APPLICATION TC LONGLINE GEAR 



100 200 300 400 500 600 

 DEPTH (FEET) 



Figure 2. --Regression lines calculated from 

 laboratory and field sounding tube data com- 

 pared with a theoretical line having a slope of 

 1. Sounding tube depths are plotted along the 

 y-axis. The x-axis represents depths r e - 

 corded by bathythermographs and the cali- 

 bration tank at Pearl Harbor. 



the experiment while the author made his ob- 

 servations as the tubes were removed from the 

 calibration tank. Also, one examiner had not 

 read any tubes during the previous 2 years. 

 However, the significant difference would sug- 

 gest that special precautions should be taken to 

 standardize readings of different observers in 

 the field. 



The regression line for the field data was 

 calculated from observations obtained by 

 Shomura and Otsu (1956) using bathythermo- 

 graphs as standards. Their data represent 

 actual field conditions in that the sounding tubes 

 were attached to bathythermographs and sus- 

 pended with longline gear on fishing stations for 

 the usual period of 5 to 6 hours. They found 

 that their data varied about the theoretical line 

 within the range of error of reading both the 

 sounding tubes and the bathythermographs. The 

 California Department of Fish and Game has 

 also compared sounding tube measurements 

 with those of bathythermographs and found tube 

 readings to vary within 2 fathoms of bathyther- 

 mograph readings (Anonynnous 1955). The 

 calibration line also agrees well with the theo- 

 retical line. Differences between the two lines 

 are surprisingly snnall with variations ranging 

 from 5 percent at 100 feet to no difference at 

 200 feet and to 4 percent at 500 feet. 



Eight albacore longline cruises have been 

 made by POFI in the central North Pacific 

 Ocean (Shomura and Otsu 1956, Graham 1957). 

 Either 40 or 60 baskets of 13-hook gear (fig. 3) 

 were set per longline station with 5- and 15- 

 fathom floatlines alternated in groups of 5 to 10 

 baskets. During 5 of the 8 cruises, extra baskets 

 with appropriate floatlines and with sounding 

 tubes attached, generally on droppers 4, 7, and 

 10, were placed in each set. Hooks were 



omitted from the droppers of these baskets to 

 prevent excessively deep readings due to the 

 diving of captured fish a n d to preclude the 

 danger fronn bare hooks during setting of the 

 gear. It was felt that catches on adjacent baskets 

 generally would not be able to submerge the rel- 

 atively large buoys or floats (1' in diameter, 2' 

 in length) used by POFI. Thus, these bore hooks 

 and were baited. Such placement of the tubes 

 afforded an opportunity to examine the confor- 

 mation of the mainline in a representative num- 

 ber of baskets of longline gear. 



3 FATHOM DROPPER 



Figure 3.--A diagram of a basket of 13-hook 

 albacore gear used by POFI in the central 

 North Pacific. Leaders, 1-1/2 fathoms in 

 length, are omitted from the above diagram. 

 For a more detailed discussion of the gear 

 consult Mann (1955). 



In an ideal situation where, with the 

 exception of gravity, no environmental forces 

 are acting, the configuration of a basket of long- 

 line should be that of a catenary, exemplified by 

 a chain hanging freely between two points of 

 support. If it were found that such a gear ap- 

 proached a catenary under fishing conditions 

 (Morita et al. 1955), then a simple method 

 could be devised that would derive the maximum 

 fishing depths of all the hooks on a given basket 

 from a single sounding tube reading. 



A scale model was constructed to determine 

 whether the configurationof a basket of gear gen- 

 erally was that of a catenary. An area 25 inches 

 in length and 12 inches in depth was marked off 

 on graph paper having 10 divisions to the inch. 

 Horizontally, the graph was scaled in feet (1 

 inch =50 feet) to a portion of the mainline 

 equivalent to a single basket of gear and 



