SAGITTA GAZELLAE 267 



It is also evident from Fig. 20 and from Table 18, Appendix, which gives the numbers plotted on 

 Fig. 20, that there is a considerable decrease in numbers with depth. This confirms the evidence from 

 the series of vertical stations mentioned above. 



The evidence from the vertical series suggests that the maximum concentration of S. gazellae is in 

 the 100-50 m. layer, and this confirms Bollmann's (1934) statement for both S. lyra and S. gazellae 

 (considered together). The data in Fig. 20, however, suggest that the main concentration is between 

 100 and 150 m. The numbers in this figure have been plotted against the mean depth of each haul and 

 the concentrations between no and 135 m. may well have been taken at a depth of less than 100 m., 

 as the nets which took them fished into the 100-50 m. layer. The data from the vertical nets may be 

 considered to be more reliable in this respect. The depth range of the species obtained from the samples 

 available extends down to 3000 m., but it is quite possible that its total range may reach to the 

 bottom; at great depths specimens are rare. 



Table 12 shows the numerical data from thirty-five daylight and thirty-five night vertical stations 

 and suggests that there is no diurnal migration in the species; Fig. 21 shows the depth distribution 

 during the 24-hour Station 461, and confirms the absence of diurnal migration. 



2230 



DARK 



HOURS 

 0230 



0430 



IO30 



Fig. 21. The vertical distribution of 5. gazellae at Station 461 showing absence of diurnal migration. 

 Data from oblique 1 m. nets. Horizontal scale 1 mm. = 2 individuals. 



Fig. 22, which has been constructed by the method described by Mackintosh (1934, p. 377), shows 

 the monthly vertical distribution of the species in the meridian of o°, and demonstrates the seasonal 

 vertical migration in winter. Table 19, p. 278, gives the numerical data from which the figures were 

 compiled. In the Antarctic (south of 51 on the meridian of o°) this migration is into the warm deep 

 layer (between approximately 150 and 1000 m) (Fig. 23 a, b confirms this), but in the Subantarctic 

 (north of 51 ) the migration is not so great, and only in August does the centre of maximum abundance 

 of the population move below 100 m. However, a study of the size-group percentages in the surface 

 100 m. as sampled by the 1 m. oblique nets in winter (Fig. 26, see p. 271) reveals that the population is 

 mainly composed of one brood, hatched out in late March and early April ; and as there is evidence that 

 breeding has been taking place throughout the summer (see p. 274) one must conclude that the larger 

 individuals have migrated to below 100 m. This is confirmed by the size of individuals taken in the 

 N70V nets at this time of year, and also by Fig. 24 which is a comparison of size-group percentages 

 from the shallow (100-0 m.) and deep (250-100 m.) oblique nets during the months of May and 

 June. 



Fig. 23 a, b referred to earlier gives the numerical data from N 70 V nets from the o° line plotted on 

 a temperature-salinity diagram, and shows the shift of the Antarctic population from the surface water 

 towards the warm deep water in winter. This would be more striking if it were not for the fact that the 

 250-100 m. nets pass through both the warm deep and the Antarctic surface layers; and although 

 it is probable that the animals in a haul come from the warm deep and not from the Antarctic surface 

 water (for example when there is a large number in the 500-250 m. net, but none in the 50-0 and 



