wherein the basket is thought to hang as a 

 catenary, but it is suggested that readings in 

 excess of fishing depth occurred during hauling. 

 This difficulty is not insurmountable and a 

 simple method is presented whereby the 

 sounding tube nnay be fully utilized on longline. 

 It is givento stinnulate the study of the vertical 

 distribution of tuna. 



We are indebted to Herbert J. Mann (POFI) 

 for his counsel on gear operations, and to the 

 captains, crews, and scientific personnel of 

 the Hugh M. Smith, John R. Manning, and 

 Charles H. Gilbert for their excellent coopera- 

 tion. We especially wish to thank Richard S. 

 Shomura and Tamio Otsu of the scientific staff 

 for their special efforts in the field. 



THE SOUNDING TUBE 



The chemical sounding tube, developed in 

 conjunction with a sounding machine by Lord 

 Kelvin (Sir William Thompson) to take sound- 

 ings while underway, has a long history in 

 oceanographic work (Knight 1945). The glass 

 tube, coated on the inside with a water-soluble 

 chemical, measures 640 mm. in length and 

 4 mm. in outside diameter and is a relatively 

 simple and inexpensive— depth indicator. Its 

 operation is based on the relation between 

 pressure and depth in a column of water. When 

 the tube, open at the bottom and closed at the 

 top, is lowered beneath the surface, water is 

 forced into the open end of the tube. The chem- 

 ical coating on the inside of the tube is dis- 

 solved, thus leaving a line of demarcation at 

 the level reached by the water. Fronn this 

 record of pressure, the depth to which the tube 

 was lowered may be read directly from a 

 calibration scale. 



Figure 1 shows the essentials of sounding 

 tube gear used in conjunction with PCFI's 

 longline fishing. When the tube is prepared for 

 use it is broken at the scored lower end and 

 then inserted in the brass casing. Small sec- 

 tions of rubber tubing are threaded upon the 

 protruding ends of the tube to retain it in the 

 casing. The casing is then strapped to the 

 longline dropper with plastic tape. Upon re- 

 trieving the gear the tube is removed from the 

 casing and placed in the slot of the calibration 

 scale. The depth in fathoms, indicated by the 

 demarcation line in the tube, is read from the 

 scale. 



— Sounding tubes may be purchased com- 

 mercially for $0. 36 each and calibration scales 

 for $1.75 each. 





A B 



Figure 1. --Essentials of the sounding tube 

 gear: (A) Glass sounding tube, (B) brass 

 casing, (C) wooden calibration scale, (D) 

 rubber hose. The tube on the reader's right 

 has been submerged and shows a demarcation 

 line (E). 



Tests performed in the laboratory and in 

 the field indicate that the sounding tube measures 

 depth with considerable accuracy. Figure 2 

 shows regression lines calculated from labora- 

 tory and field data as connpared with a theoret- 

 ical line having a slope of 1. To obtain the 

 laboratory data, 30 tubes were taken from 

 POFI's stock, which represented portions of 

 numerous orders. These were then divided 

 into 6 lots, eachof5tubes. By means of a 

 pressure tank employed by the U. S. Navy to 

 calibrate bathythermographs, pressures 

 equivalent to depths of 100, 200, 300, 400, 500, 

 and 550 feet were applied to the tubes. One lot 

 of tubes was subjected to each depth equivalent. 

 The resulting measurements of depth were read 

 by three examiners. The difference between 

 examiners was found to be statistically signifi- 

 cant (F = 3. 40, P, 05 = ^- ' ^)' ^^^ "^^ operation- 

 ally snnall; the readings of the individuals did 

 not differ more than 5 percent at any of the 

 testing depths. It is difficult to evaluate this 

 test in regard to field conditions since two of 

 the examiners read the tubes one day following 



