ON THE DEVELOPMENT AND 

 DISTRIBUTION OF THE YOUNG 

 STAGES OF KRILL (EUPHAUSIA 



SUPERBA) 



By F. C. Fraser, B.Sc. 



Department of Zoology, British Museum (Natural History) 



(Text-figs. 1-76) 



INTRODUCTION 



THIS report deals with the development of Euphaiisia siiperba during the period 

 covered by the first fifteen months of its growth. The distribution of this young 

 euphausian with reference to locality of occurrence, time of occurrence and vertical 

 position is also discussed. It has been possible to come to certain conclusions regarding 

 the life history of the animal, but it is felt that there are many points mentioned upon 

 which more research will be required in order to understand their proper significance. 

 It is hoped, however, that the results set down here will be useful in future analyses of 

 E. superba material, especially in connection with the very large collections made by the 

 vessels of the Discovery Committee, and that in this way they may help in the final 

 elucidation of the biology of this animal which is such an important constituent of the 

 antarctic fauna. 



In a report to the Discovery Committee in 1930, Dr J. F. G. Wheeler demonstrated 

 that there is a two-year developmental period in E. superba as opposed to a one-year 

 period in other euphausians whose life history is known. He measured krill from 

 the stomachs of whales throughout a whaling season, and from the analyses of length 

 frequencies was able to distinguish the two-year groups. Dr Johan T. Ruud in 1932 

 gave the first published account of this phenomenon. 



SOURCES OF MATERIAL 

 The stations mentioned below, from which plankton samples were examined, are 

 nearly all located in the Falkland Islands sector of the Antarctic. The area round South 

 Georgia was concentrated upon in the earlier stages of the investigations, for at that time 

 ice-edge whale fishing was in its infancy and South Georgia was one of the chief centres 

 of the industry. In addition to observations in the South Georgia area many were made 

 in the Bransfield Strait and on lines of stations in the Scotia Sea. It is perhaps un- 

 fortunate that the main source of the data presented is thus in a portion of the Antarctic 

 where hydrographic and bathymetric conditions are so very highly specialized. The 

 great ridge of the Scotia Arc, the proximity of Graham Land to the southern end of 

 South America, and the system of currents associated with the Weddell Sea, all con- 



