156 DISCOVERY REPORTS 



but excluding the first one. The vertical distribution is shown graphically in Fig. 74, 

 the G haul being omitted. Although the young euphausians preponderated at the sur- 

 face they were taken in appreciable numbers in the lowest nets, 660 in the sixth net of 

 the C series being the greatest. When the station was started the ship was in shoals of 

 krill which increased in density to a maximum at the time when the C flight was taken. 

 The shoals had come to an end by the time the E and F hauls were made and another 

 one was encountered at G. 



Considering the lower nets, 1 1 -VI, it is seen that with the approach of night there is 

 a distinct trend upwards of the main body of the larvae and a descent again in the 

 morning. Thus in the A flight, between 2.30 and 3.0 p.m., the larvae are more or less 

 evenly distributed below 100 m.: in the B flight between 6.30 and 7 p.m. they are con- 

 centrated between 500 and 400 m.: in the C flight between 10.30 and 11 p.m. they are 

 concentrated between 400 and 300 m.: in the D flight between 2.30 and 3.0 a.m. the 

 subsurface krill has merged with the main body of the surface shoal, and in E, with the 



TIME 1434-1457 



2236-2259 



Fig. 74. Vertical distribution of young Euphausia superha at St. 461. 



approach of daylight, there is a withdrawal of some of the krill from the surface to an 

 ill-defined point of concentration at about 300 m. 



To what are we to attribute this difference of behaviour in the larvae dividing them 

 into the two groups, (a) those that remain at the surface and (b) those that undergo a 

 vertical diurnal migration? There are several possible factors which might be effective, 

 of which the following may be considered: 



(i) Differences due to ontogenetic variation in the two groups. 



(ii) Hydrographic differences. 



(iii) Seasonal changes. 



(i) As Russell (1927, p. 239) points out, a species may be found to occur at a certain 

 depth when aduh, but while young it may have a different vertical distribution. Ex- 

 amination of the larvae from St. 461 shows that no distinction can be made between the 

 degree of development of the surface larvae and those taken deeper down ; they are a 

 homogeneous population with Furcilia 6 and adolescents occurring in all the catches in 

 proportions which preclude any conclusion that either stage prefers a particular water 

 depth (see Appendix II). 



