34© DISCOVERY REPORTS 



Mackintosh and Brown (1956) examined the records of whales observed through an organized 

 look-out system in the R.R.S. ' Discovery II ', covering nearly 47,000 miles steaming in the Antarctic, 

 and drew up a curve showing the variation in the size of the antarctic population of the larger baleen 

 whales, month by month. This curve shows a maximum in February/March and a minimum in 

 July /August and indicates that most of the population leaves the antarctic zone in winter. This curve 

 represents the total numbers of blue, fin, and humpback whales, although a few sei whales may be 

 included and some immature age groups are probably missing. From inspection of the antarctic 

 catches and the variations in numbers calculated by Mackintosh and Brown it appears that fin whales 

 spend an average period of about 4 months south of the antarctic convergence and that the main part 

 of the population is south of the convergence from mid-December to mid-April. 



The incidence of diatom infection on the skin of whales is indirect evidence of migrations (Hart, 

 1935) and suggests that while some individuals may spend the winter in antarctic waters almost all 

 migrate to low latitudes in winter. Similar evidence is provided by the parasites which attack blue 

 and fin whales in low latitudes and the partly explained presence of healed oval scars (Mackintosh and 

 Wheeler, 1929; Mackintosh, 1942; Pike, 1951). 



The chief contributing factors to the migrations of fin whales are undoubtedly food and temperature. 

 Marr (1956) states that the main diet of the southern baleen whales consists of the crustacean Euphausia 

 superba, known as krill, over 20 mm. long, and it appears that this size is very much more abundant 

 in January, February and March than at other times of the year. The main concentrations of krill are 

 confined to the East Wind drift in latitudes south of 6o°-65° S. except in the Atlantic Sector where the 

 Weddell drift brings a rich population into lower latitudes as far north as South Georgia (54 S.). The 

 whales feed heavily on these populations of krill in summer and must feed but little in the warmer 

 waters in winter. As a result, the animals in the winter catch at low-latitude shore stations show 

 progressively decreasing blubber thicknesses and they are in poor condition when they move south 

 in the spring. Once they are in antarctic waters the blubber thickness and oil production increase 

 as the season advances (Ash, 1955, 1956). 



Whales are mammals and maintain a body temperature of about 37 C. This implies that in cold 

 waters more energy is expended to maintain this temperature than in warm waters. On average at the 

 latitude of 62 S. the sea surface temperature is above — 1° C. from December to June, but at 64 S. 

 this temperature is exceeded only from January to April (Mackintosh, 1946), which is also the period 

 when large krill appear to be abundant. When krill ceases to be abundant it is presumably dis- 

 advantageous for the baleen whales to be in cold waters and by moving to warmer regions they are 

 able to reduce their energy expenditure while drawing on their reserves of fat. 



The pattern of migration which has now been described applies to the population as a whole, but 

 the movements of the different classes of fin whale differ in time and there are regular seasonal changes 

 in the composition of the antarctic population from month to month. The sex-ratio tends to remain 

 fairly constant, but Mackintosh (1942) suggests that the vanguard of the main herds of fin whales 

 arriving on the whaling grounds in January is composed mainly of males and towards the end of the 

 season females tend to be in excess of males. The percentage of immature whales in the catch increases 

 towards the end of the season and the average length of the catch decreases. Wheeler (1934) showed 

 that at South Georgia the average age declines through the season ; the majority of physically mature 

 females are taken in December, of sexually mature but not yet physically mature females in January, 

 and the peak influx of immatures is in February. This is also true of the pelagic catch though the 

 corresponding maxima occur rather later. The antarctic population of adult females also shows con- 

 spicuous seasonal variations in quality. Mackintosh (1942) showed that at South Georgia the per- 

 centage of adult females in the catch which were pregnant fell from 85 % in October to 27 % in 



