LIFE IN GREAT DEPTHS 157 



netting towed along the bottom at the end of a long, 

 tapering, steel cable. The shape as well as the width of 

 the trawl can vary within wide limits, from the otter 

 trawl used in 1910 by Hjort in great depths in the North 

 Atlantic on board the "Michael Sars," the width of 

 which was about 50 feet, to the so-called sledge trawl 

 used from the "Galathea," the gap of which, kept open 

 by an iron frame, had a width of 10 feet. The prawn 

 trawl and an otter trawl used by Nybelin from the 

 "Albatross" had a width of 30 feet.^ 



Three great improvements in the technique of deep- 

 sea trawling were utilized during the "Albatross" cruise. 

 First, the depth at which the trawl was towed was kept 

 under continuous control by means of our excellent 

 recording fathometer. With 30,000 feet of cable or 

 more it is essential to ascertain beforehand that the 

 bottom over which the trawl is moving is suitable for 

 undisturbed hauls. Second, thanks to theoretical and 

 experimental work by Kullenberg, the exact length of 

 cable required for keeping the trawl in contact with the 

 bottom could be calculated.^ Third, because of the 

 powerful electric deep-sea winch built for the "Alba- 

 tross," the trawl could be lowered and raised at the 

 speed required without undue loss of time. 



The difficulties of deep-sea trawling naturally in- 

 crease with the depth and with water movements due 

 to strong surface currents. The relatively low power of 

 the "Albatross" engine, which allowed the ship a maxi- 

 mum speed of less than two knots with 30,000 feet of 



