244 DISCOVERY REPORTS 



In referring to our results he writes on p. 96 as follows: 



Hardy and Gunther (1934) 1 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 propinqmis, C. simillimus, Metridia gerlachei, Pareuchaeta antarctica, Parathemisto 

 gaudichaudi, Vibilia antarctica, Cyllopus sp., Eiiphausia superba, E.frigida, E. triacantha, Thysanoessa 

 spp., Limacina helicina, and Salpa fustformis. 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 propinqmis, Parathemisto gaudichaudi, Eiiphausia superba, and 

 Limacina helicina. The explanation of the fact that they show little diurnal variation in the catches 

 of the N 100 B 2 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. 



The reality of vertical migration 



We must now consider the theory that has been advanced from time to time that 

 vertical migration is not a real phenomenon, and that the larger numbers of an animal 

 taken in the nets towards the surface at night are due to the animal being able in day- 

 light to see the net and escape capture. The exceptional catches of some animals at 

 or near the surface during daylight would appear to prove the rule, unless we must 

 imagine that on these occasions they are "asleep". Russell (1933) has shown that a 

 number of forms, including Calanus, Sagitta, and young fish, which show a marked 

 migration away from the upper layers in daylight during part of the year, may be taken 

 at the surface in bright sunlight in late summer. He suggests that these, belonging to 

 different broods, have a different behaviour; no one would suppose that these broods 

 are less awake or less alive to the danger of the net and so fail to escape. 



The tables giving the depth distribution of the adult and larval stages of Thysanoessa 

 and Eiiphausia frigida, from the N 70 V nets, in the section on the general distribution of 

 these forms, show that in addition to a vertical migration much larger numbers are 

 taken at night than in the daytime. This at first sight would appear to support the theory 

 that they avoid the net during the daytime, but these tables included nets from both 

 shallow- and deep-water stations. Now wehave long suspected that at shallow-water 

 stations, i.e. stations of a depth down to 250 or 300 m., the Euphausians may go right 

 down either a very short distance from the bottom or actually resting on the bottom 

 itself during the daytime. 



In Tables XL-XLV we give the day and night depth distribution of Eiiphausia frigida 

 larvae and Thysanoessa adults, and larvae for the deep-water stations only — i.e. all those 

 of 500 m. depth and over (the numbers of Eiiphausia frigida adults are not shown, being 

 too few to be at all significant). Here we see in each instance, except Thysanoessa 

 furcilia, evidence of vertical migration, but the numbers taken at night are not always 

 the larger. They are larger in the case of Thysanoessa — adults and cyrtopia — almost equal 

 in the case of Eiiphausia frigida furcilia, and smaller in the case of E.frigida cyrtopia and 

 Thysanoessa furcilia. In no case are the numerical differences of such significance to 

 warrant the suggestion that the organisms are avoiding the net during the daytime. 



1 This refers to the present report the publication of which has been delayed by additional work in 

 Part V. 



2 The 'B' refers to the 100 cm. diameter net hauled obliquely from 100 m. to the surface. 



