HORIZONTAL DISTRIBUTION, GROWTH AND DYNAMICS OF DISPERSAL 379 



(2) In November, a period of increased feeding and growth, while the majority still fall mainly 

 within the 11-20 mm. range, all but a very few have developed, notably in the second half of the 

 month, a more or less pronounced, and in some instances dominant, over 20 mm. component. 



(3) In December they tend to fall mainly within the range of the over 20 mm. class, except in water 

 of East Wind origin where they are inclined to be backward and are still, as in November, mainly 

 inside the 11-20 mm. range. 



(4) Throughout the season they display in their length frequencies a heterogeneous pattern of 

 modal values. 



Summer. The distribution and relative abundance of the 11-20 mm. class in summer (January- 

 March) is shown in Fig. 124. Taking first the position in the northern or Weddell zone, it is 

 evident that there is such rapid growth among the young, now one year old, swarms in December 

 that by January the small 1 1-20 mm. class, so prominent in spring, virtually everywhere outgrows itself 

 and, except for rare and for the most part negligible instances to the contrary, virtually everywhere 

 disappears from the plankton. In other words, in the northern zone generally — along the whole of the 

 Weddell drift, on the South Georgia whaling grounds and in the Bransfield Strait — there has been almost 

 total transition of these early adolescent forms to a whale food of the staple (over 20 mm.) size. There 

 are, however, two instances in Weddell Middle where transition has not been so complete as else- 

 where. Both were recorded in late January 1931 near a large body of pack which at that time, as 

 Mackintosh and Herdman's ice-chart (1940, PI. Lxxxiii) shows, lay far to the north of its summer 

 mean. It seems possible then, that being located in a region from which the winter ice had evidently 

 but recently cleared away, the young yearling swarms recorded there had developed more slowly 

 (p. 356, Fig. 108) than their contemporaries in warmer ice-free water and so had survived the 

 spring in a condition more commonly encountered (p. 355, Fig. 107) in higher latitudes. 



It is the East Wind drift, where the growth-rate (p. 358) is so slow, that is now virtually the sole 

 locus of abundance of the 1 1-20 mm. class. In these high latitudes the young first year swarms, having 

 spent the better part of 9 months under the winter ice, are found, now as swarms of a whole year's 

 standing, still in a very backward state (p. 355, Fig. 107), many of them with a strong to dominant 

 1 1-20 mm. component, a condition in which some of them may persist even right through to the end 

 of summer.^ These backward swarms, as Fig. 124 shows, appear to be particularly abundant in 

 the Atlantic sector, the most conspicuous of them in the highest latitudes near the continental land. 



Throughout the West Wind drift the scarcity of small whale food is most pronounced, especially 

 in the closely sampled Pacific sector. 



Autumn. The distribution and relative abundance of the 11-20 mm. class in autumn (April-June) 

 is shown in Fig. 125. By the end of summer, it is clear, the yearling swarms have practically 

 everywhere outgrown their early adolescent state with the result that throughout the ice-free zone 

 small whale food in the 1 1-20 mm. range virtually does not exist in the plankton of the autumn months. 

 Of the four very small occurrences recorded, two significantly are in the cold East Wind drift, 

 both May records, the third, an April record, in the almost equally cold Bransfield Strait and the 

 fourth, also an April record, far south in Weddell East. 



Autumn, then, is the season when the now 16-18 month old swarms belong almost exclusively 

 to the staple (over 20 mm.) class. Judging, however, from the very backward condition of a young 

 summer East Wind swarm recorded near 70° S in the Atlantic sector on 17 March (p. 396, 



^ Since in many instances the young summer yearling swarms, particularly in the East Wind drift, consist essentially of 

 mixtures of small (11-20 mm.) and staple (over 20 mm.) whale food, the detailed description of their developmental condition 

 is deferred until the section dealing with the summer developmental condition of the staple size with which it will be seen 

 the corresponding condition in the 11-20 mm. range is closely linked. Meanwhile see p. 396, Fig. 136. 



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