284 



DISCOVERY REPORTS 



knots. Nevertheless it was never possible to keep the ship's speed absolutely constant 

 during any one towing or to be sure that the speed of the ship was exactly the same at 

 successive towings, and with small variations in the speed the depth of the nets varied 

 widely — the nets rising towards the surface with increased speed and falling with de- 

 creased speed. Thus the shallowest surface haul was at St. 735, where the net fished 

 from 62 to o m., and the deepest was at St. 748, where the net fished from 180 to o m. 

 There were many gradations between these two extremes. The deep net also frequently 

 fished through depths which varied widely from those aimed at. The following stations 

 may be quoted as instances of the kind of variation which occurred : 



St. 727: 310-170 m. 

 733: 300-140 m. 

 746: 306-124 m. 

 748: 204-138 m. 

 757: 320-126 m. 



St. 769: 342-150 m. 



776: 356-170111. 



802: 320- 70 m. 



815: 314-188 m. 



830: 356-140111. 



Similar figures may be found throughout the tables. The question of applying some 

 correction to these hauls so as to standardize the catches should therefore be given 

 consideration. 



In spite of the somewhat wide variations in the depths at which the surface net 

 began towing it can be shown diagrammatically (Fig. 4) that the oblique path of the 



SURFACE HAUL 



TRACK OF SHIP STEAMING AT 5 K NOTS [SOM.PER MINUTE], I£00m APPROX 



I395mAPPR0X 



AToA. WIRE OUT ZOOM, 

 A' 



DEEP HAUL 



■ TRACK OF SHIP STEAMING AT g KNOTS [60m PER MINUTE] . IgOOM APPROX 



AtoA.wire outsoom. 

 b. closing depth 



00 M 

 '-200 m 



IOOm 



200 m 



-300m 



Fig. 4. Diagram illustrating the variations in the length of the oblique tow with variations in the com- 

 mencing depth of the surface and deep hauls and in the closing depth of the deep net. 



net to the surface was in reality not widely different at every haul. In Fig. 4 a number 

 of surface hauls have been plotted which begin at depths varying from 50 to 180 m., the 

 widest limits of error met with in the surface towings under consideration. The com- 

 mencing depth aimed at in every case was 100 m., which, as the diagram shows, gives 

 a towing of approximately 1375 m., assuming the path of the net to the surface to be a 

 perfectly straight oblique line. A haul with a commencing depth of 50 m., however, 

 gives a towing 1395 m. in length (approximately). This is only 20 m. in excess of the 

 length aimed at (1375 rn.), and in view of the length of the tow and of the circumstances 



