Transition Phase in Earplugs of the North Pacific Common Minke Whale as an Indicator of Age at Sexual Maturity

摘要

Whale age at sexual maturity is one of the most important biological parameters that can be used in stock management and population analysis. Earplugs have been widely used as an indicator of age among rorquals. It has also been accepted that the transition phase in the earplug can be used as an indicator of age at sexual maturity in fin whales, sei whales, and Antarctic minke whales. This study aimed to provide further insight into the utility of the transition phase as an indicator of age at sexual maturity in the North Pacific common minke whales, which has not yet been clarified. The relationship between sexual maturity and transition phase in earplugs was examined using 981 readable earplugs from common minke whales that were sampled at the JARPN and JARPN II scientific permit survey platform in the western North Pacific from 1994 to 2011. The transition phase was recognized in 53.2% of mature males and in 58.6% of mature females. Most whales in which the transition phase was recognized in the earplug were sexually mature. A significant correlation was found between the number of corpora and time after sexual maturation, as revealed by the transition phase, demonstrating that the transition phase is a valid indicator of age at sexual maturity in common minke whales. However, it was difficult to recognize the transition phase in whales that had recently attained sexual maturity because insufficient time had elapsed since its formation. To avoid potential bias, the use of earplugs as an indicator of age should be restricted to whales more than 12 years old.