DISTRIBUTION OF YOUNG STAGES OF EUPHAUSIA SUPERBA iii 



less than o" C, and the greatest numbers of eggs were taken from water of this low 

 temperature. 



In salinity the conditions found at St. 540 seem to indicate some approximation to the 

 optimum for the occurrence of eggs. At this station the salinity is between 34-36 and 

 34-62 parts per thousand, and at others, where the eggs are comparatively plentiful, the 

 salinity corresponds more or less closely to these limits. 



VERTICAL DISTRIBUTION 



Eggs occurred in all six nets of the vertical series fished from the surface down to 

 1000 m., but the bulk of them were found below 250 m. At stations where more than 

 one net yielded eggs, and where large numbers were found, the greater proportion was 

 in the lower nets. The depths of the nets in which eggs were found is given in Table III. 

 At all stations a series of closing nets was fished from the surface downwards, but above 

 the shallowest net mentioned no eggs were obtained. 



The percentage of the total number of eggs per 50 m. unit in each of the six hauls is as 

 follows : 



50- o m. 0-8 % 

 100- 50 m. 8-3 % 

 250-100 m. 22-0 % 

 500-250 m. 67-2 % 

 750-500 m. i-o % 

 1000-750 m. 0-7 % 



These percentages are calculated from the total number of eggs in each net haul, cor- 

 rected to make the numbers comparable. The numbers are very greatly influenced by 

 the great abundance of eggs at St. 540. At this station the sounding was 510 m., so that 

 the deepest net fished reached to 10 m. from the sea-bottom. It suggests a concentration 

 of eggs near the bottom and a possible explanation of their relative paucity in our catches 

 as a whole. 



If the eggs are laid at or near the bottom, their occurrence in the surface nets might 

 easily be accounted for by surface mixing of the water column, which in the Bransfield 

 Strait region is known to be particularly effective. 



Rustad (1930) referring to Thysanoessa macrura suggested that spawning in that 

 species might take place below 400 m., and that eggs and sexually mature animals might 

 be concentrated in circumscribed shoals. He thought it unlikely, however, that "a sur- 

 face form like Eiiphaiisia superba would spawn in deep water". Ruud (1932) discounted 

 the explanation of circumscribed shoals and, referring to E. superba, postulated the 

 hypothesis that it spawns close to, or right under, the drifting ice. In a later paper 

 Rustad (1934, p. 36) replies to Ruud's criticism and states "that the physical conditions 

 in the deeper layers, as compared with those right under the ice should be unfavourable 

 to the spawning and development of the youngest larvae, is hard to see. The chief dif- 

 ference is found in the salinity but this seems to be of less importance, our finds demon- 

 strating that the larvae may develop at the higher salinity in the deeper layers ; nor does 

 the smaller content of oxygen seem to be unfavourable." 



