Table 6. --Trolling catch of albacore in relation 

 to surface temperature 



O-O 



1 — I — I — I — I — I — r 



J 1 L 



J I 1 I L 



25° 30° 35° 



NORTH LATITUDE 



Figure 5C. --Vertical temperature sections and 

 longline catch of albacore (catch/ 100 hooks) 

 along 160 W. longitude during the three survey 

 periods, Mannin g cruise 23, December 1954- 

 February 1955. 



(8. 8 per 10 line-hours of trolling) made at 42 N. 

 on 172 E. longitude during the fall period (fig. 

 4B). In spite of the unusually low surface tem- 

 perature of 52 F, this area was within the transi- 

 tion zone. This low temperature was probably 

 the result of the mixing of surface and deeper 

 waters associated with the recent passage of a 

 low pressure cell with winds up to 65 knots. 



Vertical Distribution of Albacore 



Previous studies by POFI of the vertical 

 distribution of tuna have been based on relative 

 hook depth (Murphy and Shomura 1953a) because 

 a method of routinely determining the absolute 

 depth of the gear was lacking. On two of the 

 three cruises discussed in this paper (Manning 

 cruises 22 and 23) chennical sounding tubes were 

 used to give direct measurennents of the depth 

 of fishing. 



— Individual catches are listed to 

 show the frequency of catches made at the 

 various ten-iper atures. 



in length with an inner chemical coating which 

 dissolves upon contact with water. While in 

 use, the tubes are encased in protective naetal 

 cylinders. When subnnerged, water is forced 

 up the tube by the pressure, leaving a line of 

 demarkation which indicates the m^^ximum 

 depth when the tube is read against a scale. 



The accuracy of the calibration of the 

 sounding tubes under conditions of prolonged 

 imnnersion (i.e., 5 to 10 hours) was checked 

 by attaching thenn to the longline together with 

 a bathythermograph. The results of 26 tests 

 are shown in figure 6. The agreement between 

 the sounding tube readings and the bathyther- 

 mograms suggests that the former may be used 

 to determine the depth of the longline with rea- 

 sonable accuracy. Variation of the points about 

 the theoretical line shown in figure 6 is within 

 the range of error of reading the two instruments. 



The sounding tubes were of glass capillary To estimate routinely the fishing level 



tubing, 4 mm. in outside diameter and 640 mm. of the longline gear, sounding tubes were 



12 



