GENERAL CONSIDERATIONS 34* 



April, these figures being averaged over a number of years. A similar though less marked decline 

 from December to March has been shown in the pelagic catch also by a number of workers (Mackin- 

 tosh, 1942; Nishiwaki and Hayashi, 1950; Nishiwaki and Oye, 195 1 ; Ohno and Fujino, 1952; Kakuwa, 

 Kawakami and Iguchi, 1953). This is partly a result of the entry into the catch of resting females 

 which were recently lactating and therefore protected and under-represented in the catch, but Ohno 

 and Fujino (1952), Kakuwa, Kawakami and Iguchi (1953) and Laws (19590) also suggest that preg- 

 nant females migrate northwards earlier than the others. 



Ash (1955) showed how the rate of increase in blubber thickness was constant through the season 

 and could be represented by a straight line ; the curves describing the increase in oil content for two 

 consecutive seasons (1953/54 an d 1954/55) are nearly parallel, indicating that the rate at which whales 

 lay down fat is independent of the store already there. He suggested that the variations about the 

 regression lines representing blubber thickness and oil production might indicate waves of migration 

 coming south, and points out that such waves of migration were observed by Mackintosh and Wheeler 

 (1929) at South Georgia. In a later paper (Ash, 1956) he broke down the data on blubber thickness 

 into separate figures for males, and for pregnant, non-pregnant and lactating females. He shows that 

 males and non-pregnant females are represented by two curves which are almost identical, while the 

 curve for pregnant females is well above, but nearly parallel to them, closely following the alterations 

 in slope. He suggests that whales arrive in the Antarctic in groups which are made up of males, females 

 which are pregnant and those which are not, and that these groups maintain their identity throughout 

 the season. Ohno and Fujino (1952) have similar data. 



We may conclude then, that fin whales undertake long seasonal migrations between the Antarctic 

 and waters to the north. In the summer, largely from December to April, they are feeding on 

 Euphausia mperba in high latitudes and then move northwards to become dispersed over an immense 

 area of ocean in winter, some to tropical waters and many in sub-tropical and temperate regions. It 

 would appear that both the north and south migrations of older animals and of pregnant females are 

 in advance of those of other groups and that the sexually immature animals are later. 



THE OVARIES 



For ovaries examined since 1954, drawings and records of the size and appearance of the ovaries, and 

 the number, size, and appearance of the corpora lutea, corpora albicantia and follicles were made. 



Size 

 No late foetal ovaries have been examined, but the average dimensions of four pairs of fixed ovaries from 

 foetuses between 2 and 3 m. in length were: length 46 mm., breadth 17 mm., depth 12 mm.; the 

 mean weight of single ovaries was 6-6 g. 



Measurements of the dimensions of fixed post-natal ovaries are inaccurate because of distortion, 

 but measurements were made on 131 pairs of fresh unfixed ovaries in the 1953/54 season. 



Eight immature ovaries were of mean length 27 cm., breadth 8-2 cm., depth 3 cm. ; 52 ovaries from 

 non-pregnant mature females were of mean length 31-1 cm., breadth 11-4 cm., and depth 4 cm. Of 

 105 pairs of ovaries from pregnant females the ovary which contained the corpus luteum was of mean 

 length 33-2 cm., breadth 12-1 cm., depth 4-2 cm. (excluding the corpus luteum) and for the other 

 ovary the length was 32-3 cm., breadth 11-9 cm., depth 3-9 cm. 



Weight is a better index of the size of the ovary and there are records of the combined weight of 

 both ovaries for 1567 female fin whales from eight seasons between 1939/40 and 1955/56. The 

 frequency distribution of combined ovary weights (in 0-5 kg. groupings) for these three classes of 



