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

Whale age at sexual maturity is one of the mostimportant biological parameters that can be used in stock management andpopulation analysis. Earplugs have been widely used as an indicator of ageamong rorquals. It has also been accepted that the transition phase in theearplug 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 furtherinsight into the utility of the transition phase as an indicator of age atsexual maturity in the North Pacific common minke whales, which has not yetbeen clarified. The relationship between sexual maturity and transition phasein earplugs was examined using 981 readable earplugs from common minke whalesthat were sampled at the JARPN and JARPN II scientific permit survey platformin the western North Pacific from 1994 to 2011. The transition phase wasrecognized in 53.2% of mature males and in 58.6% of mature females. Most whalesin which the transition phase was recognized in the earplug were sexuallymature. A significant correlation was found between the number of corpora andtime after sexual maturation, as revealed by the transition phase,demonstrating that the transition phase is a valid indicator of age at sexualmaturity in common minke whales. However, it was difficult to recognize thetransition phase in whales that had recently attained sexual maturity becauseinsufficient time had elapsed since its formation. To avoid potential bias, theuse of earplugs as an indicator of age should be restricted to whales more than12 years old.