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1. From 54 female fin whales chosen at random from a greater number of animals from which data and material were collected during the Antarctic whaling season 1962/1963, records have been made of the baleen plates and the ear plugs. For the records the complete baleen plates including the part of the plates embedded in the gum are used. All ear plugs used for this study were complete and undamaged. 2. According to their ovaries and baleen records 50 animals were sexually mature, 4 animals were sexually immature. 3. In each individual the record of the complete baleen plate is entirely comparable to the record of the ear plug, in its general trend and in the sequence of peaks and hollows. Also the regular cyclic repetition found in the records of the baleen plates is present in the records of the ear plugs. The comparison of the records of baleen plates with those of ear plugs is only possible when it starts with the last formed part, forming the basis of the core of the plug and the first part of the cortex of the baleen plate deep in the gum, because these represent both the same moment in the life of the animal which is exactly known, viz. the moment in which the growth stopped due to the death of the animal. Table III “Growth periods” in the ear plug, per period the mean length number of animals i ii m rv V VI vn VIII IX 4 69 2 53 56 8 62 68 72 8 70 67 73 71 10 63 64 63 61 59 5 59 57 60 63 58 57 6 54 57 61 56 58 58 59 5 69 72 68 67 67 65 65 62 1 66 58 51 62 59 54 49 64 48 total mean 63 64 66 64 60 60 61 63 48 4. In the records of baleen plates and ear plugs of a number of immature animals the “double hump” or a part of it was found at the right hand side of both. In some of the animals an “ovulation peak” was present at the same time at the beginning (left hand end) in the record of the baleen plate and ear plug; in both in the same position with respect to the surrounding peaks and hollows. This is also true for the records of ear plugs and baleen plates of older females. 5. The records of the ear plugs can be divided into “growth periods” according to what is done in the records of the baleen plates. In each individual the division between the “growth periods” in the record of the ear plug are in the same position with respect to the sequence of the surrounding peaks and hollows as is found in the record of the complete baleen plate. In both records the cyclic repetition of peaks and hollows in the successive “growth periods” is clear. 6. In 21% of the animals examined the number of “growth periods” in the record of the ear plug is equal to the number present in the record of the baleen plate. In 17% of the animals examined the number of “growth periods” in the record of the baleen plate was lower (1 to 3 “growth periods”) than was found in the record of the ear plug. In 62% of the animals examined the number of “growth periods” in the baleen plate was greater (1 to 6 periods) than was found in the ear plug. 7. Evidence was put foreward that the increase in length of the ear plug is obstructed after the animal has reached a certain age. This moment is not the same for all animals but is probably related to the various “constitution types” present in the catch. It is shown that in the distal end of the ear plug the length of the “growth periods” suddenly decreases, so only a certain maximum number of “growth periods” can be found. In the baleen plate the same situation exists due to wear at the tip of the plate. For these reasons the exact age of a fin whale can only be determined as long as wear at the tip of the baleen plate and compression of the distal layers of the ear plug does not occur. 8. From the evidence put foreward it is clear that age determination in fin whales by simply counting the layers present in the core of the ear plug is far too subjective and does not give reliable results. In our opinion best results for age determination in fin whales are obtained by counting the corpora present in the ovaries of females. When this number is divided by the mean ovulation rate (1.25, see Van Utrecht-Cock, 1966) and by adding 6 years (mean number of years before attainment of sexual maturity) the age of the animals calculated in this way is reasonably accurate.

Comparison of Records of Baleen Plates and of Ear Plugs in Female Fin Whales, Balaenoptera Physalus (Linnaeus, 1758)