Bull. Jpn. Soc. Fish. Oceanogr. 76(2), Page 59-65, 2012   Technical improvements of the age character (earplug) collections in common minke whales Hikari Maeda1 † and Hidehiro Kato2 1 Graduate School of Marine Science and Technology, Tokyo University of Marine Science and Technology, Minato, Tokyo 108-8477, Japan 2 Department of Ocean Sciences, Faculty of Marine Science, Tokyo University of Marine Science and Technology, Minato, Tokyo 108-8477, Japan † e-mail: hmaeda@kaiyodai.ac.jp To improve the age readability for common minke whales, Balaenoptera acutorostrata, we tried to minimize damaged earplugs using newly developed techniques. Because of its soft constitution of the earplug tissue, we examined the feasibility of three different types of treatment to collect the earplugs safely from the external auditory meatus (EAM) as follows: type A treatment, impregnate EAM with gelatin stuff through needle but without opening its corium of the dermis (CD) subsequently cooling them with shaved ice to solidify gelatin stuff; type B, impregnate EAM with gelatin stuff after opening CD then cooling with the ice; type C, impregnate EAM with gelatin stuff after opening CD then cooling with coolant gas. Through a total of 214 trials on the minke whales at the platform of JARPN II programs (coastal components) in 2007 to 2009, proportions of embedding earplugs with gelatin were 50%, 71% and 80% in type A, B and C treatments, respectively. From these experiments, the gelatinized collection is so effective to prevent the earplugs from getting damaged during sampling process particularity among younger animals. This clearly leads to increase the resultant age readabilities in common minke whales, and its wider utilities are also expected in the other rorquals. The present paper also examined growth pattern of the earplug and identified the age readability would remain constant at a high level due to lengthener in both the core and the outer-covering among animals over 11 years old. Key words: earplug, age determination, common minke whale, rorquals