HORIZONTAL DISTRIBUTION. GROWTH AND DYNAMICS OF DISPERSAL 401 



of Other animals, has taken a heavy toll of the krill, the mass effect of which would only now be 

 beginning to become apparent. This too would help to account for the autumnal decline. 



In his recent studies on the diatomaceous skin-film of fin whales and its correlation with fatness 

 Ivashin (1958) finds that the 'maximum state of nutrition', as measured by blubber thickness, is in 

 February, very few whales, whether infected with diatom or not, increasing their fatness in March. 

 He therefore concludes that it is in March that a major decline in the krill takes place from its 

 'massive summer accumulation'. 



^ 68 

 % 64 

 O 60 

 O 56 



52 

 48 

 44 

 40 

 36 

 32 

 28 

 24 

 20 

 16 



NUMBER 

 MEASURED 



APRIL 



MAY 



JUN 



I 577777777777 14 15 17 I7 202026 27 27 6 7 7 8 18 2 

 42 :;07 207 207 207 207 207 2318 2322 855 1351 1359 I360 W434 



663 207 207 207 207 207 WI96 665 855 2344 2346 1360 1361 1781 

 (3)(l)(l)(8)(lX8)Cl)(8)(l)(B)(0(8)(B)(2) (2) (3) (5X2) (2)(3) 



(I) (2) 



91 30 102 69 20 33 39 57 69 53 25 lOO 44 20 

 41 39 25 50 52 55 50 66 23 370 III 46 lOO 100 



68 



64 ^ 



50 O 



tr 



56 u 



52 5 



^« I 



44 ^ 



35 g 



32 < 



28 ^ 



24 ^r 



o 



20 Z 



UJ 



16 -I 



NUMBER 

 MEASURED 



WEDDELL DRIFT 



EAST WIND DRIFT 



Fig. 138. Developmental condition of the staple whale food in autumn. For vertical scale see legend to Fig. 107. 



Taking the autumnal distribution in greater detail and beginning with the northern zone, it will be 

 seen that such minor occurrences of the over 20 mm. class as have been recorded, although widely scat- 

 tered, are confined to the Weddell drift, to the South Georgia whaling grounds, where they show some 

 small measure of crowding possibly overemphasised by the close spacing of our observations there, 

 and to the Bransfield Strait. The minor occurrence, or rather occurrences,^ in the strait may in fact 

 have been of East Wind origin, for when compared with the summer and autumn condition in which 

 numbers of the half-grown swarms occur elsewhere in the Weddell zone (Figs. 136 and 138) 

 the young 16 month old swarms in this southern channel (Fig. 138, Station 207) show a lag 

 in female development and a general inferiority in modal length. The adult swarms, too, recorded at 

 Station 207 exhibit a corresponding inferiority in modal length, a condition more typical of the East 

 Wind than of the northern zone. 



In the West Wind drift the scarcity or absence of the staple class is everywhere pronounced. 



In the East Wind zone attention may again be focused on the rather closely sampled region between 

 0° and 30° E, the so-called backwater referred to on p. 395, in which as in summer there again appears 

 to be a conspicuous scarcity of all classes of the surface population, larval, adolescent and adult alike. 

 Elsewhere, such few autumn observations as we have in these high latitudes suggest that such older 

 stages of the krill as survive the summer are now distinctly more abundant in the southern than in the 

 northern zone. It would appear, too, that by May (Fig. 138, Stations 1359-61) the adult swarms 

 having died off, the staple feeding-stuff in these as in lower latitudes is represented by the adolescent 

 (17-18 month old) generation alone. That the East Wind drift should carry, as it appears to do, 

 a somewhat richer over 20 mm. autumn population than the northern zone is perhaps not altogether 



^ In this locality a continuous series of surface nets was towed one after the other along a course extending for approxi- 

 mately II miles. 



47 DM 



