HORIZONTAL DISTRIBUTION OF ADOLESCENTS AND ADULTS 



319 



Distribution in relation to surface temperature 

 The extreme range of surface temperature over which E. triacantha has been found is -i-8o° C. 

 (St. 463) to 11-88° C. (St. 2803); two specimens only were taken at each of these stations. The 

 1 00-0 m. N lOoB night hauls have been grouped according to the season and the surface temperature, 

 using 2° C. intervals, and the mean catch per 20 min. haul calculated for each group. These means 

 are shown in Table 2. At two stations, 2807 in summer and 452 in spring, unusually large catches 

 have raised the mean considerably and the values obtained when these catches are omitted are shown 

 in brackets. It can be seen that, except during the spring, the greater part of the population lives in 

 a zone in which the surface temperatures are between 2 and 8° C. 



E. triacantha is an inhabitant of the West Wind drift and is virtually excluded from the Weddell 

 Sea water and the East Wind drift. It is clear from the surface temperature range mentioned above 

 and from the moderately large catches taken in the West Wind drift in spring that the exclusion from 

 the Weddell Sea and East Wind drift cannot only be due to an intolerance of low surface temperatures. 



Table 2. Seasonal variation zoith surface temperature of the mean number of adolescent and adult 

 Euphausia triacantha per 20 min. haul. Means calculated for 2° C intervals {see text for explanation of 



figures in brackets) 



Summer Autumn Winter Spring 



Surface 

 temperature 

 (°C.) 



— 2'00 O-OI 



o-oo-i'99 

 2-00-3-99 

 4-00-S-99 

 6-00-7-99 

 8-00-9-99 

 IO-00-II-99 

 I2-0O-I3-99 



Mean No. obs. Mean 



074 



i-6i 



970 (7-58) 



16-69 



19-11 



0-25 



0-43 



0-00 



19 



36 



37 

 16 



1-13 



6-20 



14-08 

 10-88 

 4-07 

 0-67 

 0-00 

 o-oo 



No. obs. 



16 

 46 

 36 

 17 

 15 

 6 



4 



3 



Mean No. obs. Mean 



4-40 

 10-68 

 12-36 



15-70 

 3-00 

 0-00 

 o-oo 



0-00 



35 



18 



II 



10 



8 



7 



4 



3 



7-30 

 13-09 (9-22) 

 11-37 

 5-21 

 0-36 

 0-00 



0-00 

 0-00 



No. obs. 

 69 

 33 

 19 

 24 

 II 



Referring back to Fig. 2 (circumpolar chart) it will be seen that in the east and mid-south Atlantic 

 the southern limit of distribution corresponds very closely with the mean northern position of the 

 pack-ice and it is possible that the pack-ice has some influence on the southward spread of the species. 

 That this correlation does not occur to the same extent in the Pacific and Indian Oceans suggests 

 that the pack-ice is not the sole limiting factor. 



It is possible, in a species such as E. triacantha which performs an extensive diurnal migration, that 

 the temperature at the lower level of the vertical range may have a controlling influence on the 

 horizontal distribution. It has been found that the southern limit of distribution of E. triacantha falls 

 close to the position of the 1° C. isotherm at 600 m. but it is impossible at present to examine this 



correlation more closely. • , r.- n 



Moore (1952) has suggested that the limits of distribution, both horizontal and vertical, of diurnally 

 migrating euphausiid species may be controlled by a balance between temperature and illumination 

 but to study this balance in the case of E. triacantha ^ great deal of information would be required 

 on the illumination at different depths in the Antarctic. 



Euphausia triacantha and E. superb a 

 In Fig 6 the distribution of adolescent and adult E. triacantha is plotted with that of E. superba over 

 20 mm. long. The data are derived from both day and night hauls from 100 m. to the surface with 

 the N lOoB Mr J. W. S. Marr has kindly allowed me to use his unpublished data on E. superba for 



