S4 DISCOVERY REPORTS 



made as follows: 50 m. to the surface, 100-50111., 250-100 m., 500-250111., 750-500111. and 

 1000-750 m., the net, except for the uppermost of the series, being closed at the end of each haul. 

 Not infrequently the N70V series was extended downwards to include the 1500-1000 and more rarely 

 the 2000-1500 m. layers. In shallow water less than 1000 m. deep the net was fished as above down 

 to the maximum depth permitted by the soundings. The towed nets, the coarser Nioo and TYF, 

 were fished over the stern in the wake of the ship through a fairlead on the port quarter. In the early 

 days of the investigations it was the practice to tow the N 100 horizontally, a flight of three such nets 

 being so disposed on the same warp that one would fish at or just below the surface (0-5 m.), the 

 other two at approximately 50 and 100 m. respectively. Both nets fishing at the deeper levels were as 

 a rule closed before hauling. A radical change in towing practice took place after 1927, the treble 

 horizontal flight with the NiooH being abandoned in favour of a single towing with an open net 

 hauled obliquely to the surface from a depth of about 100 m. This practice resulted in considerable 

 saving of time on station and proved to be almost as good a method of sampling the surface and sub- 

 surface population of the whale food as the more elaborate three-level horizontal towings had been in 

 earlier years. The routine surface horizontal net with which the investigations originally began was 

 reintroduced in 1932 when it became the practice from time to time to fish it on a separate line some 

 three to four fathoms long through the starboard quarter lead while the surface (roo-o m.) oblique 

 net was coming in. This practice became increasingly adopted from 1933 onwards, becoming routine 

 in 1935. At a still later stage, beginning with the upper subsurface strata in 1931, the systematic 

 examination of the whole bathymetric horizon from the surface down to 1000 m., involving the loo-o, 

 250-100, 500-250 and 1000-500 m. layers,^ was undertaken by means of flights of from two to three, 

 occasionally as many as five or six, oblique loo-cm. diameter stramin nets, all but the uppermost 

 rigged for closing and suitably disposed on the same long warp. The large Young Fish Trawl or 

 TYF, although used also in the upper strata, was chiefly employed to explore the great depths below 

 1000 m., where it was thought the krill, if present at all, would be so widely scattered as to elude 

 capture altogether by a smaller net. As already mentioned it was used obliquely, and on rarer occasions 

 vertically, and, except in the upper 250-0 m. layer, was closed at the end of each haul. Depths of 

 150 m. or less were determined by Kelvin Tube, greater depths by depth gauge. 



The fine-meshed N70V, as Fraser (1936) has shown, proved the most suitable instrument for the 

 capture of the very young stages of the krill, more especially the eggs, Nauplii, Metanauplii, Calyp- 

 topes and early Furcilias, and it is from the analyses of the catches of this net that our knowledge of 

 the distribution and movements of these stages has been mainly derived. The later Furcilias and older 

 krill including fully grown adults also appear in the catches of the vertical net but, the larvae apart, 

 only in negligible numbers. For the capture of these older stages the horizontal or oblique stramin 

 net has proved the more eflicient apparatus. 



Owing to the patchy distribution of E. superba and its tendency to gather in surface swarms, there 

 is a large element of chance in sampling the populations and special problems arise when comparing 

 samples from different nets. In view of the varying circumstances in which our nets were fished, in 

 certain instances corrections have to be applied to the catch-figures to allow such comparisons to be 

 made. These matters are discussed on pp. 57-9, and especially on pp. 278-84. 



^ I am speaking here in terms of nets fished at various levels falling within the broad limits of these several vertical 

 horizons. 



