THE DEVELOPMENT AND LIFE-HISTORY OF KRILL 131 



The prolonged spawning season, which is characteristic also of euphausians from the northern 

 hemisphere, gives rise to a very heterogeneous population, the stock being continually replenished 

 by the addition of new generations. Adults were found between the months of August and April, but 

 when the breeding season is over, they appear to die off, being absent from the catch during the 

 autumn and winter months. It should be mentioned, however, that after spawning, females were 

 found in April at the surface feeding actively, a fact which does not suggest lack of vitality, and 

 therefore, since after this month the available material becomes very scanty, their apparent absence 

 may simply be due to lack of evidence, and not to a holocaust consequent on exhaustion after breeding. 

 The material available is not enough to show a correlation between the appearance of the spring 

 phytoplankton maximum in the different regions of the Antarctic zone and the precosity of develop- 

 ment of the euphausian population, but there is evidence to show that generations hatched early in 

 the season anywhere in the zone benefit directly from the abundance of food and the rising tem- 

 perature of the surface layers, while later generations develop more slowly, partly no doubt because 

 the food supply is becoming reduced by grazing-down, and partly because of the onset of the colder 

 weather. 



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