96 DISCOVERY REPORTS 



Among the various species there is a complete gradation from those which are 

 plentiful at night and completely vanish from the catches in daytime, to those which 

 show no diurnal variation at all, or which even increase during the day and diminish at 

 night. In Table I on p. 95 the various units are arranged roughly in order of the 

 magnitude of their diurnal variations, and the figures in the first six columns are the 

 average numbers per haul, upon which the curves in Figs. 18-20 are based. Thus 

 Pleiiromamma robusta averages 145 per haul between the hours of 2200 and 0159, and 

 sinks beyond the reach of the net during the daytime or at least the afternoon. On the 

 other hand such species as Pyrostephos vanhoffeni show no clear indication of any 

 diurnal variation and in Primno macropa the variation seems actually to be reversed. 



It is now clear that observations taken in the middle of the day must be disregarded 

 in any consideration of the distribution of the more migratory species, and it must be 

 decided exactly which hauls can be admitted as an indication of the numbers of each 

 species present. Without a fuller knowledge of the details of vertical migration, as distinct 

 from diurnal variation, the best that can be done is to select, for each variable species, 

 a period of hours before and after midnight which will be appropriate to the diurnal 

 variations of that species. For Pleiiromamma, for instance, we might allow as valid only 

 the hauls between 9 p.m. and 3 a.m. (2100-0259), while we need not disregard any 

 hauls for such organisms as Pyrostephos. Column 7 in Table I shows the periods during 

 which a haul is regarded, for the purposes of the present paper, as an indication of the 

 presence or absence of a species. Those listed in the table below Rhincalanus are re- 

 garded as having insufficient diurnal variation to merit the exclusion of any of the day- 

 time hauls. It occasionally happens that a species, which on the average is plentiful 

 only at night, is caught in unexpectedly large numbers during the day. If such a catch 

 were comparable to the average haul taken at night about the same time and in the same 

 locality, it may be taken into consideration in the distribution of the species. 



Hardy and Gunther (1934) have discussed the vertical migrations of certain species 

 which are included in Table I and conclude that a more or less marked migration is 

 shown by the following species : Calanus propinquus, C. simillimus, Metridia gerlachei, 

 Pareuchaeta antarctica, Parathemisto gaudichaudi, Vibilia antarctica, Cyllopus spp., 

 Euphausia superba, E. frigida, E. triacantha, Thysanoessa spp., Limacina helicina, and 

 Salpa fasiformis. Table I shows that all except four of these species also show a clear 

 diurnal variation, and it is therefore mainly in agreement with Hardy and Gunther's 

 results. The exceptions are Calanus propinquns, Parathemisto gaudichaudi, Euphausia 

 superba, and Limacina helicina. The explanation of the fact that they show little diurnal 

 variation in the catches of the N 100 B is probably to be explained on the grounds that 

 they inhabit mainly the upper layers, and that their vertical migrations do not take the 

 bulk of them beyond the reach of the net at night. Hardy and Gunther find little or no 

 migration in Calanus acutus and Rhincalanus gigas, and this also is in agreement with 

 the results expressed in Table I. 



