Examination on Age Determination of the Fin Whale

As the age characters of the fin whale (Balaenoptera physalus L.), the following have been reported hitherto. They are body length, stage of growth, number of ovulation, white scars (Mackintosh & Wheeler, 1929), baleen plate (Ruud, 1940), christalline lens (Nishiwaki, 1950), and ear plug (Purves, 1955). Among them, laminations presented in core of the ear plug is considered as the best age character, because the lamination is considered to be formed periodically, is used for both sexes, and is recognized for all life span. Furthermore, ear plug is relatively easy to be collected and prepared to examine the number of laminations in the core. Number of corpora albicantia accumulated in overies has some weak points as an age character. There is individual variation in the accumulation rate of corpora, and number of corpora is applicable as an age character for only sexually mature females. However, it is useful for age character next to ear-plug lamination, and the data of corpora count have been collected for long seasons. Furthermore the annual accumultion of corpora is also useful for the study of reproductive physiology of the whale. Ridges presented on the baleen plates is considered to be the most excellent age character of the baleen whales in younger age groups, but unfortunately, it is not able to be used for older age groups. Now, it is very important to determine age of whale from above age characters, and many hypotheses have been published on the age determination. Concerning the ear-plug laminations, Purves (1955) first considers that the period of arrested growth corresponds to the migratory periods when physiological demands of active swimming coincide with complete absence of food, so that each lamination represents a growth period of 6 months. Laws & Purves (1956) estimate that the biannual formation of lamination is correct, compared the ear plug with baleen plate for small samples of fin whales. Nishiwaki (1957), Chittleborough (1959), Dawbin (1959) and Laws (1961) did not denied the hypothesis. The hypothesis of biannual formation oflamination now influences on the cetacean biology and population dynamics. However, recently long term marked whales have been recovered (Brown, 1962) and the investigation of ear plug of the recaptured fin whales give a question on the annual accumulation rate of ear-plug lamination (Ohsumi, 1962). Purves & Mountford (1959) state that it is very doughtful whether environmental conditions have any direct influence of the time of formation of the laminae of the ear plug in whales. Since there is strong evidence (not given by them) that lamina formation is in inherent moulting cycle, it is very probable that it takes place whether the whale migrates or not, and Ichihara (1959)